[TowerTalk] Fwd: Fwd: OT: Exterior Ethernet Cable

[HansLG at aol.com]

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 at starpower.net
To: towertalk at 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 at aol.com  
>Subject: [TowerTalk] Fwd:  OT: Exterior  Ethernet Cable  
>To: towertalk at 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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