Topband: Efficiently Matching Low-Impedance Antennas to 50ohms

[Guy Olinger, K2AV]

"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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