[TowerTalk] Elevated vertical

[Al Kozakiewicz]

Maybe a nit, or maybe not, but transmission line VSWR is determined solely by the ratio of the characteristic impedence of the feedline to the antenna feedpoint impedence.

Adding a matching circuit at the transmitter (tuner) or adjusting the length of the feedline such that a 50 ohm resistive impedence is presented to the source does not change the VSWR in the feedline.  Yes, the VSWR between the source and the tuner will be 1:1, but the VSWR in the feedline will remain unchanged.

At HF with decent coax (or open wire line) we normally don't care too much about the absolute number as long as it's a "reasonable" SWR.  Just don't be fooled into thinking that just because a tuner does its job that the VSWR on the transmission line is 1:1.  It's not.

Al
AB2ZY

-----Original Message-----
From: towertalk-bounces at contesting.com [mailto:towertalk-bounces at contesting.com] On Behalf Of Paul Christensen
Sent: Friday, December 23, 2011 10:36 PM
To: towertalk at contesting.com
Subject: Re: [TowerTalk] Elevated vertical

> "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.

http://www.qsl.net/4z4tl/pub/Feeder_tunes.pdf

Paul, W9AC 

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