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Re: [TowerTalk] Elevated vertical

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Subject: Re: [TowerTalk] Elevated vertical
From: "Paul Christensen" <>
Date: Fri, 23 Dec 2011 22:36:13 -0500
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> "Would you consider my 130' center fed dipole "resonant" on 60 or 30 m 
> because the antenna tuner inside the rig can tune the 
> antenna/twin-lead/balun/coax to a 1:1 SWR inside the rig?  Sorry to say 
> but that one hurts my head to think of as "resonant".   Tuned? yes. 
> Matched?  Yes. Resonant?  No."

How are you defining the terms "tuned," and "matched?"  What is tuned and 
what is matched?  How is your use of "tuned" different than "resonant?" 
Your antenna in this example is not resonant at the antenna feed point, but 
resonant as a system attached to a transmission line with the help of the 
rig's tuner.

Let's consider your example of a 130 ft center-fed dipole with some length 
of transmission line being used on 60 or 30m.  If the tuner in the rig is 
performing it's job and it attains a 1:1 VSWR at the input to the tuner, 
then your 130 ft dipole and transmission line are resonated, where the 
system reactance is zero. That's a resonant antenna system - where circuit 
reactance is zero.  In some circles, it's also called "tuned."  See below.

Tuned = Resonant = 0 reactance = X(0).

That's the X value but R may be a value well above or well below 50 ohms, 
but your antenna is still resonant.  However, is it also matched at the line 
input (50+j0)?

I just assisted in the design and installation of a 160m "T" radiator 
consisting of 85' ft of vertical section and a 130 ft. flat top section. 
Using 4Nec2 software, the feed-point Z was close to 50+j200.  This value was 
confirmed with a Vector Network Analyzer.  Under this condition, tuning the 
antenna to resonance and matching the antenna to 50 ohms resistive requires 
a low-pass L network, or simply a series capacitor to cancel inductive 
reactance by an equal but opposite amount of capacitive reactance.  In this 
case, one series capacitor will tune the system to resonance, while ensuring 
that the input remains matched at 50 ohms resistive (50+j0) for a 1:1 VSWR. 
It may have also been possible to achieve the same result by changing the 
feedline length to also tune the system into resonance.  But tuning to 
resonance may not have yielded an R=50 value.

Through the decades, this topic has been presented over and over again by 
QST's editors with re-prints of By Goodman's classic article: "My Feedline 
Tunes My Antenna."  Here, Goodman used the term "tune" to denote resonance. 
Rather than using line length to resonate the system as discussed by 
Goodman, the tuner in your rig is doing it for you, and it's performing the 
full tune (resonating) and match function to attain 50+j0 at the tuner input 
for a 1:1 VSWR.

Paul, W9AC 


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