Ben there, done that!
The change in the SWR due to outside current on the coax is due to the
change in impedance at the feedpoint. The coax acts a an extra radiator for
your
antenna and any change there will change the SWR. Depending on where you are
in the phase on your feedline, a quarter extra coax will make a big or small
change in the SWR.
You may utilize the coax as radiator if you add a trap at the "right point"
on the coax to stop the current from reaching you shack.
Yes, you are right though, a louse shielded or in any other way low quality
SWR meter will show that behavior even if there is not outside current. I have
used both.
73 de Hans, N2JFS
____________________________________
From: donovanf@starpower.net
To: towertalk@contesting.com
Sent: 8/12/2008 9:50:17 A.M. Eastern Daylight Time
Subj: Re: [TowerTalk] Fwd: OT: Exterior Ethernet Cable
Hans,
Not quite true...
The most common cause of a change in VSWR measurement when you change the
length of a coaxial transmission line is measurement error caused by poor VSWR
bridge directionality. Many inexpensive VSWR meters (and some expensive
ones) suffer from this problem.
Please try this simple experiment with your VSWR meter, and let us know your
results.
- Place your VSWR meter between your transmitter and a 100 ohm or 25 ohm
resistive load. Use short 50 ohm transmission lines to make the connections
(less than 1/10 wavelength long).
- Measure the VSWR. If your VSWR meter is calibrated for 50 ohms,
it should read 2:1
- Replace the short cable between the VSWR meter and the resistive load with
a 1/4 wavelength 50 ohm coaxial cable. If your VSWR bridge has excellent
directivity, the VSWR will not change from the measurement above. If the VSWR
changes, it was caused by poor VSWR bridge directivity.
73
Frank
W3LPL
---- Original message ----
>Date: Tue, 12 Aug 2008 08:49:15 EDT
>From: HansLG@aol.com
>Subject: [TowerTalk] Fwd: OT: Exterior Ethernet Cable
>To: towertalk@contesting.com
> A simple test is to add (or subtract) some of the coax between the SWR
meter and the antenna.
> If you have a change in the SWR you have current on the outside of the
coax.
>Hans, N2JFS
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