5WATT TRANSMITTER

 

By Harry Lythall SM0VPO

This is a very simple 5 watt CW TX based upon a TTL logic chip. There is just one "tricky" component and this is Cx. This component should have an impedance of about 10 - 50 ohms, at the frequency of interest. If you wish to reduce the transmitter power, increase the value of Cx. It is 'Cx' which causes the square wave from the output transistor to approximate a sine wave. The value of Cx is the price of simplicity in this TX.

STARTING values for Cx are as follows
(but there is a LOT of leeway):

1.8 MHz = 4.7nf
3.5 MHz = 2.2nf

7.0 MHz = 1.2nf
10 MHz = 820pf

14 MHz = 560pf
18 MHz = 470pf

24 MHz = 390pf
28 MHz = 330pf

It is far better to use too high a value for Cx initially, then reduce it to achieve the correct RF output power. The value of Cx will depend upon your choice of TR1. Virtually any RF power transistor will work well in this application as long as it will handle 800mA continuously. I have even used the BC108 in this application but the RF power output was restricted to about 150mW. Cx was about 5x the value quoted above.
The output tuned circuit uses a coil WITHOUT a ferrite slug. Use the usual "rule-of-thumb" formula for the tuned circuit;

Coil
Wavelength (in meters)
=
number of turns

Capacitor
Wavelength (in meters)
=
Capacitance (pf)

This will get you 'roughly' in the right area although it could differ widely with different coil formers. The coil output winding is from 5% to 15% of the total number of turns. Adjust the output winding before reducing the value of Cx. You need the least number of turns that will give you the power needed.
Connect + 5 volts to the SN7400 chip and + 12 v to the PA and you will have over 5W of power out. To key the TX put the key in the +12v lead.
You MUST use an antenna Low Pass filter with this rig if you are using a good antenna. If the antenna is tuned (magnetic loop/frame antennas etc) then you need not bother with the LP-filter. Do NOT use a linear amplifier for this transmitter. The finished transmitter will fit into a matchbox with a little care.