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I don't know if this is a good place to ask this or not, but I thought I would take a stab, just in case there are any experts running around on these forums...

We have a 900 MHz access point and, yesterday, it lost about 20 dB across the board for all radios associated with it. It sees them at about 20 dB less signal and they see it at about 20 dB less signal. I am trying to track down the cause. What I can tell you right now is that swapping out the AP with a new did not solve the problem. Swapping out the pigtail connecting the AP to the coax did not solve the problem. Bypassing the band-pass filter and the jumper between it and the coax coming down from tower did not solve the problem.

Using an Anritsu SiteMaster 332B, I swept through the band-pass filter, the jumper, the lightning arrestor, and the cable going up the tower (all the way to the antenna) doing a DTF SWR (Distance To Fault, Standing Wave Ratio) and put the end of the cable at 43.7 meters (which is where it should be). To make this measurement, I first configured the Anritsu to sweep only the frequencies passed by the band-pass filter (to get an accurate reading) and recalibrated the device.

I ran a SWR to check for impedence mismatches and found that, for the frequencies outside the range passed by the band-pass filter, the SWR was generally >2 and usually >5. For the frequencies that are allowed, the SWR was around 1.05.

Everything I have found suggests things are working correctly, execpt for the sudden drop in signal strength. Unfortunately, I have no means of generating a known signal that I can pump through the cable and antenna, nor a means of accurately measuring the signal coming out of the antenna. The SiteMaster does not have the optional power meter module in it. Short of taking down the cable and antenna and using bench equipment (which we don't have) to test them each individually, how can I verify which one is faulty? Obviously, one of them is, but which one?

About a month ago, the antenna tower was struck by lightning. We had a report from a maintainence worker that a "ball of fire and lightning" flew out of our equipment rack, out of the shed's open door, and hit some metal rebar in a sidewalk that was being installed. Amazingly, the only damage we could find at the time was a bad power supply for the radio. After replacing the power supply, everything was working again just as if nothing had happened. But as of yesterday, something has happened that is causing this new problem. So, we're not sure if it is an antenna issue or a cable issue. But replacing a $900 antenna and $1000 of cable (very large wave-guide cable) is not a fun prospect. We would like to know what needs to be replaced before we call someone out to climb this tower.

Thanks for any help anyone can provide. For those of you who have no clue what I'm talking about, take this as all as a learning experience.

Hello Eliot.

Ain't wireless fun!?

Based on about 9 months of experience doing commercial stuff like what you're talking about, and a ton of years experience in ham radio, I would strongly suspect your antenna. It's common to all radio's in your network.

You mentioned the lightning hit last month. That could have damaged/weakened your antenna. Have you looked at it closely with a pair of binoculars lately? If it's not a DC grounded antenna, it might be in shreds right now.

You also mentioned checking just about everything but the antenna. Yes, you have low SWR in the 900 Mhz area, but even a bad antenna can give you low SWR! Maybe it's a glorified dummy load now. You'd still see low SWR.

I've seen numerous ham installations using very old coax cable. Their SWR was low, but radio results were terrible. Turns out the old coax was highly lossy, killing the signal in both directions - but the SWR was great. I doubt the feedline is suddenly your problem. More likely it's your antenna. Besides, your TDR check (DTF) showed no signs of trouble along the entire length of feedline, right?

If you hire a person to replace the antenna, I'd also take a close look at the hardline connector up there too. If a pigtail is used up there at the antenna (for some strange reason), maybe it's a good time to wire the hardline directly into the antenna or replace that pigtail.

Good luck;

-Tim

Also, while they're up there, have them re-verify that all the cable and antenna connectors are watertight. The lightning hit (near-hit?) could have weakened connections and, over time, water seeping in would definitely cause problems that wouldn't be apparent until corrosion set it.

Joel

Thanks for the replies. We will definately have them rechecking the connections and cables when they go up there. It looks like we'll know sometime on Monday (assuming the climbers actually finalize the proposed time!), but even if it doesn't happen Monday, I will make sure to let you all know what the results were as soon as it does happen.

Right now, we are suspecting either a bad antenna, water in the cable, or bad cable, in that order.

Problem solved.

You'll never guess what happened....

Amazingly, the lightning strike we took last month appears to have caused absolutely no damage except for the blown out power adapter for the radio and small amount of melting on a little plastic cap at the top of the antenna.

The antenna was working just fine.

The cable was working just fine.

Everything else at the site was working just fine.

So, why did everyone drop in signal strength? Someone disconnected our antenna! Another tower company in the area was installing a UHF antenna for the traffic department in the city and simply disconnected our antenna from our cable, then attached our cable to their antenna! They just assumed that our antenna was not being used, and instead of actually following the cable to where it went to find out what it was there for, they just disconnected it and tried to use our cable to run their own antenna. And yes, this tower company is the one that our competitors use.

It certainly illustrates how much loss is involved at 900MHz when a VSWR test showed a reasonable reading when there was no antenna attached!

There is actually less loss involved at 900 MHz than at higher frequencies. The VSWR did not show anything wierd because, at the frequencies I performed the san, it actually wasn't too bad. However, if you change the frequencies (which we did today), it did come out much worse than it should have been. Remember, when I performed the scan, I scanned through the band-pass filter at the frequencies it is calibrated to. So, I saw the correct VSWR readings based on what the band-pass filter does.

egable Escribi?3:

There is actually less loss involved at 900 MHz than at higher frequencies.

Yes, I understand that, however the VSWR reading from an unterminated length of transmission line should indicate a problem.

The loss in the line will affect the reading. You can check this by putting an unmatched load at the end of a lossy line and checking VSWR. Then do the same test into the unmatched load without the lossy line. You will find that in practice a low VSWR reading can be obtained when the load is preceeded by the lossy line whereas testing the mismatched load itself would give a high VSWR. That is why VSWR should ideally be checked at the antenna ... not at the remote end of the transmission line. The only difference that the bandpass filter should have made is that it would introduce additional loss. It should not have any other affect unless tested outside its pass band ... where there should be significant mismatch.

My personal experience with commercially available bandpass filters has been that they exhibit very significant loss and impedance mismatch even at their design frequency.

Right, but first, there was an actual antenna on the cable, so it was not unterminated.

Second, the reason we weren't able to tell any difference on the VSWR reading is this:

For the frequencies passed by the bandpass filter, the VSWR of the antenna that was attached (even when not going through the filter) was below 1.2. Now, outside that range, it starts to go up a bit. About 10 MHz outside the range, it starts going up sharply. About 20 MHz outside the range, it went up really sharply. This was all seen without going through the band pass filter and with a frequency scan of 850 MHz to 950 MHz. When scanning this large set of frequencies, the VSWR clearly showed an issue. However, when I ran the scan, I originally just scanned the 914 MHz to 927 MHz with a 130 data point scan (which requires at least a 13 MHz sweep). Given the narrow frequency range I was scanning, everything showed up below 1.2. Then, when I scanned through the bandpass filter, I still saw the reading come up as <1.2 for the frequencies passed by the bandpass filter and the readings went up ever more sharply the further away from the center frequency you went. By 915 MHz or 927 MHz, the impedence was around 60 dB. This all looked normal but only because I was scanning such a small frequency range. If I had extended the range to the range we used Monday, it would have clearly shown a bad antenna (or at least the incorrect antenna).

We did also put a load on the cable on Monday to test the cable, which tested fine.

Also, there are a lot of bad bandpass filters on the market. We get ours from DCI, which I've heard from others are a very good brand. We haven't had any issue with them. When we first installed the 900 MHz system, we were having terrible problems with a very high powered paging system in the area. It's side lobes were enormous. When we put the bandpass filter in place we gained an effective 15-20 dB on the system from the reduction in interference. Before the bandpass filter, our radio was placing the noise floor at around -85 dBm. After the filter was installed, the radio placed the noise floor at -104 dBm.

Ahhh ... yes it makes sense now. They didn't simply disconnect your antenna, they replaced it with their own ... and it just happened that their antenna presented a reasonable match at 900MHz. You would have had no way of knowing there was a problem.

Regarding bandpass filters. Yes, some are better than others. In fact, 900MHz filters tend to be far better than those I've seen for 2.4GHz ... some of which have presented more than 6dB loss and a VSWR of > 5:1 at their design frequency. You're right ... you get what you pay for!

Fortunately there is little else on 900MHz in this country. :)