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

Subject: Re: [TowerTalk] Fwd: OT: Exterior Ethernet Cable
From: Ian White GM3SEK <>
Reply-to: Ian White GM3SEK <>
Date: Tue, 12 Aug 2008 16:49:05 +0100
List-post: <">>
W3LPL wrote:

>> 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

>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.

Although Frank's test with dummy loads is valid, it doesn't directly 
address Hans's point which is about antennas and feedlines.

When there is current on the outside of the coax, the radiating system 
consists of both the antenna and feedline. If you suppress the feedline 
surface current by inserting a choke, or if you change the length of the 
coax as Hans suggests, then you have changed the entire radiating 
system. The new system will almost certainly have a different feedpoint 
impedance - so yes, you *can* expect the VSWR to change.

The VSWR could go either up or down when you insert a feedline choke. 
Because you have cured an installation fault, the VSWR of a 
factory-adjusted antenna would normally be expected to improve. However, 
if the antenna had previously been adjusted for minimum VSWR while its 
feedline was radiating as well, then installing a feedline choke could 
easily cause the VSWR to increase. That simply means the antenna needs 
to be adjusted again.


73 from Ian GM3SEK

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