[TowerTalk] Coax Resiliency

[Steven Gehring]

All:

If I'm not mistaken, the megger won't generate more than 500 to 1000 volts
with VERY little current.  If the coax can't handle this basic leakage test,
it's trash.  If memory serves me right, the cautionary note on the megger
says something like 500 or 600 volts and not to crank it with your fingers
around the unit's wire leads or the line under test. Otherwise...the
generator crank will wake you up!

The Air Force uses this longstanding megger to test coax for the exact same
reason - leakage due to water and contaminant intrusion.  It's a simple and
time-tested device.

73,

Steven J. Gehring
Bothell, WA
E-mail: kz9g at arrl.net

Steven J. Gehring
19713 26th Drive SE
Bothell, WA  98012-7252
Mobile: 206-849-1218
E-mail: stevekz9g at verizon.net


-----Original Message-----
From: towertalk-bounces at contesting.com
[mailto:towertalk-bounces at contesting.com] On Behalf Of Red
Sent: Wednesday, September 17, 2003 7:30 AM
To: Towertalk
Subject: Re: [TowerTalk] Coax Resiliency


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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