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[TowerTalk] Re:TDR

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Subject: [TowerTalk] Re:TDR
From: (Stan Griffiths)
Date: Wed, 25 Jun 1997 12:18:54 -0700 (PDT)
>>> You would not normally use a TDR with an antenna hanging off the
>end, so it shouldn't be susceptible to BCI. <<
>The only case I can imagine is near a strong 25Mhz signal.   Since the
>test frequency for the AEA TDR box is 25Mhz.   It should be a fine test
>instrument for ham use....
>W8SS  Joe

As someone said earlier, I don't think AEA made a TDR box.  I think these
two guys are talking about apples and oranges . . .  

Anyway, I have found through many years experience selling Tek TDR units to
the communicatins industry that there are indeed times when you want to TDR
a feed line WITH the antenna attached.  Each antenna type has a
characteristic reflection that produces a unique pattern.  You can record
these patterns for the antenna system without knowing what every wiggle in
the trace actually means.  Later, when trouble develops, you can take
another reading and compare the graphs.  Sometimes the problem will show up
in the antenna and not in the feedline by looking at the recorded pattern
from the antenna itself and noting that it has changed from the original
picture.  Using a TDR, we found one antenna of a stacked TV transmitting
array that was disconnecting itself after about 15 minutes of operation.  In
this case, it showed the tower guy that he REALLY DID need to climb the 600
foot tower and repair the antenna.

The was in the Portland West hills where several TV, AM, and FM stations
have their antennas.  We got a lot of interferance from the other signals
while doing this measurement until I put a lowpass filter on the front end
of the TDR.  It changed the TDR display a little, but allowed us to use the
TDR in an intense RF field.  Tek TDRs are extremely sensitive and the model
1502 we were using uses a pulse that is only about 225 millivolts in amplitude.


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