"I'm looking for advice about how to efficiently match
my 160m inverted-L to 50 ohms." ...
" My receiver noise
bridge shows that its impedance has a resistive
component that's around 20 ohms," ...
One single-band technique I have not seen mentioned in this thread is
to use a two section series matching transformer in combination with a
series coil/cap if needed to tune out the reactance.
50 ohm coax from shack...X feet of YY ohm coax...Z feet of 50 ohm
coax...series device...antenna.
A handy special case is used to do a one-band conversion from 50 to 70
ohm coax, using ~1/12 wave of each as above, and assuming both are
well-matched.
W8WWV has a calculator program for the general case that can be found
at
http://www.seed-solutions.com/gregordy/Software/SMC.htm
You specify the frequency, the matching goal, and enter the two
characteristic impedances and velocity factors. Hit compute and it
returns the series section lengths, if a solution is in range.
Effectively 25 ohm coax is made by running two 50's side-by-side,
connecting shield to shield and center conductor to center conductor
at both ends.
For Ron's problem particularly, after tuning out the series reactance
to get a resistive Z, using 50 ohm coax and 25 ohm coax and 20 ohm Z,
the SMC program gives a solution. Using coax with .66 velocity factor:
50 ohm line to shack, 38.8 feet of double 50 ohm (25), 21.9 feet of
50 ohm, what should be a series cap, Ron's wire.
For 160, that's really not a lot of coax.
The method will tune out a limited amount of reactance. If the complex
Z is known, it can be entered to the program, but a solution may not
exist.
73, Guy.
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