I have been using power-mosfets to produce
several hundred Watts for some years now so it seemed a good idea to employ
them on 136kHz. It would be possible to make a linear similar to the 160m one
but, in the interests of efficiency and ultimate power output, I decided to
make a CW-only class-D (or is it E?) TX.
Two versions are detailed
below. One is a 12V, 350W version with low-power keying, intended for portable
use from a car battery. The other is the full power version producing up to
900W output, mains powered.
Various updates have taken
place over the last few months, make sure you have the latest version of the
articles (1/7/200) before attempting to follow them.
The Portable version
The diagram for the portable
TX is below. It is built on the KISS principle so there's not much to it but it
is capable of producing nearly 400W with the engine running! The constuctional
details are in the article 136tx.doc which can be downloaded below.
Here's a picture of the
portable version. I've managed to fit it in a case 125mm wide x 95mm high x
More photos of the /P
Download the article on the portable rig in Word2 format.
See bottom of page for
zipped-up version and diagrams...
The base station
Using high voltage mosfets
means that the power supply for a kilowatt amplifier becomes a bit more
manageable. I started off with low voltage fets running at 20V but this
required 60 Amps or more which meant everything was big! With an 80V
rail we need only 15A to get a kilowatt as the switching design is so
efficient. The fets used here are 500V rated so they can easily stand the 100V
off-load voltage and no PSU regulation is required... More weight and heat
saved. It is theoretically possible to get the full output with just a pair of
fets but I have played a bit safe and used four. I have also added protection
circuitry to cope with over current and mismatch, these refinements could be
added to the portable version if desired, as the drive circuit is basically the
same. In the version pictured I use a CB synth rather than a VFO, the black
window on the right is the 7 segment "channel" display.
As you can see, the circuit is
basically the same as the simple version but a few refinements have been added.
I have not yet produced a PCB for the drive board but I may do one day. It can
be built on strip-board in one evening.
Here is the front view of my
version. The inside is not so neat! The meters are not the Maplin ones given in
the parts list but some 1mA ones I got at the Leicester show. I have added a
row of idiot-lights to make it look more high-tech and added switching for
external receive aerial, preamp and filter on/off etc.
Download the article on the base stn. version in word2
See bottom of page for
zipped-up version and diagrams...
Bits for both rigs
This is the mosfet clamping
system pinched from the 160/80 linear. You don't need the copper heat spreaders
for these rigs as the dissipation is not so great. When using four fets, I put
some rubber cut from an old inner tube between the fets and the clamp-bar to
equalise the pressure. Everthing else is a total bodge and it all works OK so
there shouldn't be any particular layout problems. Just keep earth returns
short and use good quality Cs and thick wire....
Not much need be said about
this, I only include it for completeness. It's a Colpitts thing made of bits
from the junk box. The capacitors around the tuned circuit and choke are
polystyrene and the coil is wound on an old quarter-inch IFT former. The
resistor values aren't critical.
It's all in a metal box with
a reduction drive on the tuning C. Stability is very good because the final TX
frequency is a tenth of the VFO frequency so the drift is ten times less. You
can check it's working with a medium wave radio.
1.37MHz crystals are very
expensive whereas 13.7MHz ones can be made for about £8 each. Another
divide by ten chip does the required frequency reduction and the output is
connected straight to the "VFO" input of the main circuit. If VFO
facilities are not going to be required the output from pin 2 can be connected
straight to pin 14 of the 4017 in the main rig cct.
It is very important to use
Philips Locmos (HEF4XXX) chips in this part of the circuit as they are the only
CMOS chips that go fast enough!
I am now using a CB rig
synthesiser to give better stability in case anyone is using ultra-low
bandwidth receivers. It's a Rotel RVC220 (useless piece of kit!) legal CB rig
whose synth runs at half output-frequency. Unfortunately this is about 13.8MHz
which, when divided by 100, puts it outside the top of the band. Replacing the
10.240MHz reference crystal with 10.075 MHz gives almost full coverage of the
band in approximately 50Hz steps when divided down. I removed the TX
components, strapped it onto TX and re-tuned the "doubler" stage in
the TX strip to 13.6MHZ. This gave about 12V pk to pk which will just-about
clock an HEF4017 decade counter (only this type seems to work at 13.7mHz...
perhaps 74HC series would be better). The 1.36MHz from its' carry output is
then fed into the TX divider strip as above. A DDS synth suitable for this
design is shown on G0MRF's site at
A lot of your suggestions have
gone into the current design that you see here, if you have any more let me
John G4GVC has had some
problems with the 4001 squarer stage oscillating of its own accord with no
drive applied, this defeats the logic which only works if the drive stops. A
470pf across the input terminal stopped it on another example.
Thanks to Rik ON7YD for the
idea to simplify the interlock circuit by shutting-off the 4013.
A 73kHz version of the 12V rig
is working fine, 8 turns on the output transformer and double the values of L
and C in the tx filter.
Some have suggested that the
lack of any "dead time" between the switch-off of one fet and the
turn-on of the other is a bad idea. I pondered the idea of using SMPS
controller chips which have this facility, to fire the mosfets but so far I
haven't actually tried it. It could yield better efficiency and less
The latest version of the 1kw
TX uses a series transistor for the keying function. This gives better keying
envelope and less key clicks.
Any good ideas? Let me know
and I'll try them and/or publish them here.