[TowerTalk] impedance measuring antennas

[W5LT]

Well, I have a small advantage:
My tower folds over, plus I used a temporary remote control coax switch to
choose between 'short' and 'open'.
I created a group of cal tables for all bands of interest for the two coax
runs that go up the tower.
Those are then stored on the computer for recall as needed. 
I was able to characterize the feed point Z of my 80m sloper as a function
of the SteppIR band setting and found a nice 50-jX point for 160m. From
there I made a series inductor I could switch in or out. Built it, and
Viola! VSWR at 160m design frequency was 1.08:1. 
I also found the appropriate SteppIR settings to tune the sloper across 80m.

Bob, W5LT


-----Original Message-----
From: John Lyles [mailto:jtml at losalamos.com] 
Sent: Friday, March 06, 2009 3:09 PM
To: W5LT; n8de at thepoint.net
Cc: towertalk at contesting.com
Subject: RE: [TowerTalk] impedance measuring antennas

Exactly Bob. But I wouldn't want to be the one who has to run up the tower
for short, back down to press 'SHORT DONE' and back up for open, down to
press 'OPEN DONE'....you get the idea!

I remember (oops, dating myself) when the big antenna ranges (like Harris'
in Hannibal, MO, for television broadcast) all had the old HP8754
"portables" strapped up on their rigs. Now you couldn't give one of those
VNAs away. 


-----Original message-----
From: "W5LT" W5LT at verizon.net
Date: Fri,  6 Mar 2009 13:19:06 -0700
To: n8de at thepoint.net
Subject: RE: [TowerTalk] impedance measuring antennas

> Accurate measurement of the antenna from the shack depends on the
> instrument. With a VNA (Vector Network Analyzer) and the short, open,
> termination procedure it calibrates out the cable and refers the measured
Z
> to the end of the coax.  The AIM-4170 and all lab grade VNAs can do that. 
> Bob, W5LT
> 
> -----Original Message-----
> From: n8de at thepoint.net [mailto:n8de at thepoint.net] 
> Sent: Friday, March 06, 2009 1:34 PM
> To: jtml at vla.com; John Lyles
> Cc: towertalk at contesting.com
> Subject: Re: [TowerTalk] impedance measuring antennas
> 
> Isn't it a bit difficult to haul all that up to the top of a tower to  
> make measurements?
> Measuring at the shack end of coax is NOT measuring the antenna.
> Don
> N8DE