[TowerTalk] New Cushcraft XM240 Trouble

Dan Hammill kb5my at starband.net
Thu Apr 13 20:31:31 EDT 2006

Given a long enough feedline, one can make anything at the antenna end look
like a perfectly matched load in the shack...even an open- or
short-circuited coax.  That's why we can use 100 feet of open-ended RG-58 as
a well-matched 50-watt dummy load at 1296 MHz.

Here's a question to ponder:

Given two equal-electrical-length coaxial lines around 1000 feet long,
terminated at the opposite end with a non-reactive 75-ohm load, and measured
with a $128,000 Agilent VNA at 1000 MHz:

Line 1: RG-213
Line 2: 1-5/8" HELIAX

Assuming that the lines' electrical lengths are such that we present the VNA
with a non-reactive load, which line is most likely to show a better match
to 50 ohms on the VNA?

I've posed this question in a slightly different form to many degreed RF
design & test engineer interviewees over the last 18 years.  Only ONE has
given me the logical answer.

Dan  KB5MY

-----Original Message-----
From: towertalk-bounces at contesting.com
[mailto:towertalk-bounces at contesting.com] On Behalf Of K7LXC at aol.com
Sent: Thursday, April 13, 2006 6:38 AM
To: towertalk at contesting.com; RadioIR at charter.net
Subject: Re: [TowerTalk] New Cushcraft XM240 Trouble

In a message dated 4/12/2006 8:44:01 P.M. Pacific Daylight Time,
towertalk-request at contesting.com writes:

>  Your analyzer could also be lying to you. 
    After working on over 200 ham tower/antenna  installations, the only
thing that matters to me is how the antenna looks in  the shack at the end
of the feedline. That's my bottom line. Sometimes an  antenna looks a little
funny when measured up on the tower but looks FB in the  shack ( e.g.
Steve     K7LXC
Cell: 206-890-4188


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