[TowerTalk] N4XM's X Match Tuner Info
Jim Lux
jimlux at earthlink.net
Tue Dec 2 14:51:50 EST 2003
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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