Re: [TowerTalk] N4XM's X Match Tuner Info

[Jim Lux <jimlux@earthlink.net>]

At 10:29 AM 12/2/2003 -1000, Jim Reid wrote:
> <snip>
> The XMatch series capacitor which is switched to be in
> series with either the tuner input or output connector is
> a "bank" of doorknob type capacitors which total 575 uF,
> rated at  5 kV.  The two sections of the LARGE
> variable capacitor  (which is switched to the opposite
> connector from the bank of fixed capacitors) are
> in parallel,  forming a large amount of variable C,
> I presume enough C to equal, or come close to
> the same 575 uF,  but perhaps not.  The instructions
> suggest that if you find the variable C approaching
> high numbers,  something is amiss and you should
> change the length of your feedline, or change the antenna
> particularly if you are going to be QRO.

I assume you mean 575 pF, which would have a reactance of about -140j ohms 
at 2MHz (half that on 80, etc.)

I'd imagine that the usual situation is feeding a physically short antenna, 
where it presents some fairly small R with a big -jX.  A matched monopole 
would be 36 ohms, and since relatively few folks have a 130 foot high 
monopole, it's likely that the usual situation would be an R around 10-20 
ohms with a few hundred ohms of -jX in series.  Adding another 140 won't 
make much difference, and you'll tune it out with the shunt L 
anyway.  However, the combination's going to be pretty high Q.  Say 300 
ohms of reactance and 30 ohms of R, that's a Q of 10, and, more to the 
point, 10 times as much reactive power flowing around the circuit as active 
power.  If you're putting that kilowatt of active power in, the reactive 
components need to handle better than 10kW.  I suspect that the important 
rating on that capacitor is not it's voltage rating, but the RMS current 
rating.  10kVA at 140 ohms implies 70+ RMS amps.  I haven't seen many RF 
capacitors that have that kind of current rating.

This is all sort of loose ballparking, but, it goes to show that designing 
a rugged matching network is non-trivial.


The recommendation to change feedline length implies that you'd hope to use 
the feedline as an impedance transformer of sorts, to get the impedance 
presented to the tuner to a reasonable value.  This would imply, then, that 
there's a fair amount of reactive power circulating in the transmission 
line. Hopefully it's a pretty low loss line!




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