Batboard

Here's a dilema for you RF interference experts.

Our radio:
462.45 repeater ouput
467.45 repeater input
DPL 364

Engineering radio:
464.875 repeater output
469.875 repeater input
DPL 412

If our repeater is transmitting at the same time as an engineering portable on the 10'th floor, the engineering radio bleeds over onto our repeater. When our unit stops, the engineering portable on the 10th floor will also hold our repeater open with their audio, even though the DPL is different.

Both repeaters (Quantars) are on the roof of a 12 story building. The engineering portables that cause this problem are on the 10th floor of the building next to the one with the repeaters.

What gives? Anyone?

This sounds like a classic problem with close-in radios on a multi-repeater site or system.

There are a number of factors that combine to cause this problem:
1) Two or more repeaters in close proximity
2) Both repeaters having the same offset
3) Both repeaters keyed up simultaneously
4) The unit keying one of the repeaters is in close proximity to the transmitter site. (Actually it's a matter of field strength rather than distance)

Here's what happens:
1) Well designed and installed repeaters should include appropriate protections to prevent or supress the generation of IM products.
2) Most portables and mobiles do not include any real IM protection.
3) When a close-in field unit transmits on one of the repeaters while they other is also transmitting, the output frequencies from both repeaters can combine in the transmitter of the field unit with the field unit's TX frequency. As long as both repeaters are using the same offset, the third order IM product will always fall exactly on the input of the second repeater.
4) The reason this appears in close-in units is that the field unit is receiving a high field strength from both repeaters, and in turn is radiating a strong third-order IM product, which in turn is strong enough to affect the nearby receiver.

Here's how the actual math works out in your particular case:
462.45 + 469.875 - 464.875 = 467.45
If it's any comfort, they could very well suffer the same fate from your radios.

The only true cure for this problem is to lower the near field strengths from the repeaters, either by lowering the output power, and/or (where appropriate) increasing antenna gain to direct more energy towards the horizon and away from the area under the antenna. Lowering the output power on the portables may also help to reduce the amplitude of the IM product.

It doesn't matter that you are using different DPL codes, since your repeaters output tone is part of the IM mix. You can mask the problem by going to split tones on the repeaters so that no output can bring up an input, but this will not solve the problem, only hide it. The real danger, in your particular case, is that the IM product from a close-in engineering portable could capture the input of the security repeater, and keep a more distantly located officer from being heard.

This is why it is important to use only as little power as absolutely necessary on a repeater. There are bigger problems than talk-in / talk-out range balance lurking when you put up an "alligator" (all mouth, no ears) repeater.

For reference, Andrew has some great calculators available on their web site.
http://www.andrew.com/products/antennas ... rmulas.asp

" Both repeaters (Quantars) are on the roof of a 12 story building. "

Why are the repeaters on the roof of the building when you need coverage IN the building, bad idea.

Thanks humhead! That's along the lines of what I was suspecting, I just don't know the theory & math behind it. Now I do. At least now I can call the local moto and say "HEY! FIX THIS!" And you are absolutely right. It does blow our further away portables out of the water. Picture this... "I need backup at--- Bob, there is a clogged toilet in ER!"

Will- believe it or not, they work great up there. The only places we have trouble getting in and out of is the underground parking garage (cement cave) and some of the MRI areas (lead cave). There are actually no less than 4 repeaters up there that are used by portables in the building, all UHF.

Matt

If your looking for a cheap way to solve the intermod problem, you might try looking into placing a notch filter on both repeater reciever lines. Make the notch for the other repeater input. Make sure that both repeaters are using double shielded coax or a "heliax" type cable.

As for the cement dungion as you call it, one option would be to run a coax cable from the repeater all the way down the building and into the garage. Place a low gain or omni antenna there. Put a power spliter at the antenna port of the duplexer. Only bear in mind that the spliter will cause the receiver level to also be cut in half. You might gain service in the garage, but loose it in another part of the building due to the 3 db split on the receiver signal.

Jim

Unfortunately, in this particular situation, it sounds like the IM product is being produced in a field transmitter, not in the repeaters. Because of the frequencies and spacings involved, the offending IM product is showing up exactly on the repeater's input frequency. No amount of filtering on the repeater's inputs or outputs can do anything to fix that (not that good engineering practices aren't critical- it just doesn't sound like the issue here).

The only fix is to get the field strength where the field units are operating down to a level where IM stops being strong enough to cause a problem. The easiest way is to drop the output power of all involved repeaters. You could probably drop the power quite a lot before you noticed any real degradation in your TX coverage. The same goes for the field radios.

Jim's suggestion about doing multiple antennas is one valid approach. Another might be running "leaky" Radiax throughout the facility. Also, if you split the TX output, you can use an asymetrical power divider to apportion the signal. It wouldn't necessarily have to be a 50/50 split.

The really slick solution would be to multi-couple all of the repeaters onto a combined antenna / Radiax system, along with a receive multicoupler and appropriate filtering, and run everything at fairly low power. However, engineering such an undertaking is not a project for the faint of heart.

If we were to replace their portables with tone-remote to the repeater, that would make the problem go away, correct? I mean, yea it will still exist if someone walks by with a portable but the engineering dispatcher working 24/7 won't be causing it all day and all night. That would be a lot more economical than $10,000 woth of combiners and radiax.

If that is the only radio that you have noticed causing a problem then replacing it with a wired station should hide the problem.

Is Engineering dispatch using a portable, or a mobile on a power supply?

The 3-4 people up there use portables. The reason this has just now cropped up is because they just moved to this location from another far away building. I swear on all that is holy, this problem started the exact moment in time that they moved into that office.

Of course, MSS's reply to this was "We put multiple UHF repeaters at the same site all the time, but we would be happy to come check your equipment for problems", basicly denying that IM could be occurring.

And IS this new office near the woof?

That new office is where these portables on the 10th floor are located. That is the 10th floor of the building adjacent to the building with the repeaters. So, yes they are VERY close.

Here's a hypothetical situation. Lets we took all these repeaters and put them on a combiner with circulators. The problem would go bye-bye correct?

Unfortunately, if you are seeing a 3rd order IM product being produced in a field radio, it will not.

The problem (assuming that my initial diagnosis is correct) is that the field radio is producing an IM product that falls directly on the repeater's inputs.

As long as the repeaters are functioning on their input and output frequencies, the 3rd order IM math will remain unchanged, regardless of how much filtering you put on the repeaters.

The only realistic thing you can do at the repeaters to help the problem is to lower their output power until the IM level gets low enough to not be an interference issue.

That or get your friends in Engineering off of their radios...

Crap, thats what I thought. I was with Motorola for 5 hours today nit picking the system and troubleshooting. They don't seem to think it is IM. They think it is just "co channel interference" and the circulators will kill it. To me, that logic isn't working.

To me, that would require intput-to-input since the problem is clearly coming in our input and coming only from the near field portables making it from their input also. So I'd love for someone to explain how you can have input-to-input when one of those inputs isn't even there. This happens even when using the console transmit and our repeater input is not even open. With a different DPL, it couldn't open the input if it wanted to. It would require something legit to open the input and it would go from there.

The only thing as I am seeing it right now that can cause this is IM. It is the only thing that can create a carrier on our input when we are using the console transmit.

While it is great fun to sit and try to solve this at a distance, what it sounds like you need at this point is someone with a Spectrum Analyzer (and a clue) to come out and spend a couple of hours on the site hunting the problem.

If I were going to be out there looking, here is how I would probably go after it:

1) Go up to the repeaters with the SA, and look at the spectrum with both repeaters off the air.

2) Locally key the repeaters in question, first individually, and then together, and look for any spurious signals on the input frequencies. (This would also help rule out a non-linear junction somewhere else on the roof)

3) If steps 1 & 2 are clear, have someone key one of the suspected portables in the Engineering dispatch, and check the SA.

4) With the Engineering portable keying the Engineering repeater, key the Security repeater locally and see if a spurious signal appears on the Security input. If it does you can be fairly certain you have your culprit. Depending on the dynamic range of the SA, the signal may or may not be visible at the repeaters. It may be easier to find it by moving the SA to the Engineering office.

5) If you found an IM product in step 4, try again with another radio farther away. Also try reversing the situation, using a Security radio, to see if the IM problem also appears in the opposite direction.

From the information that you have provided, this really sounds like an IM problem, however, the only real way to tell is to go out and physically investigate the cause.

Time to bring this one up again!

Update from last year since I guess I never said anything aftewards... They finally submitted to my pressing the 3rd order IM explanation after hours of troubleshooting yielded nothing. The put in tone remote desksets to replace those culprit portables and the problem went away just as fast as it came. Victory for batlabs on that one.

Now we are having another issue. Another agency with a repeater on our roof is causing the same damn thing. But in this case, the other agency's portable field unit is a much greater distance away. I'm figuring 2 or 3 blocks away and inside a brick building at ground level. I'm pretty sure it is only those field units that cause it when we are transmitting simultaneously but sometimes it is hard to guess where they are located.

Is that kind of distance causing 3rd order IM product normal??

I'll have some audio recordings tommorow morning.

It would certainly be interesting, although by no means definitive, to hear some recordings.

You have already picked up on the first question that you need to answer: Is the problem global across all units on the channel, or is it tied to only one or two units? If it is isolated to one or two field radios, you are probably chasing another IM problem. If it's global, you probably need to be looking at filtering and/or desense problems with the repeaters.

If you can validate that the problem is tied to only one or two radios, here's a cheap test that you could try (It's a low-budget version of what I had suggested previously): 1) Lock your repeater into transmit. Go out to the suspect field units location with a scanner or radio programmed to receive on your repeater's input. 3) To really prove the point, go ahead and take the antenna off of your test receiver! 4) While standing near the suspect radio, have them key their repeater. If you get a carrier on your input frequency when they key up, you may have found your culprit. If not, it's time to keep looking.

2-3 blocks is a pretty good distance for an IM product, but this is a game about field strength, not pure distance. If you are transmitting 100 watts out of your Quantar into a high-gain antenna on top of a 12 story building, they may be getting hammered pretty well by your main lobe 2-3 blocks away.

A couple of months ago I was in Detroit dealing with some festivities around a certain football game, and had to deal with a site that was near a 30 story building with an FM broadcast station on the roof. That beast was pumping out 45.5KW ERP 24/7! I was getting slaughtered with IM and audio rectification problems in everything from DC to daylight for several blocks around the transmitter site. It was absolutely insane.

My philiosphy is to never say never until you have the problem narrowed down some more.

If this isn't a third order IM problem confined to a small number of field radios, I would also start looking at the repeaters themselves. Are the duplexers good quality bandpass or pass / notch units, or did someone go and put a cheap notch / notch duplex in the middle of a crowded site? Are there isolators between the repeater transmitters and the duplexers? Is all of the transmission line quality double shielded coax or heliax, or did someone sneak some RG-58 up there?

If you are, in fact, again causing yourself these kind of IM problems it is time to take a long hard look at your antenna configuration and output power. The ultimate solution may well be to cut the power, lose the gain, and add some Radiax to ensure solid coverage. Everyone may end up a lot happier in the long run.

Here are two recordings. #1 first plays the other agencies audio by itself for reference. Then it plays our recording with the simultaneous interference. #2 is a second incidnet with just our recording and the interference.

http://www.nmvfc.org/Interference1.wav
http://www.nmvfc.org/Interference2.wav

Our repeater is co-located with five other repeaters on a TX Combiner and RX multi-coupler. There are no duplexers. There is a TX antenna on the combiner and an RX antenna on the RX multi-coupler. The offending other agencies repeater i on it's own with it's own antenna and duplexer.

Have you had your Duplexars checked lately?