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[TowerTalk] Shunt-Fed Tower for 160 m

Subject: [TowerTalk] Shunt-Fed Tower for 160 m
From: Kim Elmore <>
Date: Sat, 01 Mar 2008 10:41:58 -0600
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I've been trying to determine the feed impedance of a relatively 
short, shunt-fed tower for 160 m. As configured, the system appears 
to be resonant at about 9.6 MHz.

I borrowed a friend's MFJ-259B analyzer to supplement my readings 
from my noise bridge, as I didn't think those made sense. I kept 
getting an inductive reactance at a frequency well below what I knew 
to be the system's resonant frequency.

The MFJ-259B showed a reactance of about 360 ohms, but without sign 
information. To remedy that, I put a T-connector on the port, put the 
load on one side and piece of RG-8 coax about 19" long on the other 
side. A 19" long piece of RG-8 should have about 48 pF of 
capacitance. The 259B said it had about 60 pF, so that seemed close 
enough, counting the connectors. Thus, I added about 60 pF in 
parallel with the unknown reactance.

I figure that if the impedance is inductive, adding a capacitor will 
increase the reactive component by cancelling out the inductive 
impedance since I'm essentially approaching parallel resonance. 
Alternatively, if the reactance decreases, then the load reactance 
must be capacitive.

The added capacitance increased the reactance, so I conclude the 
antenna impedance must be inductive. Does this sound right?

Kim Elmore, N5OP


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