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Testing coax/antenna assemblies?

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gavifer

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Mesa, Arizona
I am NOT a radio expert. I come from a digital background. I'm easily befuddled by anything rounder than a square wave. I have what I hope is a simple question about a simple problem.

I'm trying to correct a fault in a neighborhood WiFi tower whose short-haul point-to-point feed went bad. The usual fix is to replace the sour radio card, but this time the card was not the culprit. We also replaced the pigtail, since it was easily accessible, but no joy.

The symptom is that neither the old nor the new card can see any access points in the 5GHz WiFi band (there are several strong ones within 100 yards, including the one it should be using). Both cards do detect several local APs in the 2.4GHz WiFi band, so we know they're not both broken. (The 2.4GHz APs share the tower, so I suspect the card would would pick them up with no antenna at all.)

Ruling out the these components leaves only the directional 5GHz antenna or the (long) connecting LMR-400 coax. We had rain recently (it's rare here), so the coax could be the culprit (we do goop up the connectors, but stuff happens). I've never had an antenna just stop working before, but I suppose I can't rule that out either.

Getting someone up on this tower is particularly troublesome. I don't have specialized RF test equipment, but I'm not averse to investing in some if our budget allows.

I'd like to be able to determine which component is at fault so I can do something smarter than just a shotgun whole-blood transfusion. Plus it would be nice to have on hand whatever equipment is necessary to test new and used antennas and cables as they rotate in and out of the shop.

Can someone explain to me how to go about such testing and what equipment I will need?

Thank you so much.
 

popnokick

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You're lucky this setup ever worked in the first place. LMR-400 is not a good coax choice at 5gHz due to very high losses in that freq range. You should be using LMR-1200 or maybe 900... whichever will give you the lowest loss per foot for 5 gHz (I'll let you look it up... there are other coax choices possible too). With LMR-400 having a high loss per foot (you didn't say how many feet of 400 you have) all it would take is a little moisture ingress (i.e. water) in any of the multiple connectors involved n the setup.... or in the coax...and poof! 5 gHz signal gone.

Since you said the 5 gHz "card" (in computer? AP?) had been replaced and is apparently OK you'll need to look at the antenna path.... all of it from the point it leaves the "card", through the pigtail, out over the LMR-400 to the antenna. To troubleshoot, you'll need a 5 gHz signal source that you can move around to different locations. Simplest is probably a known good WiFi AP that you know is putting out a signal at 5 gHz.

First, connect a 5 gHz antenna as close as you can to the "card" (back of computer? On AP?). Doesn't matter what kind... you're just trying to verify that you're getting a 5 gHz signal. If the signal from your test AP is received by the card, then attach the pigtail but DETACH the LMR-400 to the main antenna and attach your test antenna. If you get the 5 gHz signal again on the card, you know the pigtail and connectors are OK... and that the problem lies somewhere in the LMR-400 and/or connectors. If you can do so, go to the antenna/far end of the coax and replace it with your test antenna. This may be inconclusive since your test antenna may not have enough gain to overcome the high losses in the LMR-400. So the antenna-end test is optional. But if it should happen to work then you may have a bad (original) antenna. They can get water in them, too.

If you determine the signal is good out through the pigtail and up to the LMR-400..... and it is the LMR-400 and/or connectors that are bad.... rip out the entire run of LMR-400 and connectors and replace it with a brand new lower loss coax that is appropriate for 5 gHz, and shiny new connectors that are properly weather-sealed.
 

prcguy

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LMR400 is fine for use at 5GHz as long as the link budget for the system includes the anticipated loss. Since the system was working fine at one time the choice of cable is not the problem and something has changed like water ingress in the cable or antenna or ??

To properly diagnose the problem takes suitable test equipment like a signal generator and power meter or scaler network analyzer or spectrum analyzer with tracking generator that covers your frequency range and/or an antenna analyzer or directional coupler to use with the other instruments to verify the antenna is in good shape.

If you get good performance from bypassing the cable as previously mentioned its a good indication the cable is the main problem but if the antenna is then in a bad spot for reception it would be hard to draw a conclusion. You really need proper equipment to get the right answer or exact replacements for the cable and antenna to swap out piece by piece.
prcguy


You're lucky this setup ever worked in the first place. LMR-400 is not a good coax choice at 5gHz due to very high losses in that freq range. You should be using LMR-1200 or maybe 900... whichever will give you the lowest loss per foot for 5 gHz (I'll let you look it up... there are other coax choices possible too). With LMR-400 having a high loss per foot (you didn't say how many feet of 400 you have) all it would take is a little moisture ingress (i.e. water) in any of the multiple connectors involved n the setup.... or in the coax...and poof! 5 gHz signal gone.

Since you said the 5 gHz "card" (in computer? AP?) had been replaced and is apparently OK you'll need to look at the antenna path.... all of it from the point it leaves the "card", through the pigtail, out over the LMR-400 to the antenna. To troubleshoot, you'll need a 5 gHz signal source that you can move around to different locations. Simplest is probably a known good WiFi AP that you know is putting out a signal at 5 gHz.

First, connect a 5 gHz antenna as close as you can to the "card" (back of computer? On AP?). Doesn't matter what kind... you're just trying to verify that you're getting a 5 gHz signal. If the signal from your test AP is received by the card, then attach the pigtail but DETACH the LMR-400 to the main antenna and attach your test antenna. If you get the 5 gHz signal again on the card, you know the pigtail and connectors are OK... and that the problem lies somewhere in the LMR-400 and/or connectors. If you can do so, go to the antenna/far end of the coax and replace it with your test antenna. This may be inconclusive since your test antenna may not have enough gain to overcome the high losses in the LMR-400. So the antenna-end test is optional. But if it should happen to work then you may have a bad (original) antenna. They can get water in them, too.

If you determine the signal is good out through the pigtail and up to the LMR-400..... and it is the LMR-400 and/or connectors that are bad.... rip out the entire run of LMR-400 and connectors and replace it with a brand new lower loss coax that is appropriate for 5 gHz, and shiny new connectors that are properly weather-sealed.
 

gavifer

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Dec 24, 2012
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Location
Mesa, Arizona
Testing coax/antenna assemblies

Thanks for the discussion.

The calculator the original engineer used (COAX CUTOFF FREQUENCY) indicated that 5GHz was wall within the capabilities of LMR-400. The cable is about 40' long; however, the radio shot itself is a mere 250', and he figured he could eat a lot of loss. And the equipment did work perfectly for over a year, until it suddenly went belly up.

To properly diagnose the problem takes suitable test equipment like a signal generator and power meter or scaler network analyzer or spectrum analyzer with tracking generator that covers your frequency range and/or an antenna analyzer or directional coupler to use with the other instruments to verify the antenna is in good shape.

Ouch. That sounds like about $5,000 worth of equipment that I would have no idea how to use or in which order.

Sounds like what I'm hearing is that there is no "clever" way to diagnose this other than to climb the d*n tower, take down both components, test each separately on a known working router box, then climb the tower again to reinstall good replacement parts. Back to brute force, I guess.

Is a multimeter capable of indicating water in a coax cable?
 

prcguy

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So Cal - Richardson, TX - Tewksbury, MA
An ohmmeter can give you an idea of a very serious water problem, it should read in the megohms across an unterminated piece of coax and if the connectors or dielectric are soaked it may read in the tens to hundreds of ohms. There is a lot more to consider with coax at 5GHz and an ohmmeter will only take care of a very basic measurement.

If you were using Belden 9913, which is a mostly air dielectric coax I would say that could be your problem because 9913 is notorious for water ingress. Otherwise each component needs to be tested or swapped with a known good item until you find the problem. Don't be surprised if there are not multiple problems that have gone unnoticed until now.
prcguy


Thanks for the discussion.

The calculator the original engineer used (COAX CUTOFF FREQUENCY) indicated that 5GHz was wall within the capabilities of LMR-400. The cable is about 40' long; however, the radio shot itself is a mere 250', and he figured he could eat a lot of loss. And the equipment did work perfectly for over a year, until it suddenly went belly up.



Ouch. That sounds like about $5,000 worth of equipment that I would have no idea how to use or in which order.

Sounds like what I'm hearing is that there is no "clever" way to diagnose this other than to climb the d*n tower, take down both components, test each separately on a known working router box, then climb the tower again to reinstall good replacement parts. Back to brute force, I guess.

Is a multimeter capable of indicating water in a coax cable?
 

popnokick

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Northeast PA
I agree with the original systems engineer that only 250' of distance to cover between the 5 gHz radios should have provided a very generous fade margin in the overall link budget, even allowing for (some) losses in the 40' coax.
Which probably now means that something is grossly wrong, and hopefully also means that a swap out method of troubleshooting will find the problem vs having to go the network analyzer, directional coupler, power meter, tracking generator, etc. route. I figured you had all that gear just laying around on your service bench but didn't feel like hauling it out ;-)
Seriously... start at the end of the transmission path that is simplest to get to and see if you still have signal at each test point you swap in a known good component. And don't overlook the obvious like using your multimeter to check for shorts/opens in the coax and connectors (as appropriate).
 
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