Forgive me if this becomes lengthy, but this very topic is something that I have been dealing with in the ham and commercial world for 25 plus years. It sounds like what you are experiencing is signal mixing with your own repeater's transmitter that is producing the phenomenon known as intermodulation, or "intermod."
You will need to do some tests first to determine if this is the case or not. It most likely has nothing to do with the repeater itself. The reason that digital mode doesn't seem to be affected by it is because you don't hear the noise itself in digital mode.. however.. be certain that whatever degradation you hear on analog mode is affecting the system in digital as well. It is a common misconception that digital FM operation is "immune to noise issues that analog has." The same RF problems that analog operators have are on digital mode as well, but again, you just don't "hear" them like you do on analog.
Ideally, you need a signal generator set to the repeater's input frequency and PL tone, and a way to inject the signal into the duplexer/antenna system via an isolated injector. If you don't have such an injector, a signal generator hooked to a 1/4 wave mobile magnetic mount antenna on top of the repeater building will usually suffice. You need to first disable the repeater's transmitter but keep the receiver speaker enabled so you can hear the signal generator's signal. The best way to do this is put the DR-1X in SETUP mode. This will disable the transmitter, but leave the receiver enabled so you can hear. Crank up the RF output of the signal generator until you just obtain a full quieting signal on the receiver. Then, exit setup mode so the repeater transmitter is enabled. If you hear ANY degradation of your received signal when the transmitter kicks on, you have an issue that needs to be corrected. If the received signal becomes noisy, but can be cleared with a slight increase in signal generator output, then you have DESENSE, which is usually a symptom of not enough isolation between the transmit and receiver in the duplexer or antenna system. If you must increase the signal generator's output to a more significant amount, but you hear weird noises, like arc welding, or the famous hollow pipe growl, then you have intermod, or mixing of the transmitter with another signal at the site somewhere. You mention that the condition sometimes "locks up the repeater." This can be temporarily remedied (or masked) by not running PL encode on the transmit. The sheer fact that you mention the repeater locks itself up on the noise tells me right away that you have an intermod mixing problem at the site.
If you don't have a signal generator, phone a friend who can give you a very weak, yet consistent signal, like a distant mobile on low power that can simulate a weak signal generator as a worse case scenario.
Intermod and desense can certainly be caused by the antenna itself. You didn't mention the type of antenna or feed-line you are running. Dual band antennas, like Comet or Diamond, or God forbid you use a Tram or knock-off fiberglass stick, usually don't work well for repeaters in duplex mode.. although many people have success with them, it isn't the best engineering practice especially at a busy RF side. Braided cable ANYWHERE on the output of the duplexer to the antenna can be a very liable suspect. Loose metal on the tower, even several feet away from, or up and down from your antenna can cause havoc on your system. Loose metal can not only cause random "scratchies" but also what sounds like rhythmic buzzing or arcing noises similar to what you are describing.
Another issue with 90% of off-the-shelf repeaters is the lack of an isolator or circulator on the transmitter output. Such a device allows RF to pass in one direction only. It's commonly used on the transmitter output to prevent other signals from making their way down the feed-line and into your power amp and mixing there. In other words, your own transmitter final amplifier can be the culprit and you may not even know it. Ideally, EVERYONE's transmitter at the site needs an isolator so that mixing doesn't occur in anyone's transmitter at the site.
Does the issue seem to change with weather? Typically mixing issues on the tower can go away when the tower is soaking wet, and there isn't much place miniature arcing of signals can occur on the tower. If the issue is worse when it's dry and windy, it's likely loose or dissimilar metals at or near the site. If weather doesn't seem to make a difference, the problem could be elsewhere (like your own power amp, or someone else's at the site, too).
One other thing to check when doing the desense test is your power output. Does the intensity of the noise increase when you run higher power, and decrease when the power is on low? Such findings can help you detect the problem. Most likely, you will find that when the signal generator is running but your local transmitter is OFF, the signal will be quiet with minimal interference. The moment you turn your transmitter ON while trying to receive at the same time, you'll probably get eaten up with noise. I would like to know the following details and maybe we can solve your problem by determining what you need.
1. What kind of antenna are you using exactly, and where/how is it mounted in your tower? What kind of tower is it (broadcast, commercial public safety, etc?)
2. What other transmitters are running at your site? Is there a commercial system there with a control channel that transmits all the time? Are there multiple transmitters at that site? If you know the frequencies, we can plug them into an intermod calculator to see if they're causing an issue.
3. What kind of feed line are you running? If you say LMR400 is running anywhere between the duplexer and antenna, then that's a likely suspect, unless it's flooded direct burial cable which is better.. but solid aluminum or copper NON-BRAIDED cable is best for duplex operation, especially at a busy site. Are there any jumpers, adapters, etc. in your feed-line?
4. How much power are you running out of the duplexer? What kind of duplexer and/or filtering are you running and is it in good shape?
5. Has the problem as a whole started recently, as if there were a change made somewhere at the site, or have you experienced this problem ever since your repeater has been on the air here?
I am currently fighting a battle at a TV broadcast site. My antenna is a UHF DB-413 at 1000 feet, right under the TV broadcast antenna. It was working beautifully until they did an upgrade at the site. Something up there got knocked loose and it's creating all kinds of problems on my receive, but only when the transmitter is keyed. It's loose metal up there somewhere, which causes classic intermod problems, especially when there's high powered transmitters nearby. Hopefully the next time a tower audit is done, there's a slim chance the problem can be found, and corrected.
Reply :
Always test repeaters in FM! Your ear can determine a lot of things. On digital, the range goes down and people that get into the repeater fine don't notice it.
About 90% of the time when people complain about bursty noise, it's the antenna system. If you're running a duplexer and there IS ANY kind of bad connection, that bad connection will act like a diode. You get the diode effect whenever two dissimilar metals are in contact! What often happens is that as antennas are blown about they develop bad connections between sections. Really good repeater antennas have everything welded, don't use a bunch of different metals (like solder, brass, and copper), and pretty much don't bend in the wind. (And they cost $600 - $1,200.)
So see if you still have the problem into a dummy load. This is basic troubleshooting. If not, it's your antenna system.
Keep in mind these bad connections can be anywhere. It can be caused by coax, coax connectors, or you may have a bad filter/duplexer. In this last case, the dummy load comes in handy again by testing with it placed at the output of the duplexer instead of the output of the transmitter.
More power makes this worse. Sometimes you'll have some arcing at 100 watts and none at 25.
When I last had this problem, it took me months to solve it. I had gone through everything in the rack. Bonded all the metal together. Checked and verified all coax cables, duplexer, etc. Tried different antennas. It was still there.
I finally ended up figuring out how to DF the source of the noise. The antenna was on the roof of this big building that was full of bolts holding things together. I fixed a Verizon antenna that had a loose N connector and aming bolts (really Verizon?). That wasn't it. I went around tightning down everything I could. About 100 bolts later, the noise went away.
So the point is this can also be caused by a nearby antenna, the tower, etc. Had this problem on one repeater that was on its own tower. We moved the antenna to the broadcast tower (and much closer to the broadcast antennas) and the problem went away.
You will also find that the problems at 2 meters are much, much worse than at UHF. Because of the proliferation of computers and networks, two meters has just plain gotten real noisy. In the incident on the roof of the building, we couldn't even operate a 2 meter repeater because of all the interactions with the hundreds of miles of CAT cable and thousands of computers and network devices. Unless you are truely miles away from civilization, my preference is to run a repeater on UHF.
Reply :
I must have spent years tracking down the 'rusty bolt effect' causing intermittent 'crackling audio', so annoying and would disappear for weeks and then return for no apparent reason. But of course, there is always a reason and always a solution to a problem.
I bonded everything in sight and to ground with copper tape and rods, heavy conductor and thought it was fixed, until one day it came back! This was on a UHF FM repeater and the effect can be demonstrated by simply scratching a screwdriver blade against any metal associated with the repeater, even when I resolved the problem. As you say Chris, electrical and RF connections must be perfect and using only top quality connectors, cables and testing for continuity and insulation resistance using professional test equipment.
The only effective solution in the end, was to opt for separate aerials for transmit and receive frequencies and with vertical separation on the tower. With a duplex system and one aerial, the noise would always return. It was all down to the fact that the building housing the repeater was constructed using steel reinforced concrete and which was deteriorating with the effects of time and weather and 'blowing' of some concrete panels. It was impossible to electrically bond all the structural steel.
The repeater is now Yaesu Fusion AMS, plus WiRES X and all is well. Hopefully, this discussion will help others with similar problems.
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