[TowerTalk] Understanding Antenna Impedance Measurements (was Foam cables and velocty factor)

[Robert Chudek]

Jim, Roger, Tom,

Okay, it's starting to sink in now!

Yes, I was using a 1/2 wavelength feedline. That's where this thread started, I cut and tuned some RG-8x to 1830 KHz. So when I analyze the antenna through the feedline at 1830, the readings will be valid.

Where I was being led astray was when I scanned a range of frequencies (1650 ~ 1900 KHz in this case). So the issue is the further the instrument deviates from 1830, the more the feedline is going to camouflage the true values of the antenna. Understood now!

In order to scan a frequency range, I need the analyzer attached at the antenna feedpoint (just like you said in your message). This is "doable", although not real convenient!

73 de Bob - K0RC


Message: 8
Date: Mon, 14 Nov 2005 19:28:50 -0600
From: "Jim Brown" <jim at audiosystemsgroup.com>
Subject: [TowerTalk] Understanding Antenna Impedance Measurements (was
Foam cables and velocity factor)
To: "Tower Talk List" <towertalk at contesting.com>
Message-ID: <20051115012901.39BAC7D3C at gw1.nlenet.net>
Content-Type: text/plain; charset="us-ascii"

First, please change the subject line when you change the topic. It's a 
real PITA to keep track of what to read. 

On Mon, 14 Nov 2005 19:11:45 -0600, Robert Chudek wrote:

>I have two questions:

>a) Where in the spectrum is this 248 foot hunk of wire resonant 
anyway!?
>  iv) can't be determined from the data, 

It can't be determined from the limited data you've put in your email. 
Remember my dissertation a few weeks ago about Smith charts. You can 
measure Z, R, X, and VSWR at the end of the coax, but the coax 
transforms that impedance. You said X was zero, but that is only the 
definition of resonance AT THE ANTENNA. You've measured at the 
transmitter end of the feedline, so that's not a meaningful result 
unless the feedline is precisely a half wave long. What Z did you 
measure at that low VSWR point?  Was it within 25% of 50 ohms?  If so, 
that's probably the resonance of the antenna. 

In general, for a dipole connected to coax, resonance is generally 
pretty close to the minimum VSWR frequency. 

>b) What will provide the best "performance" for this wire at 1830 KHz?
>    i) Build a 2:1 matching device for the feedpoint,
>   ii) Trim the legs shorter to move the minimum vswr to 1830 KHz,
>  iii) Adjust the coax length to move the minimum vswr, 

You first need to answer your first question, then move the resonance 
to 1830, then measure Z. If Z is within about 50% of 50 ohms, a decent 
antenna tuner ought to be able to handle it. If the VSWR relative to 50 
ohms is below about 1.5 or 2:1, you may not need a tuner. 

73,

Jim Brown  K9YC