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[TowerTalk] Re:TDR

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Subject: [TowerTalk] Re:TDR
From: (John Brosnahan)
Date: Wed, 25 Jun 1997 18:47:27 -0600 (MDT)
>>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

I am not familiar with the guts of the AEA TDR but normally a TDR puts
out a fast-rise-time DC pulse and there is no sine wave RF component.  The
PRF (pulse repitition frequency) is usually much lower than 25 MHz to
allow for reasonably long coax runs.  So I don't know where this 25 MHz
"test frequency" comes from.  

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

YES, AEA made a TDR, but no one knew about it.  It is/was called the CableMate
and looks just like their  HF Analyst.  It was priced at $995, but the QST ad
p.7, Sept. 1996 shows it "discounted" to $499

>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.

This IS a valid use of a TDR for reference patterns with antennas attached,
but it is hard to discern cable problems accurately when operated in this
mode.  Different antennas have different response characteristics and
testing a
cable this way after changing antennas--even ones on the same band, but with
different types of matching--will be difficult.  Since the fast-rise pulse has
a broadband spectrum, the different types of matches will reflect different
spectral components in different ways.

The TDR is designed for cables without a frequency sensitive termination.
It CAN be used to gain some insight into changes of a system with a 
narrow band antenna attached, but it makes it difficult to make quantitative
measurements--you can only make qualitative insights into system changes.

>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


The TEK 1502 is a great instrument that I often use one at the radar I
built in
Taiwan, since the University owns one.  Unfortunately I can't afford one so I 
still use a couple of old HP 1415A TDR plugins in 140A scope mainframes.  
Works fine in the shack where AC power is available.  A real pain to take
into the field, though.

73  John  W0UN

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