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

[Jim Brown]

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