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Re: [TowerTalk] Impedance matching dilemma - 80M Inverted L

To: <>, "Towertalk reflector" <>
Subject: Re: [TowerTalk] Impedance matching dilemma - 80M Inverted L
From: "Al Williams" <>
Date: Sat, 2 Oct 2010 13:08:10 -0700
List-post: <">>
Could it be that the wrong question is being asked?  Why not just put a 
fixed capacitor in series with the input (or output) lead of the tuner?


----- Original Message ----- 
From: <>
To: "Towertalk reflector" <>
Sent: Saturday, October 02, 2010 7:56 AM
Subject: [TowerTalk] Impedance matching dilemma - 80M Inverted L

> Hello,
> 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
> dilemma.
> 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
> subject!
> Thanks all...
> 73, Stokley W3SB
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