sceiv

Number 7 on your Feedback e.rd
Build Your Own
20 Meter Transceiver
Work the world with this easy-to-build rig.
by Gordon Young WB6NKJ
T
his project is perfect for the low-power
enthusiast who wants full break-i n
(QSK), sidetone and diode antenna switching
features. The parts for this project are common
and even a first-time builder should have
little or no trouble assembling a complete
transceiver. You will be surprised at the results
and happy with the simple features this
rig provides. I have already logged many
stations and have put on many hours of use,
talking across the states and around the world
with just a few watts. An advanced builder
may consider adding RIT, audio filtering, a
frequency counter, a drive control, or other
features.
The power output will easily drive a pair of
high-power transistors to a level greater than
100 watts, but alone it is enough power to
work almost any station you can receive.
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Figure J. Transceiver block diagram.
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With some tinkering. you can bri ng the
MRF-433 transistor to its full tz-wan capability.
I am always surprised at the performance
of simple direct-conversion rigs. and this
one is no exception. The circuit is simple.
construction is easy, and normal operation
is a pleasure considering that rhe transceiver
is so simple. I' ve worked all states before
with j ust 5 watts; this rig should do the same
for you.
The Circuit
Figure I shows the block diagram of the
transceiver and it is readily apparent that it
contains the basics: a signal input, a detector.
VFO. audio amplifier and RF PA. This started
as only a weekend project, but I wanted to
make sure that it had some ofthe nice features
Igrew to depend on. such as a sidetone, diode
antenna switching, and audio output to drive
a speaker (headphones begin to get uncomfortable
after hours of operation on Field
Day). The other annoying aspect of fu ll
QSK is the " thumping"
you normally
get switching from
transmit to receive.
The cause of this is
the DC dumping of
the audio stage. To
help overcome this problem I used a simple
transistor switch betweenthe detector and AF
preamp. This switch doesn't really tackle the
mismatchofimpedances at this point; an FET
would be better.
Another objective was that the power amplifier
have enough guts to drive a linear stage
10 later follow rhe transceiver project. Ifyou
have a pair of 8O-watt devices. they will require
at least 5 watts ofdrive. and pushing the
MRF4 33 to 12 watts would mean that an
attenuator would likely be needed somewhere
in between.
The outcome ofthe entire effo rt is shown in
the figures. In my prototype, each part ofthe
rig was originally air-wired on plates of PCB
board. However.etched and drilled are available
to case your construction efforts (see the
Parts LiM).
Starting with the receiver PCB. the frontend
bandpass fi lter helps a 101 to reduce the
foreign broadcast interference that would
overpower receivers such as this. To make up
for the few dB lost in the filter and to add
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PhOIO A. The completed WB6NKJ transceiver.
26 73Amateur Radio Today . June, 1992
Photo B. Inside view ofthe transceiver.