[ARC5] " ARC-5 " Transmitter Antenna Matching.

[Mike Hanz]

>Stinson's method is part-complete and unsupported with an explanation
>
>Your problem is to adapt the thing to run into something beside
>a short random wire that looks like low-R and big Xc
>
>Scroll the loading coil back to left to discard the Xc compensation (with Xl)
>
>Then connect a broadcast (370pf) variable in series w. 50 ohm resistive
>load and spring-terminal on front.  This'll be adjustable Xc to tune out
>Xl of swinging link in ARC-5 tank coil. Same boolah-boolah for a T19 or a
>T-22.
>
>Connect "command set" to dummy load & muck with broadcast variable
>& link adjustment 'til you get, say, 40W into dummy load when 600V
>on 1625 plates.
>

Insertion of a series Xc is an old technique, and is precisely what Dave 
suggested.  It is also part of the tuneup procedure cited in the command 
set manuals for certain antenna length and frequency combinations.  So 
far I don't see a different pony in the pile above.  The idea of using a 
/variable/ cap is a good one, at least for establishing a reference 
point - it will give you a ballpark on a particular antenna for a 
particular transmitter at a particular frequency.  The risk in using it 
permanently is arcing in the receiver capacitor if the antenna VSWR is 
high, though - several of us have experienced that.  I suspect that's 
why Dave was recommending using a 1 kilovolt minimum cap.  The MOPA 
design is fairly sensitive to changes in antenna impedance, and using an 
NPO cap simply makes things more stable.

>Don't screw with the Un-Un hooey since it doesen't address the reactance
>questions at all.  The Un-Un procedure is as effective as putting
>a fotograph of a parakeet in with the 1626
>

Well, let's think about this a bit.  I wouldn't call an unun transformer 
'hooey' unless I had solid technical reasons to do so, and I don't see 
anything except denigration without a basis - sorta "part-complete and 
unsupported with an explanation..."   :-P   That raises my professional 
hackles a bit.  I'll just chalk it up to having a bad hair day that 
caused you to be unusually didactic, not to mention discourteous 
concerning Dave's practical guidance.

I seem to recall that your mentor told you that you needed a conjugate 
match for maximum power transfer.  I also seem to recall you saying at 
one time that you weren't overly impressed with that, but the math 
doesn't care what anyone thinks about it - it's just physics.  Sure, you 
don't have to have maximum power transfer to get a good signal out - the 
QRP guys communicate with tiny power levels all the time.  I have 
absolutely nothing against folks who enjoy playing slop pool, or 
shooting a few baskets in an offhand manner, or throwing an antenna over 
a tree branch and using the system at a fraction of its capability.  
Lots of fun doing that sort of relaxed activity - all of us have enjoyed 
the instant gratification element.  But sometimes it's just as much fun 
to chase perfection as it is to throw a wire over a limb and call it 
good.  That's one reason I have a problem with dismissing a feasible 
engineering solution for those who are trying to get the last watt out 
of a system - at least without having solid technical reasons to do so.  
For those who pursue the goal of maximum power transfer, I'm afraid I'd 
have to disagree with your assertion about its effectiveness.  Just so I 
don't leave this unsupported by any explanation, here's my view:

Ideally, the PA tank coil and rotating link at resonance in this 
particular case is simply part of a transformer, matching the plate 
resistance of the parallel 1625 tubes with the 5 to 12 ohm resistive 
component of the aircraft antenna.  Like many aircraft sets (the ART-13 
is a partial exception,) the command transmitters used the capacitance 
of the aircraft antenna as part of the tank circuit.  Its presence is 
also important in reducing harmonic components - one of the reasons the 
command sets developed a bad reputation for TVI when fed into a straight 
50 ohm resistive antenna.  The extreme in this practice is the Bendix 
TA-2 series, which had no tank capacitor whatsoever - it depended 
/completely/ on the antenna (and residual capacitance) to furnish that 
component. 

If you have a 50 ohm pure resistive ham antenna fed by a 50 ohm 
transmission line and are lucky enough to have tuned out capacitive or 
inductive reactance at the driving point where it would connect to the 
transmitter, you'll still need a capacitive reactance to load the 
command transmitter properly.  You will also still have as much as a 4:1 
pure resistive mismatch with many of the command transmitters that does 
not satisfy the conjugate match requirement for pure R.  You can solve 
that in a number of ways - with outboard network components (like a pi 
network), winding more turns on the rotating link, or putting an 
outboard wideband transformer (an unun or balun, depending on the 
transmission line and antenna you're feeding) at the junction of the 
transmitter output and the transmission line.  Failure to do so is like 
feeding a 16 ohm speaker with a 4 ohm transformer output - you may hear 
the music fine, but you won't be able to pump the room with full power 
until you get the strapping correct.  Frankly, for me the unun is a lot 
easier solution than the others, but it's not the only solution.  The 
other thing you may have forgotten is that a wideband unun can also 
transform the reactive component appearing at the driving point - not 
just the resistive component.  That sometimes brings the oddball antenna 
system impedance that most of us have from a practical standpoint at our 
QTH  into the range that can be accommodated by the command transmitter 
tank and output network.  It also makes the antenna current meter stand 
up straight and start working again, which is another reason I favor 
getting the Ro match close to correct.  Not a big thang, but it's neat 
to watch it swing up to 30-40% of FS with every key stroke.

So, by all means, don't use an unun if you think it's hooey - doesn't 
make any difference to me.  But don't mention parakeets along with it or 
you'll get another boring dissertation from me again.  Next time I'll 
force you to endure the math - complete with derivation from Maxwell's 
equations...aauugh...

Hmmm...come to think of it, I wonder if putting a parakeet picture next 
to the M.O. would enhance the transparent highs and effortless lows I've 
been trying to find?  I'll have to try that...heh...

73,
Mike