recommend constructing over pieces of un-etched PCBs. They are
cheaply available everywhere. See the pictures as a guide to component
layout. We recommend the following rules:
► Keep your leads
short. Short connections are more important than components that
are at right angles to each other. What might look neat to you
might look unstable to the RF design.
► Keep the outputs
and inputs isolated from each other. We have taken care to keep
the high impedance points down to a minimum. But still, maintain
► Make one module
at a time, test it completely, then move to the next one. Construct
the transceiver in the following steps:
► Make the VFO. Check
the RF output using an RF probe. Check the stability on a regular
receiver or a frequency counter. With the tuning capacitor fully
closed (the plates inside each other), set the trimmer so that
the VF0 frequency is exactly 3.9995 MHz (keep 5 KHz margin at
the band end)
► Make the BFO. Check
the output on the RF probe.
► Calculate the ladder
filter values and make the IF strip along with the audio preamplifier.
► Connect the BFO, VFO,
IF strip and an external audio amplifier together. When you power
on and attach a piece of 2-3 meter long wire to the input of the
IF amplifier you should be able to hear the atmospheric noise.
Tune the BFO coil by fully screwing the slug in and then slowly
tuning it out until the IF noise sounds right (not too shrill
and not too muffled).
► Wire up the receiver
mixer, connect the VFO. Peak the mixer output and the RF input
coils for maximum output. Then tune to a weak signal on the band
and tune for the best signal. Be careful to tune for best quality
of signal and not for maximum loudness. Take a break,
spend a day or two listening to the band with your receiver. Nothing
is more enjoyable than using a crisp receiver that you have homebrewed.
► Wire up the modulator.
If you have an oscilloscope, you can check the modulation. The
modulated output will be too low for you to be able to measure
on the RF probe.
► Wire up the linear
chain. DON’T solder the IRF510 yet.
► Put the transceiver
in transmit mode. Whistle into the microphone and peak the transmit
mixer output coils for about 6 volts peak RF voltage on the probe
at the 56 ohms resistor
where the gate of the IRF510 would be.
► Solder in the IRF510.
ATTACH A DUMMY LOAD. We used four 220 ohms two watts resistors
► Keep the bias trimmer
totally down towards zero. Attach VOM in series with point X in
the power amplifier. Apply power in transmit mode and slow increase
the bias until you have 80mA flowing through the IRF510.
► Connect the RF probe
across the dummy load.
► As you whistle,
You should get about 20-24volts of peak RF on the probe. When
you pull out the microphone from the jack, the RF output should
drop to complete zero. What if your transmitter is unstable?
- Don’t curse your fate. All transmitters start
out as unstable beasts. Relax.
- Start disconnecting power from the stages starting
from final IRF510 and working backwards. When you have located
the unstable stage, there are a number of things you can do
to fix it.
- Try increasing the value of the 10 ohms resistor
used in the emitter degeneration OR
- Strap a resistor of about 1K across the output
transformer of the unstable stage to ‘load’ it.
- Move the linear amplifier away from the rest of
- Redo the board. This time spread the stages out.
We guess that the linear chain should occupy about 6 inches
of space, all laid out in one line.
The BF195 transistors
can be substituted with any other HF transistor like 2N2222 etc.
The 2N3866s are best not substituted. The circuit works with slight
increase in the noise figure if BF195 or equivalents are used
in place of 2N3866s in the IF stages. The output power on the
transmitter absolutely needs the 2N3866s. Substituting them with
other switching transistors didn’t give good performance.
The IRF510 should not be substituted
with any other transistor. The other IRFs, though rated higher,
have higher input capacitance which makes them a bad choice for
The LM380/LM386 can be substituted
with almost any other audio amplifier. Our first amplifier was
Cambridge SoundWorks Sound System. If you turn down the bass,
they are an excellent system for the shack. We have tried a TBA180,
an LM386, an LM380 and even a glow-bug guitar amp. Feel free to
The first contact
we made using this rig was DF6PW. He reported us 57. Within the
first evening we had worked four continents. The rig is regularly
used at VU2PEP. People are often surprised at how the transmitter
quality is ‘just like a commercial rig’. Many refused to believe
that it is a seven watt rig.