VU2VWN's 7 on 7 Qrp

 

Submited By Nadisha 4S7NR

This is a VFO control, easy to build 7 watts QRP on 7 Mhz from Vasantha VU2VWN. If you are in close VU, you may hear hundreds of hams using this QRP on 7MHz - AM. They just modulate the whole Driver/Amp section using a Audio transfomer. It is very populer (specially) in South India.

VFO works with two MPF102 FEts. It should be enclosed. Many VU hams put this VFO on a Card-Bord box and wrap it with Aluminium foil. And they oparate the VFO using 6V battery and 5.1v Zener diode. I change it with 7805 three pin voltage regulator and still works okay (I mean with out any hum).

BD139 should be mounted on a large heat sink. Once everything is finish, Supply 12v to the Amp (not 24v) connect a 24v 5W bulb as the load. C4 should be tuned to the minimum brightness of the LED. Total DC current to the RF amp should be 200-300 and it should never exeed 400mA (at 12V). On its full power 28V power amp will take about 300-400mA (max 500). Use 75 ohm coax, to feed the dipole.

A sensitive RF probe

A sensitive RF probe is very useful in a situation where an oscilloscope is not available. This design allows measurement of the peak-to peak amplitude of an RF signal in the 100 kHz to the VHF range at up to 50 volts P-P. 50 Volts P-P is approximately 14 volts RMS. The 50 volt P-P limit is due to the 60 volt breakdown rating of the diodes

The upper frequency range limit of the probe is due to the length of the ground wire and the stray junction capacitance of the germanium diodes. It is designed for use with a high impedance DC voltmeter, or VOM. Instruments with FET inputs are preferable.Avoid using a long ground lead. Keep the ground lead as short as possible, and attach it as close as possible to the ground point of the circuit under test.

A thirty-six (36) inch length of RG-174 coax is recommended for the probe’s cable. RG-174 is well suited for this task because of its small diameter and flexibility. Also, #16-#18 guage insulated stranded hook-up wire can be used. Twist the wires together.

The RF probe is a charge pump. The peak crest value of a signal is stored in the stray capacitance of the coaxial cable. The stray capacitance of the cable is about 50 pF. The 50 pF capacitance of the cable, and the 10 mega-ohm input impedance of an FET VOM implies a DC filter time constant of 500 m Seconds, Accurate measurements are possible at frequencies as low as 100 kHz.

Noticeable errors occur at low signal levels because of the forward bias drop of the diodes. Assume a total diode voltage drop of about 0.3 volts.If a standard 20KW /Volt meter is used, then you may have to add a 1000 pF capacitor across the input of the VOM to improve DC filtering. Expect more circuit loading effects, and an increase in the probe’s lower cut-off frequency.

Modulation Transformer for QRP AM transmitter projects

One the stumbling block for QRP enthusiasts, interested in building AM transmitter related projects is need of modulation transformer. I have simple solution for this. You can use audio output transformer from old radio receivers.

Low Cost ATU for QRP rigs

The circuit shows simple low cost ATU for QRP transmitter projects. The coil is wound on a 12mm ferrite rod, which can be junked from old radio receivers.The wire will be 20swg and  the winding should occupy 4cm long.VC1 and VC2 are 365 pf Variable condensers used in radio receivers.