Thursday 10 July 2014

Build a RF Dummy Load

 

A RF dummy load is quite useful when working on transmitters. It allows you to test and adjust the transmitter without an antenna, eliminating interference to other radios on your test frequency. It also presents your transmitter with a proper 50 Ω load so as to not cause any damage to its final RF amplifier stage.

A recent project required me to modify and align twelve UHF transmitters. The transmitters had a 25 watt output and the alignment session on each would be short. Rather than buy a dummy load for this project, I decided to build my own.

The central part of the dummy load is a resistor (or resistors) with a total resistance of 50 Ω and a wattage equal to or greater than your transmitter. The resistors also must be non-inductive which eliminates all the common wire-wound power resistors. Acceptable types of resistors include carbon composition and thick film.

For the resistor in this dummy load I chose a #32-1007 50 Ω flanged termination unit from Florida RF Labs with a rating of 40 watts. Other parts include an aluminum case, heatsink and SO-239 connector.

Dummy load parts

Construction is rather simple. First the heatsink and SO-239 connector are bolted to the case. Be sure to use heatsink compound between the heatsink and case.

Dummy load case

Next holes are drilled and tapped for the flanged termination unit. The flanged termination unit also gets a thin coat of heatsink compound before installation. Its lead is extended with a short piece of wire to reach the SO-239 connector.

Flanged Termination Unit

A quick check with a SWR Meter shows a VSWR of 1.1, an excellent reading.

Sunday 6 July 2014

Protect your MOSFET final stage

by G3YCC

Protect your MOSFET final stage

This idea was described in Sprat 52 in 1982 and basically shows how, by the addition of two cheap components, a resistor and zener diode, mosfet PA's can be protected from destruction by over driving. This is described in the article by Alan G3UZU who mentions seeing this recommended in Radio Spares data sheet 5342 on power mosfets. The resistor limits the dissipation in the zener diode in series with it from gate to earth and can be 10 ohms, 1/4 watt. The zener voltage is found from data sheets. For example the commonly used VN46AF needs a zener of less than the maximum drive volts of 15, say 13v, 400mW. The ubiquitous VN10KM would need a zener of 4.7v, 400mw. For other devices, look up the data in many books and catalogues and note the maximum gate voltage quoted and choose an appropriate zener. This simple modification will be found very useful in homebrew transmitters.


Steve Quest's Transmitter

by S.Quest

In various newsgroups Steve Quest has described an AM transmitter for mediumwave and shortwave that has produced great results and is quite inexpensive. Here is the verbal description he offered:

However, for now, just picture a standard Pierce oscillator, crystal controlled on your frequency of choice, with the output connected to a standard class C amplifier stage. The only modification to the standard class C amp stage is the insertion (series, so cut the trace and insert) of a modulation transformer between the power supply line and the isolation inductor on the collector. What works great is the $1.95 at my local audio shop, SPECO line matching transformer. Connect the transformer backwards! In other words, the input side goes to the class C amp, and the output side, normally the side connected to the speaker becomes input. Transformers work both ways you know.
Use the 500 ohm and common taps on the class C amp side, and the 8 ohm and common tap on the input side.
Now hook up a power audio amplifier to the (now) input of the transformer, and a mic or other audio source to the line input of the power audio amp. Adjusting the volume on the audio power amp will increase and decrease your MODULATION! Use a scope to set your modulation, just lay the probe near the antenna to see the AM waveform. Adjust to about 80% modulation, don't overmodulate, it sounds like crap AND is quite inefficient.
*IMPORTANT* Use a chebichev filter between the output of the class C and your antenna! If you don't your harmonics will be over a watt for sure!
This configuration, when powered by 12 volts will generally give you an input power of about 5 watts. That's based upon the class C amp giving you a times 10 gain, and the input power from the Pierce oscillator of half a watt. Sloppy design of the Pierce, and poor tuning on the class C will cause the power to go DOWN. However, if you're good, you can get 10 watts or more by tweaking the Pierce up to a 1 watt output into the x10 class C stage.
About a 4 to 5 watt input signal to a 150 foot random-wire antenna on the 41 meter band during the average sunny winter day resulted in a clear signal copied at greater than 200 miles and a weak signal copied at 1000 miles. This range example is reproduceable by several successful tests.
And here is the schematic:

Schematic of Steve Quest's Transmitter

Parts List:
R1 = 47k 1/4w
C1 = .001uF
C2 = 470pF
C3 = 470pF
C4 = 100pF
C5 = .01uF
C6 = 470pF
C7 = 820pF
Y1 = 7.3728 Mhz series type (off the shelf, CPU application)
Q1 = ECG or NTE282 or similar
Q2 = ECG or NTE235 or similar
T1 = Speco type T7010 70 volt line matching transformer
L1 = 28turns around 1/4" ferrite overwrapped with 9turn secondary
L2 = 28turns around 1/4" ferrite
L3 = 1.5uH choke
To make L1, tightly wrap 28 turns, then wrap 9 turns over that tomake the secondary. Space the windings out to evenly cover thefull length of the 28 turns. Use light gauge enamel wire for all.
To connect the modulation transformer, wire backwards using the normal secondary as the primary. Connect the red wire of theinput side to Vcc and the black wire to the class C amp choke. Connect the output side white wire to the amplifier speaker jack positive, and the black wire to the speaker jack negative. Use the amplifier volume control to adjust modulation level.

A few more details of this circuit:

... since the rig operates in the 41 meter band, I just used generic CB radio transistors. The Pierce oscillator uses a generic CB radio driver transistor, and the final is a CB radio final. Thesetransistors are cheap, readily available, and probably will never beoutlawed (like I suspect VHF RF components will eventually be). Since CB radio is 27 Mhz, running them at 41 meters (7 Mhz) is very easy on them.
Any type NPN, TO-220 package CB radio driver and final will workfor this application. There are probably _better_ transistors out there,but these are cheap, available at the local radio shop, and already instock for those of us who fix a few CB radios from time to time.

And here's some additional detail:

I get my forms from a distributor, and my inductors as well. However, I know there are ferrites that are "close enough" in the Radio Shack assortments. You should be able to find one fairly close.
The question this time is could one substitute say 5/16" core for L1 & L2 and adjust the windings to come out with the same results. If the answer is yes, then could you recommend a good starting point to make the alterations?
Sure, use a 5/16" core, and the same number of turns. Remember, I didn't specify the wire gauge, and I should have said "about 1/4" ferrite rod" instead of being exact. Use a heavy enough enameled wire that you don't vaporize it with current flow, I used the middle gauge out of the Radio Shack 3 pack for my windings. This fact should probably be added at the website. :) You may have to adjust secondary turns between the oscillator/driver and the final to tune for maximum power. Don't glue the windings and leave plenty of extra (not looped but curved up) so you can do that. Once you find the right amount, glue them down. I use wax usually, but have also used shellac, varnish, silicone, you name it.
On the inductor in the final, an extra few turns will not hurt, but don't take away! Same goes for the primary on the osc/driver. However if you add turns, you'll have to adjust the secondary turns and capacitors. One thing to remember with HF verses VHF, the windings are NOT as critical. 3 turns in an air wound VHF oscillator tank coil will get you, let's say 88 Mhz, and one more turn and you're down to 64 Mhz, and another and you are in the 40's. It isn't like that with HF, it's not so touchy.

Steve Quest's Transmitter

by S.Quest

In various newsgroups Steve Quest has described an AM transmitter for mediumwave and shortwave that has produced great results and is quite inexpensive. Here is the verbal description he offered:

However, for now, just picture a standard Pierce oscillator, crystal controlled on your frequency of choice, with the output connected to a standard class C amplifier stage. The only modification to the standard class C amp stage is the insertion (series, so cut the trace and insert) of a modulation transformer between the power supply line and the isolation inductor on the collector. What works great is the $1.95 at my local audio shop, SPECO line matching transformer. Connect the transformer backwards! In other words, the input side goes to the class C amp, and the output side, normally the side connected to the speaker becomes input. Transformers work both ways you know.
Use the 500 ohm and common taps on the class C amp side, and the 8 ohm and common tap on the input side.
Now hook up a power audio amplifier to the (now) input of the transformer, and a mic or other audio source to the line input of the power audio amp. Adjusting the volume on the audio power amp will increase and decrease your MODULATION! Use a scope to set your modulation, just lay the probe near the antenna to see the AM waveform. Adjust to about 80% modulation, don't overmodulate, it sounds like crap AND is quite inefficient.
*IMPORTANT* Use a chebichev filter between the output of the class C and your antenna! If you don't your harmonics will be over a watt for sure!
This configuration, when powered by 12 volts will generally give you an input power of about 5 watts. That's based upon the class C amp giving you a times 10 gain, and the input power from the Pierce oscillator of half a watt. Sloppy design of the Pierce, and poor tuning on the class C will cause the power to go DOWN. However, if you're good, you can get 10 watts or more by tweaking the Pierce up to a 1 watt output into the x10 class C stage.
About a 4 to 5 watt input signal to a 150 foot random-wire antenna on the 41 meter band during the average sunny winter day resulted in a clear signal copied at greater than 200 miles and a weak signal copied at 1000 miles. This range example is reproduceable by several successful tests.
And here is the schematic:

Schematic of Steve Quest's Transmitter

Parts List:
R1 = 47k 1/4w
C1 = .001uF
C2 = 470pF
C3 = 470pF
C4 = 100pF
C5 = .01uF
C6 = 470pF
C7 = 820pF
Y1 = 7.3728 Mhz series type (off the shelf, CPU application)
Q1 = ECG or NTE282 or similar
Q2 = ECG or NTE235 or similar
T1 = Speco type T7010 70 volt line matching transformer
L1 = 28turns around 1/4" ferrite overwrapped with 9turn secondary
L2 = 28turns around 1/4" ferrite
L3 = 1.5uH choke
To make L1, tightly wrap 28 turns, then wrap 9 turns over that tomake the secondary. Space the windings out to evenly cover thefull length of the 28 turns. Use light gauge enamel wire for all.
To connect the modulation transformer, wire backwards using the normal secondary as the primary. Connect the red wire of theinput side to Vcc and the black wire to the class C amp choke. Connect the output side white wire to the amplifier speaker jack positive, and the black wire to the speaker jack negative. Use the amplifier volume control to adjust modulation level.

A few more details of this circuit:

... since the rig operates in the 41 meter band, I just used generic CB radio transistors. The Pierce oscillator uses a generic CB radio driver transistor, and the final is a CB radio final. Thesetransistors are cheap, readily available, and probably will never beoutlawed (like I suspect VHF RF components will eventually be). Since CB radio is 27 Mhz, running them at 41 meters (7 Mhz) is very easy on them.
Any type NPN, TO-220 package CB radio driver and final will workfor this application. There are probably _better_ transistors out there,but these are cheap, available at the local radio shop, and already instock for those of us who fix a few CB radios from time to time.

And here's some additional detail:

I get my forms from a distributor, and my inductors as well. However, I know there are ferrites that are "close enough" in the Radio Shack assortments. You should be able to find one fairly close.
The question this time is could one substitute say 5/16" core for L1 & L2 and adjust the windings to come out with the same results. If the answer is yes, then could you recommend a good starting point to make the alterations?
Sure, use a 5/16" core, and the same number of turns. Remember, I didn't specify the wire gauge, and I should have said "about 1/4" ferrite rod" instead of being exact. Use a heavy enough enameled wire that you don't vaporize it with current flow, I used the middle gauge out of the Radio Shack 3 pack for my windings. This fact should probably be added at the website. :) You may have to adjust secondary turns between the oscillator/driver and the final to tune for maximum power. Don't glue the windings and leave plenty of extra (not looped but curved up) so you can do that. Once you find the right amount, glue them down. I use wax usually, but have also used shellac, varnish, silicone, you name it.
On the inductor in the final, an extra few turns will not hurt, but don't take away! Same goes for the primary on the osc/driver. However if you add turns, you'll have to adjust the secondary turns and capacitors. One thing to remember with HF verses VHF, the windings are NOT as critical. 3 turns in an air wound VHF oscillator tank coil will get you, let's say 88 Mhz, and one more turn and you're down to 64 Mhz, and another and you are in the 40's. It isn't like that with HF, it's not so touchy.

Low-pass filters

 

"Station WRNY is equipped with the latest device known to the world of radio engineering, called a harmonic suppressor, the effect of which is the elimination of the signals of WRNY from every frequency except that to which it has been assigned by the Government, which means, in fan language, that the station may be tuned in on only one setting of the dial." Radio News magazine, August 1925

A low-pass filter attentuates the energy above a specified cut-off frequency. These filters are used to reduce the intensity of harmonics so that they don't interefere with other signals. Most of this filters on this page were designed for various ham radio gear operating in the bands from 1.8 to 14 MHz.

Unless otherwise noted these filters have 50 or 52 ohms input and output impedance. Capacitances are expressed in picofarads, inductances in microhenries. Most authors recommend using silver-mica capacitors.

filter1.gif

for transmit frequencies
in the range of         C1     C2                           L1 
1500-2000 kHz    1800  1800   30 turns #26 wire on T-50-2 toroid 
3500-4500 kHz     680    680    21 turns #22 wire on T-50-2 toroid 
5500-7300 kHz     470    470    14 turns #22 wire on T-50-2 toroid 
5500-7300 kHz     820    820                           2.2

filter2.gif

approx.
cutoff freq.      C1              C2            C3           L1, L2      
1200 kHz    .0039 uF    .0056 uF   .0039 uF       6.8 

1800 kHz    .0033 uF    .0043 uF   .0033 uF       5.6 

2000 kHz    1592 pF     3184 pF    1592 pF      3.98 
7300 kHz    470 pF       1000 pF     470 pF     42 turns
                                                                         #26 wire
                                                                        on T-50-2  

filter3.gif

cutoff  frequency
(in MHz)             30 dB attenuation
                             point (in MHz)      C1,/C4    C2,/C3    L1,/L3    L2 
2.16                           4.0                     820         2200        4.44     5.61 
4.12                           7.3                     470         1200        2.43     3.01 
7.36                         12.9                     270          680         1.38     1.70 
10.37                       15.8                     270          560         1.09     1.26 

Based on an article by Ed Wetherhold published in UK Short Wave (Dec. 1983).

Simple Modulators for Solid-State Transmitters

 

Simple AM modulators work by varying the amount of power flowing through the transistor which is serving as the RF output amplifier. By imposing an audio waveform on the power supply, amplitude modulation is achieved.

Rather than giving detailed examples, this page gives simplified schematics, followed by links to circuits that actually use the various techniques.

mod1.gif

An external audio amp sends its output to the 8-ohm side of an 8-to-1000 ohm (or similar) audio transformer. The other winding of the transformer is inserted in the power supply going to the final RF amp transistor. The audio transformer must be rated to handle the level of power going through it; an inadequate transformer will produce a bad-sounding signal. In some cases it takes a lot of searching and experimentation to find the best transformer.

technique #2-A

mod2a.gif

A lightly amplified audio signal is fed to the base of an NPN transistor. This transistor is inserted into the power supply going to the RF amp transistor. A choke between the two transistors keeps RF out of the power supply and audio circuitry. The RF choke must be rated to handle the level of current going through it.

technique #2-B

mod2b.gif

A line-level audio signal is fed to an audio amplifier I.C. The output of this chip is used as a power supply for the RF amp transistor.

This circuit is physically smaller and lighter than designs that use a modulation transformer. The audio quality is good.

technique #3

mod3.gif

The RF output transistor is an FET (often an IRF510 or similar). The FET's source is grounded. The FET's drain is connected to a transformer, which has the modulation (audio) amp on one side and the RF output taken from the other side.

technique #4

mod4.gif

This is the system used in Charles Wenzel's circuit. This design has had many re-incarnations, for example in a proposed 13.5 MHz transmitter circuit. The Wenzel modulator is capable of high quality modulation at levels approaching 100%. For 100 milliwatt transmitters, suitable transistors are 2N4401 or 2N5551.

Finals

Call 'em PA's, finals, or output amps, these are the last chance for the signal to get amplified before it is hurtled out into the world. Shown here are some partial schematics of finals taken from a wide variety of sources. Unless otherwise noted the input is on the left side, output on the right.

final2.gif

The circuit above will output 1.2 watts with a 13.8 volt power supply. Value of RFC not critical, but must be able to handle some current, try 15 turns of wire on a toroid core.

final3.gif

The circuit above will output 4 to 5 watts with an adequate power supply. The design shown is optimized for 7 MHz; the transformers may need a bit of modification to optimize for other bands. Provide heat-sinks for the transistors.

Crystal Oscillators

 

The circuit below is a standard oscillator of the Colpitts variety. Similar circuits have been used in many ham radio homebrew transmitters. This particular circuit should function well at frequencies from 1500 kHz to 8000 kHz. For use on lower frequencies, the values of C1 and C2 might need to be increased.

[diagram]

C1: 100 pF ceramic disc or silver mica
C2: 680 pF ceramic disc or silver mica
C3: .01 uF ceramic disc
C4: .001 uF ceramic disc
Q1: 2N3904
R1: 220 K
R2: 1 K

beginner's assembly instructions:

[diagram] If you've never built a circuit from a schematic before, this might be a good one to start with. The diagram below shows how you can arrange the parts on a prototyping board such as Radio Shack catalog number 276-175. (A prototyping board, also called a solderless breadboard, contains groups of holes that are electrically connected. Each hole has a little spring/clamp thingy in it that grabs ahold of the component leads. This is a great way to experiment with circuit designs and learn about the building process.)

Attach a 10-inch (25 cm) piece of bare wire to the output, then sit a radio next to the circuit and tune to the crystal frequency. Apply power. If everything is connected correctly and all the components are in working order, you will hear the carrier (or the silence caused by it) on your receiver. Now all you need to do is add a buffer amp, a modulation stage, a final RF amp, a harmonic suppression and output matching section, and you've got an AM transmitter. :-)


Below is another version of the circuit with a couple of enhancements. The variable capacitor between the crystal and ground allows you to adjust the frequency slightly. (More capacitance equals lower frequency.) Q2 serves as a buffer amplifier which stabilizes the circuit and boosts the output power. This circuit was developed independently by another MWA member and uses very different values for R1, C1 and C2 compared to the first circuit on this page; don't let those differences scare you.

[diagram]

C1, C4, C5: .001 uF
C2: .0033 uF
C3: 20 to 50 pF variable
Q1, Q2: 2N3904
R1: 22 K
R2, R6: 1 K
R3: 18 K
R4: 270 K
R5: 470 K


Another common variation of the Colpitts circuit involves adding a resistor parallel to the crystal, as shown below. What's the advantage? I have no idea. The circuit shown has been tested and works fine at frequencies from 1.5 to 20 MHz.

[diagram]


To round out the collection, here's a Pierce oscillator using an FET. The version on the left is from an electronics textbook. The version on the right is from the "Grenade" shortwave pirate radio transmitter designed by "Radio Animal."

[diagram]


Some people might prefer an oscillator that uses a crystal at 4 times the operating frequency and then divides by four to produce the carrier wave. Advantages are 1) your signal will have a super stable frequency with immeasurably low drift, and 2) you can order a custom-made crystal without it being obvious to the manufacturer that it will be used for a broadcast-band application. The disadvantages are 1) the circuit is more complex, and 2) the output of the oscillator will be a square-wave rich in harmonics. If you would be interested in this approach, you can consider building a circuit inspired by the design of the oscillator section of the Wild Planet toy transmitter.

1 Watt QRP Power Transmitter

 


1 Watt QRP Power Transmitter
The 1 watt 20 meter QRP transmitter with VXO. This is a nice QRP transmitter that can be used in combination of one of the simple receivers. Normally these designs have only two transistors: one is the X-tal oscillator and the second the final amplifier. A good example is my first QRP rig that is also described somewhere on this site. Here the VXO (Variabele X-tal Oscillator) has a tuning range of 16 kHz. This VXO is buffered with an extra driver stage for a better frequency stability and a varicap diode is used instead of a variabele capacitor. An extra transistor is added for keying the transmitter with a low keying current. What you can do with such a simple 1 watt QRP power transmitter. This is a real low power transmitter, so do not expect that you can do everything with it but... When conditions are normal, you can easily make many QSO's during one afternoon with stations with distances upto 2000 km with a simple inverted V wire dipole antenna! From Europe, I did even make QSO's across the Ocean!

Friday 4 July 2014

QRP Transmitters

 

7W on 7Mhz Vasantha VU2VWN's QRO? VFO controlled 40m Tansmitter. Loved by South Indian hams and they modulate the whole tansmitter (and the 40m band - hi!) with AM. My friend Sunil VU2TES who send this circuit said that he worked about 60 countries (in 1993) with 5W CW.

LM317 Voltage Reguletor TX What? Yes, extreemly simple single chip, tansmitter. Ideal for low frequency hamming. If you can get 5W/AM by modulating this simple TX, that will be your Top Band Chat-Box.

Logic Chip Tx Very simple Transmitter that can be used on 20, 15 or 10 meter band. Out put is about 500mW. Xtal Control. Only active component is a 74HC240 Octal inverting buffer!

5Watt Transmitter Simple Transmitter that can be used vertually on any band. Out put is about 5W. Single transister and driven by a TTL Logic chip. Xtal Control.

Tuna Tine Transmitter40m Xtal control two transister nice little transmitter that was orginaly built in a Tuna Fish Can. That does not mean that you too should built in one of those, but it is a nice idea for a simple case isnt it? How about a Pepsi or Coke Can (I have seen AM/FM BC radios made in those - factory stuff!)

Pippin 40m 2Transister TX40m Xtal control two transister nice little transmitter by G3PTO. It is simple but gives 1W Output in to a 50ohm load. The author says "The PA transistor has a "Stove Pipe" heat sink attached and has been left running continously for more than 1 hour without any complaint from the PA stage"

A Simple Receiver - the DC40

 

Last year my cousin had to submit a class project towards her engineering degree and I suggested that a direct conversion receiver might be a good idea. This receiver is the result of that project.

Usual construction write-ups only deal with a finished design that is logically explained and the performance is discussed clinically. Such texts overlook the unpredictable path that an experimenter takes during a project. The final form is often a result of initial specifications, personal choices, accidents and availability of time and material.

Hence, this article: a journal that follows a receiver's birth. This receiver was made over several evenings, spending less than an hour each day.

For those real hams who never read the entire articles, I follow my convention of key caveats and some tips. Here is the entire circuit:

Remember this while assembling the receiver:

  1. Shield the VFO. It should be inside a metal box. I soldered pieces of copper clad board (blank PCB material) around the VFO circuit.
  2. I don't think you understood how important it is. So, I am saying it again, SHIELD THE VFO. This is a magic bullet cure for common ills of a direct conversion receiver.
  3. The receiver layout is uncritical. But keep it clean. Q1, Q2 and Q3 form a tricky circuit. Double check the connections.
  4. It might be tempting, but don't use a battery eliminator. It will produce hum. Use a regulated power supply with adequate filtering. A 15 volt supply with 2200 uF capacitor followed by a 7812 regulator is recommended.
  5. I have included the voltages to be expected at all the transistor leads. Use that as a guide to trouble-shooting the receiver.
  6. Double check the two transformer windings polarities.
  7. Usually, the FETs have a very odd pin out, with the drain lead often put between the gate and source leads. Be careful and sure that you solder it correctly.
  8. If the oscillator doesn't oscillate, swap the two ends of the VFO coil and try again.

Advice on building this receiver

This receiver is for someone looking for a weekend project. But it is quite a performer.

The receiver has just six transistors and a very common audio IC. You can substitute the audio IC with any other audio amplifier circuit. The oscillator too can be substituted by another design. A bipolar oscillator as a VFO (look at my BITX design) can be a lower cost substitute. The FET transistor used costs as much as all the other components put together. The entire receiver cost is still under Rs.100 (about two dollars) in India.

I have noted the voltages at all the transistor leads to help you trouble shoot your receiver. But don't be surprised if it works straight off. The only critical thing is to get the VFO inductance right. You can use another receiver to set it for proper coverage.

This receiver can be adapted to 80 meters as well as 30 meters. With some sort of VFO stabilizing scheme it can work on higher bands too. Nothing is sacred. Anything goes.

Why build a receiver?

Why do you want to build it? These are available at the Dubai Duty Free asked Harish, an old friend, when he spotted us struggling over the DC40 one evening. I didn't have an answer to this question and considering the amount of work piled this quarter, it appeared to be a sensible thing to ask.

I think this question is answered by us all in different ways. My personal answer would be because we human beings are fundamentally tool builders. We have an opposable thumb that allows us to grip the soldering iron.

For an engineer (by the word ‘engineer', I don't just mean those who have a degree, but anyone who applies technical knowledge to build things) the act of building a receiver is a fundamental proof of her competence and capability. It is much easier to put out 1 watt signal than it is to receive a 1 watt signal.

A simple definition of a good receiver is that a good receiver consistently, clearly receives only the intended signal, such a definition hides a wide range of requirements. The receiver has to be sensitive enough to pick up the weakest signal imaginable (note: clearly), it has to be selective enough to eliminate other signals (only), it has to be stable enough (consistently).

For a ham or an engineer, building a usable receiver is a personal landmark. It establishes a personal competency to be able to understand the very fundamental operation of the radio and mastery over it.

A direct conversion design

A direct conversion design is simple and pure. A signal arrives from the antenna; it is converted to audio by simply mixing it with a local RF source and played to your ears through an audio amplifier. The principle can be explained to anyone on a piece of paper in a few minutes. But having the principle expounded so simply, the issues of sensitivity, stability, dynamic range, etc. that confront a builder are exactly the same as those that will confront anyone building a far more complex system.

We chose a direct conversion design because:

  1. It could be assembled easily from parts already available in the junk box.
  2. A direct conversion receiver needs as much design consideration as superhet does to achieve acceptable performance.
  3. I hadn't experienced a direct conversion receiver for a long time.
  4. It would be an innovative project submission.

Initial Design

We didn't plan the receiver minutely. We knew that we required something like in the figure below:

So, the initial design was to fill up these four boxes in the following way:

  • RF Filter: Doubly Tuned Circuit centered at 7.050 Mhz
  • Product Detector: a two diode singly balanced detector
  • AF amplifier: discrete transistors used as audio amplifier to drive Walkman headphones.
  • VFO: Bipolar transistor based, low noise oscillator.

We reasoned that a doubly tuned circuit will prevent strong out-of-band signals from AM breakthrough. The singly balanced detector would be simply enough (just one trifilar coil to wind) and an audio amplifier based on discrete transistors would be better than the LM386 (that I was beginning to hate the LM386 for high noise).

Having read in the EMRFD (Ref. 1) that the local oscillator radiation can cause tunable hum and microphonics, it was contended that an RF amplifier might be required.

Day 1: Beginning at Audio End

We began the receiver construction by fishing out a copper clad board that was about 8 inches by 2-1/2 inches. Using an old razor blade it was scrapped until the copper looked bright. This was to be our new receiver's base.

We had decided to use discrete transistors in our design instead of the standard LM386. We fished around in the junk box for an NPN-PNP transistor pair to be used in the output stage. While looking around, an LM386 turned up. After a bit of head scratching, hate against the LM386 quickly gave way to an old familiarity. We decided to use the LM386 after all.

A few seconds on Google turned up the datasheet of the LM386. The LM386 comes in many flavors according to the datasheet, we were not sure of what the top operating voltage of our chip was (our version was not from National Semiconductors), so we decided to shoot for a conservative 6-9 volts range for our design. This was simply achieved by placing a resistor from the 12 volts supply line to the pin 6 (the power supply pin) of the IC.

We were not too sure of whether we required the additional gain required by adding a capacitor from pin 1 to pin 8: It was left out for the time being. The guiding principle being "Don't ask for more gain, unless you absolutely need". In any case, the extra gain, if required would be only a capacitor away.

It took us 15 minutes to complete the audio amplifier. We immediately wanted to fire up the amplifier. No suitable speaker was available. Looking around the shack, it was noted that the PC was hooked up to a Cambridge Sound Works' System. Those speakers were connected to their outboard amplifier using RCA jacks. One speaker was disconnected from the PC sound system, an RCA jack was soldered to the output of the amplifier.

Laziness took over, and the 12V power line was directly soldered to amplifier. Power was applied and a finger on pin 3 of LM386 produced a loud buzz (with a trace of the local AM station). It was alive. Pin 6 was expectedly hovering around 8 volts. A wet finger applied to the LM386's body revealed no heat. That wrapped up the initial tests of the audio amp.

To test it further, the output of the PC's sound card was soldered directly to the LM 386's input and the PC was booted. LM386 proved to be quite sensitive to the PC's output and the sound volume had to be cut down from the PC's end.

The evening ended with some Jim Morrison flowing out of the LM386. While the party mood had set in, the engineer in me realized that the pumping bass of Lonnie Mack on Roadhouse blues was intermodulating Robby Krieger's bluesy guitar. Note to myself: keep your eyes on the distort and your hands upon the gain!

Day 2: The Oscillator

The key issue plaguing us at this time was: what kind of tuning mechanism should we use? My junk box has a fast depleting stock of variable capacitors (just four more to go). On the other hand, varactor tuning would mean going out and buying a decent 100K linear potentiometer. Varactor tuning also adds noise to oscillator. It was decided to park this issue for the time being and get on with the rest of the oscillator.

A Hartley oscillator based on a FET was decided upon. It is easy to get a Hartley oscillator working. The numbers of crucial frequency determining components are few.

We started the work with winding a coil. A nylon tap washer was used as toroidal former to wind the oscillator coil. A toroidal coil's inductance in micro-henries is given as (k x square of number of turns). The k for these nylon toroids varies between 1nH per turns squared to 1.5 nH per turns squared. A coil with 80 turns (with a tap at 20 turns) was wound to give an estimated inductance of 3.6uH.

The oscillator was fabricated at the other end of the main board. Some space was left out to accommodate a big tuning capacitor if required. It was discovered that we didn't have a Zener diode or a voltage regulator chip in the junk box. We decided to go ahead without it for the time being. We didn't find a polystyrene capacitor either, so we used a disc ceramic 100 pf instead. To keep the circuit simple, an elementary emitter follower was used as a buffer to the VFO. The output of the emitter follower was hooked to an oscilloscope probe and power was applied to the VFO. No output was noted.

Power was removed immediately and the board was take out into the sun and examined carefully. Fortunately the Hartley is such a simple oscillator that one can easily visually verify the entire oscillator's connections. The board was brought back into the lab, power was applied again and the drain voltage was noted to be around 9 volts. The entire VFO was powered through a 470 ohms resistor. Hence a 6mA current appeared to be normal for the FET. The emitter follower was disconnected and the still no oscillations were observed at the FET's source.

At this time, just as I was fighting the urge to use the board as a projectile, the coil started looking suspicious as the rest appeared proper and the DC voltages around the oscillator appeared normal. It was decided to wind a new coil. This is an advantage of using the tap washers: They cost 50 paise each (about 1 cent) and you can throw away a bad one. A new coil was wound and soldered in. The old one's enamel was probably not scrapped well at the tap.

The oscillator was now working well. The output of the VFO was connected to a homemade frequency counter and the frequency was found to be around 5.5 MHz instead of 7 MHz. It was also noted that the oscillator was quite stable considering that the power supply to the VFO was not stabilized. The 100 pf tank capacitor was changed to 47pf to get the frequency down to 7.6 MHz.

An air variable capacitor was soldered across the coil with hookup wire to make it tunable. This made the oscillator unstable. Any hand movement near the VFO would make the frequency change. Even a light tap on the table would make the oscillator jump its frequency. It was clearly due to the variable capacitor's addition. The variable capacitors are quite temperature stable, and this behavior was probably due to the loose hook wires that connected the tuning capacitor to the oscillator. Clearly, we needed a front panel that would mount the tuning capacitor and hold it firmly. At this point, we declared the day close.

 

Day 3: The Box

It was decided that an air variable capacitor without a slow motion drive will be used to tune the receiver. A second piece of copper clad that was 8 inches by 2 inches (I got a large collection of them as discards from a local PCB making shop) was selected to be the front panel. Approximate location of the tuning capacitor was selected such that it would not cover the oscillator components and keep the shaft at mid height from the bottom of the panel to allow the biggest knob possible.

Using a hand drill, three holes were drilled for the tuning capacitor, the volume control and the ear-phones jack. The tuning capacitor required two screws to affix it to the panel, these were marked after inserting the shaft through the main hole and they were also drilled out. From a small piece of scrap copper clad board, three right-angled triangles about an inch to a side were cut and smoothened with a flat file. These were soldered in standing position on the main board and the front panel was in turn soldered onto these angles (see the pictures).

The main tuning capacitor was screwed in and so were the volume control and the phone jack. The oscillator was found to be oscillating at 7.6Mhz so a 22pf trimmer was added in parallel with the 47 pf to 'net' the oscillator at 7.000 MHz. Now the tuning range started at 7.0 MHz and went up to 7.5 MHz. This was quite a wide range especially because we were not using a slow motion drive. A 47 pf capacitor in series with the variable capacitor reduced the tuning range to 180 KHz. The oscillator was set using the preset to tune from 6980 to 7160 KHz.

Day 4: The audio pre-amp and the Big Bang

The W7EL (I am not sure if it is his original design, he is one of the most creative RF designers I have come across) audio pre-amp is now a classic. It consists of a common base stage biased at 0.5ma to give a 50 ohms input impedance that matches the output impedance of a diode product detector. This stage is directly coupled to a common emitter amplifier. Both these stages are powered through another transistor used as an active decoupler. The common base stage provides a stable 50 ohms matching impedance to the detector output, the active decoupler keeps power supply hum and noise out of this crucial stage and the second stage provides bulk of the gain.

We didn't have any text books at hand to refer to, so all we knew was that we should bias the first stage to 0.5mA current which we did and that the second stage should be biased well within the active region to handle at least a swing of 1V signal at the output. This is what we did achieve with the audio pre-amp. A 50 ohms resistance in series with a 0.1 capacitor was fashioned as 'pauper's diplexer' at the input of the audio preamp. The diplexer is supposed terminate all frequencies above the audio range that could come out of the diode mixer.

Now, we had everything in except the diode mixer in place. We couldn't wait to get the first noise out of the receiver. So we hastily slapped together a two diode mixer (singly balanced), connected it to the oscillator, hooked up the audio chain, tack soldered a single piece of wire from the 20 meter dipole's coax to the center tap on the diode mixer and powered the receiver on.

We well did receive - a number of AM stations. Tuning the receiver around resulted in whirrs and whooshes, some of them distinguishable as RTTY stations and some constant carriers. There was a strong hum-like noise across the band.

Until now, our progress was more or less smooth, our performance problems were quite distinct and their solutions quite distinct. But our current problems were not easily measurable and 'knowable'. All we knew at this point was that the receiver was not receiving.

I suggested two things. First, introduce some selectivity in the front-end. Second, get a known signal that you can tune to.

We quickly fashioned a coil and cap tank for the front end that was coupled lightly through 22pf capacitors to the antenna lead and the diode mixer. A 7050 MHz crystal was soldered into a test oscillator and used as a signal generator.

We found that we could tune in the signal. So the receiver was basically working. At this point, my cousin was losing courage. This was not the way she saw it. From her point of view, we had designed and build the circuit. All blocks were working as expected but due to some evil spirit we were not having a working receiver.

At this point, we decided to sleep over the problem and revisit it the next day. We needed a plan for the next day to keep the spirits up.

Day 5: Completion

Our problems were clearly centered around our product detector, we decided to do something about it. It could have been any of the following:

  1. Insufficient drive from the VFO: We had a simple emitter follower as a buffer to the oscillator. The oscilloscope showed that it had insufficient bias and it was clipping on the down-swing.
  2. Singly balanced: The RF input was not being balanced out at the detector output. We needed a doubly balanced detector.
  3. According to Rick Campbell's texts, VFO leakage into the RF input is responsible for tunable hum and microphonics.

A trip was made to the market and 9.1V Zener was procured that was soldered between the power line of the VFO and the ground. The supply resistor was changed from 470 to 220 ohms so that the Zener had reasonable bias current.

The VFO's buffer amplifier was changed from the skimpy single stage design to using a two stage amplifier with feedback with unity gain and the VFO was completely shielded from all sides by soldering copper clad boards all around it. (See the pictures).

A doubly balanced detector was fabricated using two trifilar coils and four diodes. A simple diplexer of a 0.1uf in series with a 50 ohms resistor was added to the detector output to properly terminate the mixer.

A simple low pass filter was added on a trial basis to the input of the detector. The receiver was fired up.

Success at last! The receiver was now working just as well. Connected to a long wire (actually, the shield end of the 20 meters inverted V), it pulled signals from all around. The noise floor was sufficiently low to mark a dramatic jump in the noise when connected to the antenna.

Performance

We do not have the tools to make performance measurements on the receiver. Hence, we rely on our actual receiver experience instead. The receiver is clearly far more sensitive than is required for this band.

We used a regular disc ceramic capacitor in the VFO, but still, it is quite stable. The local SSB nets could be monitored continuously without needing to retune. Even the initial warm-up took less than a minute and the drift was less than 1 KHz during this period (not measured).

The initial audio bandwidth was much greater than 3 KHz. A 0.1 uF at the output of the audio preamp brought the band-width down to acceptable levels. Ideally, an active first or second order low pass filter based on op-amps should have been used. However, the current arrangement is sufficient for us. I personally prefer a wider than normal bandwidth.

At rare instances, a slight trace of AM breakthrough can be heard, if we had an RFC, we could have inserted it between the AF amplifier and the detector to keep the RF out of audio stages. The AM breakthrough, when it happens, it hardly perceptible and it takes a bit of concentration to discern it above the band noise.

Conclusion

We had started with an over-engineered design that included an RF amplifier, etc. Our first cut was a sub-optimal design, and the final version was a reasonable compromise between performance, effort and component availability.

The receiver performance is very clean and the stability is quite good, it is now the standard night-stand receiver by the bed.

Thursday 3 July 2014

Construction of an Emergency Transceiver

 

A few years ago, I took a short boat tour off the coast of Hawaii. As Murphy's Law would have it, our ship was hit by a major typhoon. It seemed like the typhoon lasted for an eternity and when it was over we found ourselves washed up on a deserted tropical island. Unfortunately, our GPS and ship-to-shore radio was destroyed during the typhoon. We had no means of communication to the outside world and we didn't even know where we were located. Somehow, we had to use the local materials to develop an emergency transceiver. I thought that I would document this design in case you ever find yourself in a similar predicament.

The receiver: As you review this reference design, please remember that we only had to work with the local plants and minerals from the tropical island. The objective of the receiver was to keep it simple and build a single diode AM receiver. Figure 1 shows the schematic for the receiver. Although the design seems relatively straight forward, the problem was that we did not have any wire, resistors, capacitors, diodes, or earphones. Fortunately, we were able to construct these circuit elements using local resources. This section will provide how-to guide for building these elements.

Figure 1:  Simple One Diode AM Receiver

Figure 1: Simple One Diode AM Receiver

Coconut shell diode: Probably the most challenging problem we had to overcome was to develop a semiconductor diode. As luck would have it, however, the coconut fruit is actually a pure semiconductor (see Figure 2). Furthermore, sea salt and lime make excellent P and N type doping impurities (see Figure 3). You might wonder why the simple coconut diode isn't used commercially. The answer is simply that coconut has a limited shelf life.

Figure 2: Excerpt from periodic table

Figure 2: Excerpt from periodic table

Figure 3: Coconut Shell Diode

Figure 3: Coconut Shell Diode

Clam shell capacitor: The clam shell capacitor is also worth noting because it makes an excellent natural capacitor. The top and bottom shells act as a parallel plate capacitor, and the capacitance can be easily adjusted by opening or closing the shell to varying degrees. Figure 4 shows the clam in two different positions and the associated capacitance. Please note that the voltage coefficient of a clam shell capacitor is extremely low which minimizes distortion. In fact, I think the clam shell capacitor should be used in commercial high fidelity audio applications.

Figure 4: Capacitance vs. Clam Opening

Figure 4: Capacitance vs. Clam Opening

All the rest of the components: The remaining components are common applications of tropical flora, and consequently, we will not focus on these. For further information, consult the Electrical Characteristics for Flora and Fauna in Tropical and Subtropical Regions [3].

The transmitter: Figure 5 illustrates the simple transmitter design that we used to call for help. This section of the reference design provides details on the components used in the transmitter.

Figure 5:  Schematic Diagram of Simple AM Transmitter

Figure 5: Schematic Diagram of Simple AM Transmitter

Gourd amplifier: It was of key importance that the transmitter had sufficient power to transmit long distances. The gourd from the talahoobaloo tree is an excellent natural amplifier. Typically, the voltage gain of this gourd is between 1,000 and 10,000. The power output is strictly dependant on the batteries connected, so we placed 100 limes in parallel.

Coconut microphone: Considering the principle of reciprocity, it can be shown that a coconut not only acts as an excellent earphone, but also can be used as a microphone. In fact, the coconut approximates an electret microphone and can be modeled as such.

Crystal oscillator: The crystal oscillator was fashioned out of a piece of quartz found in the islands cave. Unfortunately, one of the members of the castaway group was captured by some natives and boiled in a large caldron of water. Fortunately, the natives became distracted when the island's volcano erupted and he was able to escape with the needed crystal.

Final outcome: I am happy to say that the transceiver worked quite well and we were rescued a few months after the typhoon. We were all very happy about the rescue as we were getting sick of coconut cream pie, coconut on a stick, and fried coconut. Unfortunately, we were fined by the FCC for violating AM transmission power restriction. It seems that the lime batteries were even more powerful that we expected. Nevertheless, I hope you can make use of this design in the eventuality that you ever end up stranded on a deserted island. (See Figure 6 and Figure 7).

Figure 6:  Photograph of Transceiver with its Components Labeled

Figure 6: Photograph of Transceiver with its Components Labeled

Figure 7: Operation of the Transceiver

Figure 7: Operation of the Transceiver

References:

  1. Wenzel, Charles, "Crystal Radio Circuits," Crystal Radio Circuits, TechLib.com, 1995
  2. Field, Simon Quellen, "Building a very simple AM Voice transmitter," Building a very simple AM Voice transmitter
  3. Hinkley, Professor Roy, BA, BS, MA, PhD, Electrical Characteristics for Flora and Fauna in Tropical and Subtropical Regions, Island Press, New York, 1964

Sunday 29 June 2014

A QRO HF DUMMY LOAD

image 16

100ohm power resistors for use in Wilkinson power splitters are often available at low cost. A pair of these resistors will make an excellent dummy load. I recently acquired a power film resistor which has four 50ohm resistors mounted on a large copper heatsink flange. As all of the resistors are connected to the mounting flange at one end, it was not possible to connect the resistors in a series/parallel combination to make 50ohm. The solution was to connect the four resistors in parallel to make a 12.5ohm load and use a 4:1 transmission line transformer to provide a 50ohm match. The transformer consists of two turns of RG58 on a Maplin N90AB ferrite core. This gives a perfect 1:1 SWR from 1.8 - 30MHz. The finished circuit is in Figure 6. Note that the DC voltage from the diode is only half the peak-input voltage because of the 2:1 voltage ratio of the transformer. I haven't been able to make much of an impression on this load during testing. Several minutes of SSB speech at 500W PEP will bring it from room temperature to lukewarm. I'm sure my transmitter would catch fire long before this load gets hot!

Saturday 28 June 2014

5W 14MHz (20m) AM Transmitter

 

 

This is a simple audio transmitter for the 14MHz (20m) band. Its principle is seen from the diagram, so I will describe very briefly: The first stage is a simplified oscillator, the other three stages are amplifiers. The coils are wound on the 4mm diameter core with screw ferrite core to fine tune to the highest brightness of the light bulb. As a modulation choke, a secondary of small transformers (eg 220V/9V) is used. The tuning capacitor is also tuned for the highest brightness of the bulb. Then you can replace the bulb by a short. When you connect other antennas However, it is necessary to re-tune the output. Never operate without antenna!! Current consumption of the transmitter from the 12V supply during the normal operation of about 800mA. The coils have 30, 25 and 12 turns.

    


5W 14MHz (20m) AM Transmitter schematic

Homebrew Antenna Tuner

 

 

Steve Yates - AA5TB

Tuner Schematic


For years I've thrown together simple antenna tuners to get me on the air when needed. Whenever I would go to a sidewalk sale or hamfest I would pick up any components that might someday come in handy to make an antenna tuner. Commercial antenna tuners are very expensive and often of questionable quality. They are so very simple to make that I find it hard to justify purchasing a store bought unit. I realize that good antenna tuner components can be quite expensive when purchased new if you are even lucky enough to find a supplier. If you are not in a hurry all of the necessary components can be found used for very reasonable prices.

There is nothing special about the antenna tuner described on this page. It is a simple T-network and I have found that it can match any unbalanced antenna system I've ever put on it. I've always used PI-networks but component values can become unwieldy at the lower frequencies for such a network. Below are some more photos of my T-network.

Front View
Front View

Rear View
Rear View

Inside View
Inside View

The components were all found on the surplus market and even though it looks very old it's really only a few years old. The metal enclosure is probably 50 years old even though it had never been used when I purchased it at a sidewalk sale. The roller inductor is silver plated and incorporates a very good turns counter. It is WWII surplus and I had been saving it ever since I came across it as a kid. The two capacitors are 500 pF each and I removed them out of a defunct automatic antenna tuner that was once used at a shore station for ship to shore communications. I purchased all of the insulators from a local surplus outlet. Even the knobs and dial plates are ancient surplus. After all is said and done, I have about $10 invested in this tuner but to purchase one of similar quality the price would probably be about $300.

I don't have plans for you to follow in order to replicate this antenna tuner but as you can see via the schematic it is very simple. I would suggest obtaining the components first and then design everything around them. Use short, fat conductors if possible to interconnect the components. Ther required capacitor plate spacing is determined by the transmitter power and the impedances involved. For 100 W or less the plate spacing of most available air-variable capacitors is probably adequate. The capacitors that I used here have a maximum capacitance value of 500pF each but I probably could have gotten by with 250 pF units. For the inductor try for around 25 µH if you plan to use the tuner on 80 m and maybe 160 m. The larger the inductor's conductor, the better. If you can't find a roller inductor then a tapped inductor can easily be made assuming an adequate switch can be obtained. Of course for simple open-air designs, a wire and alligator clip will suffice.

If the power you plan on using is at QRP levels, say less then 5 W, then the components can be greatly reduced in size. The polyvaricons that are often found in less expensive AM radios can be used for the capacitors and a small tapped coil wound on a toroid will suffice for the inductor. It should be noted however, that the efficiency of this antenna tuner is inversely related to the losses in the inductor. Therefore, even though a small inductor will not burn up at QRP levels, a dB of loss is a dB of loss at any power. What I am getting at is that at some impedance ratios, the RF currents in the coil can become relatively high. In these cases the losses within the inductor can become high unless care is taken to keep the Q of the coil high. This can be done by making the inductor out of the largest conductor possible and by making sure any contact resistance, such as the alligator clip, is kept at a minimum. It should be noted however, that this contact resistance will usually swamp out the RF resistance of the conductor. In other words, if you are using an el cheapo alligator clip or tap switch, there isn't too much sense in going to a 00 AWG conductor ;-)

I only created this page to hopefully encourage others to try and build their own antenna tuners instead spending good ham radio budget on an expensive commercial tuner. I apologize for not having exact instructions but this is an easy project to do on your own with the components that you have available.

40 Meter QRPp Transmitter

 

Steve Yates - AA5TB

40m QRPp Transmitter


I built this simple, crystal controlled 40 meter CW transmitter back in 1996. It sports full break-in operation and 250 mW of output power. The final output transistor is a mighty 2N2219A. The N6WG QSL card laying against the lid is just an example of one of the many contacts I have made with this transmitter. The station was located in California and my antenna was an inverted L (bent monopole) operated against a very poor ground system (a couple of ground rods). Considering the losses in my antenna system, I thought the contact was pretty good.

My design was based on one of the late Doug DeMaw's (W1FB) designs with many of my own modifications. The rig is built into an old file box I bought at a garage sale for $1.00.s

Inside View
Inside View

Schematic Diagram

Friday 27 June 2014

SSB signal generated by phase shift


 


SSB signal generated by phase shift
Home SSB transmitter is a challenge to the HAM and a test to its own power. But up to now little HAM can self-make SSB transmitter, the main reason is the core components of the transmitter SSB-crystal filter and the matching crystal (such as crystal filters are equipped with 9MHz the 8998.5kHz, 9001.5kHz crystal) is difficult to purchase in the country, in this case, the phase shift method can generate SSB signals, comparing with the commonly used filter, but it has the advantages of easy make, low-cost, particularly the output frequency of VFO (should have a very high degree of frequency stability) can be directly taken as the firing frequency, it does not have to go through one or more frequency, greatly simplifying the whole circuit , the circuit is shown as the chart.

Wednesday 25 June 2014

The Moxon-Beam

 

The Moxon-Beam was introduced by L. Moxon (G6XN) in his book "HF Antennas for all Locations" (RSGB- Publications, Great Britain 1993). This beam is a 2-Element-Yagi with radiator and reflector and reduced size to about 75% of a normal beam. The 2-Element-Yagi with reflector has normally a 0,2-lambda-boom and an impedance of 50 W. The Moxon-beam has a 0,18-lambda-boom and still 50 Ohm. This is a good impedance for wire- beams.The ends of the two elements are bended backward (radiator) or forward (reflector) and act as a capacitive load. That is much better than inductive loading with coils. So we have greater bandwidth and lower losses.Through the reduced size we get a 0,5-0,7 dB lower gain than with a fullsize beam.

This type of a 2-Element-Yagi has an unbelievable F/B-ratio on the design frequency of >= 30 dB. That is higher than with any other 2-Element-Beam.

The gain is higher on the beginning of the band and lower at the end. The bandwidth for a SWR < 1,5 is great enough for the range of 28,0-28,7 and 21,0-21,45 MHz if the beam is built up with aluminium tubes. Wire-beams of the Moxon-type have a smaller bandwidth.

The design frequency should be for a frequency 1/3 from the beginning of the band, because the SWR raises more below the design frequency. For example look for the SWR of a tube-Moxon for the 15-m-band:

Designing a Moxon- Beam is very easy with a useful little program by D. Maguire, AC6LA with the name "Moxgen".

This freeware can be downloaded at:

www.qsl.net/ac6la/

Moxgen generates an output file for "EZNEC" for modifications (e.g tapering) and shows you the dimensions for  building a wire-Moxon.

http://www.qsl.net/dk7zb/Moxon/Moxon.htm

Tuesday 24 June 2014

ADRIAN KNOTT PROPOSES TUNED VLF ACTIVE ANTENNA

 

Hi to all! Today I found a very interesting project that using a small whip for hear the long VLF waves. Adrian Knot says that with this simple and inexpensive system you have good results, but suggests the using outdoor far from electric sources.
References: (http://www.hard-core-dx.com/nordicdx/antenna/special/vlfactive.html)