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[TowerTalk] Impedance matching dilemma - 80M Inverted L

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Subject: [TowerTalk] Impedance matching dilemma - 80M Inverted L
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Date: Sat, 02 Oct 2010 10:56:00 -0400
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I have an 80M inverted L that I want to use as a multi-band
antenna. It is center fed with 96 ft of 450 ohm window line to a 4:1
current balun and then 35 ft of RG8 coax into the tuner (T-network). Each
leg is 68 ft of #12 AWG insulated wire; the horizontal leg is approx. 80 ft
in the air. I have measured the lumped impedance (window line, balun plus
coax) with an AIM 4170C (measured at the input or shack end of the line)
for all the bands below 80M (except 60M - no interest). I use the R and X
values from these measurements to preset the caps and inductor on the tuner
using CS, CL and L values calc'd from "L-C Matching Networks" by WY2U for a
Highpass T network. The technique works wonderfully on 80M but here's the

The C(in) and C(out) range on the tuner caps is 48 to 575 pf
(measured); the L range is 0.05 to 26 uH (measured). On the other bands
(except 10M and 12M) the calc'd CS and CL values (35 to 42 pf) are below
the caps range of the tuner. The L values needed are well within the range
of the tuner. I changed out the balun to a 1:1 current balun but with
similar results. 

The question is how do I calc the length of coax needed
at a particular frequency to transform this lumped circuit into a Z range
that the tuner can handle? I'm sure there's a way (Smith chart?) but this
retired chemical engineer has reached the limit of his knowledge on this

Thanks all... 

73, Stokley W3SB 


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