[TowerTalk] Fwd: OT: Exterior Ethernet Cable

[donovanf at starpower.net]

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