n4fbz
New Member
Posts: 1
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Post by n4fbz on Mar 17, 2017 13:59:20 GMT -5
I've noticed something strange when using the TDR function.
When I scan one of my cables, when I use "High Resolution", the scan shows a good cable -almost flat at between 50 and 52 ohms, the entire distance (86 feet).
When I turn "High Resolution" off, the response varies as much as 10 ohms or a bit more... showing the cable varying between around 55 ohms and 35-40 ohms.
After doing some repairs to the antenna, the curves are approximately what they should be (multi-band fan Inverted V). I am pretty certain that the coax is fine.
While I'm at it... I have an Ascom VHF Test Meter on my 2m/440 rig. I just replaced the coax to the antenna (it got water in it) and discovered that with the AIM 4170 (Old version) the response of the antenna was OK (it needs minor adjustment) but the meter is showing a higher SWR (3 vs 1.4 for the AIM). This has me a bit confused... could the meter be that far off? (I have some rather precise 50 ohm wide bandwidth terminators I use for calibration for the AIM and think that's not the problem.) This is one of those times I wish I had a newer AIM, as mine will only go to 150 (and I have 440 radios as well) - so I can't say about the UHF side of the antenna/radio/coax.
Thanks! Bob
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Post by Bob on Mar 17, 2017 15:24:04 GMT -5
Hi Bob,
The TDR response can be hard to interpret when the antenna system is complex, like multiband antennas and baluns and stubs, etc.
To test the coax, it's best to disconnect it from the feed point and attach a resistor to the coax. It's helpful to use a resistor that does not match the line exactly, for example, a 100 ohm resistor. Then you get a good indication of where the end of the coax is when the TDR trace steps up to 100 ohms.
The AIMuhf is good for testing 440MHz antennas and the TDR function has higher resolution too.
--73/Bob
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