antenna issue/msf5000/cable fault sweep

[talviar]

OK folks. . . .
Here is what I have. . . .
A tower with a UHF antenna mounted at the 175' mark (if it matters celwave PD458-2 450-470) with a motorola MSF5000 225W repeater station operating on 463.300 MHz. Hardline is very old (at least 25-30years best to my knowledge) and is 7/8" pressurized air-line. (Pressure gauge usually kept around 7 PSI charged with dry nitrogen) Recent checks have shown line will loose pressure within 24-48 hours of recharge. (I know this indicates a problem in the line or connectors) Before I send a crew up to swap out the coax I would like to sweep the line and want to know anyone's experience with the following:

Using a Aeroflex 2975 Service monitor with a Return Loss Bridge to sweep the line/antenna to know where the fault is. (If sweep shows in the vicinity of a connector it may be easier to fix than replace)
IE have you done it before? How accurate are the readings? Any suggestions to do this as I have never done this before. (Other than call someone with a Sitemaster to sweep as it is going to cost dearly to get one for a sweep using the Sitemaster).

Thanks in advance for any help
Tony

[Wowbagger]

Well, actually, you don't need the return loss bridge to do a distance to fault, just a T connector.

The accuracy will depend upon:
1) How well you know the velocity factor of the cable in question.
2) How well you locate the nulls in the return loss.

We routinely get accuracies of a couple of percent when we do the tests - call it a couple of feet for your situation.

Steps to perform test:
1) Place a T connector on the antenna port.
2) Connect antenna to the GEN port with a SHORT co-ax.
3) Connect cable under test to the antenna port.
4) Select Spectrum Analyzer (Mode 6).
5) Set top of scale to -10 dBm
6) Turn on the Tracking generator (option 6). Set it for GEN output (softkey 3), and set the level to -10dBm.
7) Increase the span until you see 2 nulls. There will be regularly spaced nulls, you can pick any 2 that are convenient.
Turn on Markers 1 and 2 (Option 7). Set them for vertical markers (the default). Make sure the cable fault is set to use markers 1 and 2. Set the Velocity Factor (VF) to be the correct velocity factor for your cable.
9) Close the marker setup window.
10) Position marker 1 on the low null, and marker 2 on the high null. (move the selection cursor until it highlights the flag at the top of the marker, then hit ENTER and spin the spinner to move the marker, OR us a mouse to drag it over).
The more accurately you position the marker, the better the reading. You can move each marker to center, and bring the span in to get a better fix on the bottom of the null.
11) re-open the Marker config window (option 7) again. Read the distance to fault from the window.

Also, you can tell if the fault is a short or open by looking at the behavior around 0MHz: if the first null is as far from 0 as the second null is from the first, then the fault is a short (or a reduction in impedance), if the first null is half the distance from 0 as the distance between nulls, the fault is an open (or an increase in impedance).

[talviar]

Wowbagger-
Thanks for the response!

Will be at the site on Sunday and test.
Thanks again!

Tony

[Wowbagger]

You used the "magic words" - Aeroflex 2975. I cannot fail to respond to that.