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Re: [TowerTalk] Coax Resiliency

To: Towertalk <towertalk@contesting.com>
Subject: Re: [TowerTalk] Coax Resiliency
From: Red <RedHaines@centurytel.net>
Date: Wed, 17 Sep 2003 09:30:27 -0500
List-post: <mailto:towertalk@contesting.com>
The line loss is one of the characteristics that may be degraded by moisture intrusion or other degradation of coax. It is easily measured by measuring impedance of the coax with one end either shorted or open. If open, the nulls appear at odd multiples of the 1/4 wavelength of the line. If shorted, the nulls appear at multiples of the 1/2 wavelength.

As frequency is varied, note the minimum Z at the nulls. The cable loss at that frequency, in dB, is equal to 8.69 x minimum Z/ line impedance.

For 50 ohm line, the loss is 0.17 x minimum Z.

For example, if you measure a 4 ohm minimum Z with 50 ohm line, the loss is 0.68 dB.

The loss thus calculated is for a line terminated in the line impedance, SWR = 1.

This information is taken from the instruction manual for the Autek VA1 antenna analyst. I use this method of measurement to assess deterioration of my transmission lines. I also measure DC insulation resistance and DC conductivity of the conductors. I'm reluctant to use a Megger. Exceeding the breakdown voltage of a line can permanently damage it, depending upon the kind of insulation that breaks down.

73 de WOØW


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