The ARRL transmatch
as implemented by ralph klimek aug 2009
is not an inovative design at all. Its just another
implementation of the good old ARRL transmatch design, popular because
you do not require the now impossible to obtain (and unaffordable)
I began this project in 1974. I had
just moved into a new house and was wondering the neighberhood to get a
feel for the place. There was a local council hard rubbish collection
where the the good burghers of Melbourne are encouraged to dispense
with their surplus goods and chattels by leaving them out for removal
and curious passers-by.
I passed by one pile of junk and noted
that it contained the remains of some piece of military radio
equipment. It has been heavily cannibalized and only a few of its
components had remained. I knew nothing of military equipment back then
or what this radio was or its history, only that it was junk,
unrestorable and for me , merely a source of hard to get radio
The radio set in question, as I learned decades laters,
was the famous Radio Set Number 19. All of the remaining
components that I had laboriously removed from it had been slowly
used or lost in the decades since that rainy winter afternoon except
this variable capacitor.
I kept it because it had ceramic insulators
and I knew enough electronics at this early age to know that this was a
good thing. The capacitor had a shaft that continued out the back,
and two full bearing races. The normal broadcast tuning
gang had only one ball race and only the one shaft. I knew that
this had to be usefull for something !
That something was to be one arm of a DIFFERENTIAL split stator variable capacitor,
rendered tractable by having the shaft coming out both ends. It
could be mechancially coupled to another broadcast gang in such a way
that as one capacitor increased its capacitance, the other would
decrease . This can, and is used in the transmatch circuit to
serve as both a variable reactive element in a tuner and as a fully
variable impedance tap, think of it as an variable capacitive
autotransformer (variac anyone ?). With such a device in
resonance with a parallel tank inductor it is possible to tap
down any and feed (or source) any resistive impedance.
can see the old capacitor now in service again, after lying idle for
60+ years as one arm of my differential capacitor. It is electrically
isolated from its companion capacitor by a ceramic shaft coupler
and both mounted on a sheet of perspex. I do not force RF
currents from the capacitor rotor wiper. I dont think this is a good
thing. The circulating RF currents are quite high and I do not trust
the old wipers in this duty. Instead, I feed the RF current via both
stator sections, effectively putting the two capacitor halves in
series, doubling their total RF breakdown voltage and forcing the RF
current to travel via the rotor shaft and avoid the wipers alltogether.
This has to be a Good Thing. It affords a serious improvement of the
tuned circuit Q. Allways use a proper shaft coupler ( universal
joint ) for coping with the lack of colinearity of the capacitor
shafts. No matter how skilled you are at metalwork it it impossible to
get the two shafts perfectly collinear.
The capacitance of each
section is normal 415pF, in series is half that which is exactly
what the standard transmatch design requires.
frames are effectively floating for RF, thus avoiding parasitic
resonances. I do ground, via some 1M ohm resistors the capacitor
shafts, and frames , for the elmination of static electricity. I did
this to raise the flashover potential of the capacitors because they
were never designed for transmitter use having a plate seperation of
about 0.5mm. It would not be hard to accumulate 1KV of static
charge that would iniciate an RF flashover, so just get rid of
one potential source of flashover. My guess is that this
arrangement will make my
transmatch safe for power use up to between 100 and 200Watts and maybe more with a well matched load.
only refinenment in my implementation is the use of copper sheet metal
for creating real soldered RF grounding points. Dont think for one
millisecond that you can create a good low impedance RF ground by
grounding to Aluminium with a grounding lug and screw. It just wont cut
it for RF. It is actually very difficult to connect to Aluminium,
especially at RF. I got my copper sheet metal from a discarded
copper hot water service. Double sided PCB would also work well.
Soldering to copper sheet is easy, but you really do need a gas powered
iron, at least 100 watts. Underneath the small inductor (not
visible here) is a large copper strap that forms the positive RF earth
for the tap shorting front panel selector switch and input capacitor.
Do not rely on RF current reliably going through the switches
rotating shaft to chassis.
Dont forget that the variable capacitor shafts are live with RF, you must use plastic rods to couple them to the tuning knobs.
inductors in my transmatch also have a slightly less colorful history.
The low inductance coil is a copper heat exchanger I found in an
industrial waste skip made from quarter inch hydraulic copper pipe. The
medium coil came from a Burroughs mainframe power supply switchmode
regulator, I just added extra taps. The taps are sheilded with teflon
sheet to prevent shorted turns.
The high inductance coil came from
the surge current limiting coil from the capstan drive of a long
forgotten Burroughs tape drive. I knew that I would find a use for that
coil, I kept it because I cannot do the PI winding by hand. I had
to wait twenty years at the bottom of my deepest junk box where it
served its duty as a retirement home for large angry ugly elderly
deadly ghastly red back spiders!
Internal conductors are
either copper strap cut from a sheet of copper flashing or the outer
braid stripped of from heavy duty RG58. The heavier the better. RF
loves thick conductors ! All RF joints should be real metalurgical
joints, solder everything !
One thing that allways puzzled me about the ARRL transmatch was the use of selecting taps by shorting them to ground. I thought,
that making a shorted turn, and isnt that bad ? One advantage in
having multiple coils is that the magnetic flux one one coil is not
sensibly shared by other coils. But what of the same coil ?
Because these are High Q inductors, the resulting circulating
currents are not readily dissipated. It becomes a scenario of putting
coils in parrallel, which reduces inductance. The other rational of
shorting the non selected coils is to reduce the possibilty of exiting
parasitic resonances. Common sense suggests that there are just as many
potential parasitic resonances in a shorted coil as an open one, except
that they would occur at higher frequencies, hopefully well away from
the intended usage frequency. I have yet to check mine with a
grid dip meter to see where the parasitic resonances occur, but they do
exist. The laws of physics demand it. I have also added a sheet
of copper flashing the to lid. The box I used was some surplus
junk, it was just the right size, and I think a little too small. High
Q solenoid inductors really should have lots of free space around them,
because free space does not dissipate energy. If you cant have free
space, then, the confining metallic surfaces should have the least
resistance possible and that means copper. In an ideal world you
would make the box out of silver metal or copper sheet but we do not
have access to Defense Budgets ! (or live in an ideal world)
But does it work? Yes!
|the completed transmatch||taps on the medium inductor. a piece of teflon is removing a self inflicted shorted turn.||top view, copper backplane for positive RF grounding|
|top cover with copper flashing shield near the inductors||differential capacitor at 2/3rds mesh||there was space so I put in both PL259 and BNC and terminal posts|
|antenna output capacitor.output strap is soldered to capacitor frame. I will take my chance with the rotor wiper||medium inductor and output strap, static drain resistor||"high" inductor|
|after 60 years , a No 19 set tuning dual shaft capacitor finds a new job||homemade sharft coupler and universal joint||upper differential capacitor|
|another view of the venerable cap||heavy duty tap selector switch from an old electric blanket control||a tangle of tap conductors made from heavy RG58 braid|
note-to-self insert schematic diagram here
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page created at Tue Sep 1 18:17:04 EST 2009