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noise on a 900 repeater

This forum is for the discussions targeted at converting various models of Motorola equipment to operate in the 900MHz Amateur Band.

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rrfd43
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noise on a 900 repeater

Postby rrfd43 » Thu May 08, 2008 9:29 am

I have a click, clap, clp sound (best I can describe) on my repeater (927.4875). This sound was present on the first machine that i had (maxtrac/spectra home brew). I reciently switched to an MSF5000. I have tried different duplexers and different frequencies (927.5875 and 927.5125) with the same noise. It is worse the more distant the remote user is. I can't seem to find a way around it, and it really sucks!

It is not on transmitt during the tail. It seems to mix in on recieve. I have not gotten a chance to get any serivce equipment at the site. The best idea is a spread spectrum transmitter in this area?

I have a MSF5000 test at home on a different frequency and can just barley hear the problem (very acceptable and only on very weak signal). This set up is running a freq. pair and duplexer used at the main site without any luck there. I am about 5 miles from the primary site.

Any idea what this could be or ideas to track down? Any ideas on how to filter it?

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Re: noise on a 900 repeater

Postby Jim2121 » Thu May 08, 2008 10:11 am

Does anyone with a mobile/HT hear this sound? Because if its a SS transmitter, and you can hear it on the reverse? Then I would take a drive in the direction of where your hearing the sound... Of course I can think of 5 other things that might make that sound. 1.I had a bad power backup unit. 2. A hardline problem.... Is it 24/7 this noise.... I'd also run this past that yahoo group... but "noise on a 900 repeater" you have your work cut out. Jim
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Re: noise on a 900 repeater

Postby N2OXV » Tue May 27, 2008 6:00 am

Are you using a ctcss tone on the rx all the time, and if so have you tried to run with carrier squelch to see if the noise will bring up the repeater on its own? If in carrier squelch operation the noise dosen't bring up the repeater, try having a weak signal on the rx, and then shut off the tx does the noise go away? Are you using the same antenna for rx and tx?

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Re: noise on a 900 repeater

Postby MSS-Dave » Wed May 28, 2008 11:44 am

I can't seem to find a way around it, and it really sucks!


It's more than likely Spread Spectrum across your RX frequency. You can filter out of band noise but you can't filter on frequency energy without losing what you are trying to hear. I had a 927/902 machine going for 3 years with little use. When I first got it going (MSF 5K) in 2002, things worked great. It got worse over time, finally took it down a couple of years ago 'cause the RX noise was ridiculous. My site was fairly close to a industrial area that had lots of SS links that probably was the source of the noise. I tried so many different combinations of bandpass and notch filters that I lost count. In some cases, the RX noise floor was -75 dBm on pulses and -85 constant and that was through 1 MHz wide preselector and a cavity. I would have battled on but it was a waste of time with 2-3 users and they went to another machine 15 miles away that was lower that didn't have the RX issue. Some areas of the country may be great on that band but my experience in this area of the country has proved otherwise.

Dave

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rrfd43
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Re: noise on a 900 repeater

Postby rrfd43 » Wed May 28, 2008 12:15 pm

Well it has been some time before I got a chance to reply.

There is a pl on recieve and transmitt. I run a duplexer and one feedline/antenna. The duplexer is a BPBR with andrews 1/2 and a Motorola antennna.

The noise is NOT on the TX signal when the station is forced to tx without an input. The same noise is on two different machines used at this site. (same freq pair). The noise appears on any signal not for the imediate recieve site.

I hooked a scanner up to the system after the duplexer and turned the schelch all of the way down. I then scanned 902-903 with nothing heard. Silence. I tried reprogramming from 902.4875 to 902.5875 with a different duplexer and got the same annoying noise. I do not have a scope to bring to the site.

I would believe it to be spread spectrum ut don't know how to find out how to confirm or what further steps to take from here.

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Re: noise on a 900 repeater

Postby tvsjr » Wed May 28, 2008 12:30 pm

Most likely spread spectrum crap. Unfortunately, you're going to need a spec-an and a service monitor to do any detailed work toward identifying the source.

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Re: noise on a 900 repeater

Postby MTS2000des » Wed May 28, 2008 12:52 pm

we have the same thing on the Sweat Mountain (just north of Atlanta) 927.575 repeater which is also an MSF5000. It sounds like either SS or iDEN crap, we just put up with it, if your signal into the machine is strong enough, it isn't noticeable as much as when users of portables use the repeater. Being that Sweat is an RF hell hole, with everything from DC to microwave, cellular, iDEN, paging and TV and FM broadcast it's a miracle it works as well as it does.
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rrfd43
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Re: noise on a 900 repeater

Postby rrfd43 » Wed May 28, 2008 3:54 pm

What luck would I have running to a lower or higher pair? Would I seem to have this across the whole band, or would one think it gets better or worse the higher or lower I can go?

I would assume the true answer is a spec-an and serivce monitor but that is not a simple solution on this end....

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Re: noise on a 900 repeater

Postby tvsjr » Wed May 28, 2008 4:35 pm

The spread spectrum stuff runs the whole 902-928 band. You might get lucky, you might not. Of course, you are also limited by what your subscriber units will do... most are pretty limited to the 902/927 split.

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Re: noise on a 900 repeater

Postby Jim2121 » Wed May 28, 2008 5:08 pm

see if you can change the pair of frequencies you have? But like "tvsjr" said its a crap shoot. 50/50...
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Re: noise on a 900 repeater

Postby xmo » Wed May 28, 2008 6:25 pm

Just like MSS-Dave, I was an early adopter on 900. I found a source for Maxtrac filters. I modified a couple dozen radios. Figured out the hack for the Spectra RSS, built repeaters. I even scored an MSF5000 back in 2001 and put that up.

It seemed to work pretty well. Most of the 900 band incumbents were dodads like cordless phones and baby monitors - stuff that used ordinary FM - stuff you could move to get away from. Over time is seemed to work less well and all the users drifted away.

When I first saw this thread I thought I should re-visit the idea of a 900 repeater. I was thinking that most of the cordless phones have moved up to 2.4 and 5.6 GHz. Maybe the band is quieter now.

NOT.

I went to one of our sites where we have a 900 antenna. Mind you - this was a brand new commercial [Decibel Products] 10 dB gain antenna we put up just for the 900 repeater. A first rate system all the way - located in a suburban area.

I set up a spectrum analyzer & took a noise floor reading from a 50 ohm termination:

Image

Then I connected the analyzer to the antenna [through a tunable bandpass preselector] Here is what the receive part of the band looks like:

Image

The upper trace is a peak hold accumulation over less than one minute. The lower trace is the live trace. The marker shows the absolute noise value but it changes up and down. We also hooked up a 900 receiver and conducted a receiver desensitization [effective recevier sensitivity] test - we saw consistently 30 to 40 dB of receiver degradation.

That's caused by an aggregate of the spread spectrum trash. No filtering in the world can make that go away.

We also checked other areas of the band - all were the same.

The FCC allows all that unlicensed stuff on a non-interference basis. Hah!

They created an RF toxic wasteland.

Technically an amateur license has precedence over the unlicensed stuff. The buyers got a notice when they bought the gizmo - a notice that says what the rules are - that they have to accept any interference and won't cause any to licensed services. [a notice which they threw away without reading]

Imagine trying to track down hundreds of devices and tell the owners they have to turn them off. You bet...

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Re: noise on a 900 repeater

Postby tvsjr » Wed May 28, 2008 6:33 pm

All we need is some type of high-speed data link that would saturate the band (hey, a new ATV, now with hi-def!) but be legitimate ham traffic. A couple KW should do. Eventually, everyone would buy 2.4/5.8 stuff since their 900 gear magically quit working.

Then we move up to 2.4 and have some *real* fun. :lol:

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Re: noise on a 900 repeater

Postby MSS-Dave » Thu May 29, 2008 3:56 am

xmo...

Thanks for posting that info and the Analyzer shots. I too had somewhere between 20 and 30 dB desense on the MSF 5K. I built several other repeaters out of old MAS equipment, of course they worked worse. I haven't looked at the 909/921 split here but I suspect it's as bad...

Dave

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rrfd43
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Re: noise on a 900 repeater

Postby rrfd43 » Thu May 29, 2008 4:41 am

Just so I understand (beacuse I drank too much beer in college instead of going to electronics class and now protect lives and property instead) this noise is what gives me the clickity clack noise? It is just a clicking.....everything else seems to work very well. The range is a good as expected and the audio quality is good, but just the clicking.....I need a beer again.....nothing else seems to bother the signal...

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Re: noise on a 900 repeater

Postby N2OXV » Thu May 29, 2008 7:38 am

It sounds like SS crap. My only suggesstion is make sure you have a pass cavity on rx to remove any off freq ss noise (this may reduce your irratation not remove it), and if possible move your antenna into a spot that dosent hear this noise at all or as well.

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Re: noise on a 900 repeater

Postby MSS-Dave » Thu May 29, 2008 1:14 pm

I guess to answer your question, if you are happy with the performance and you can live with the noise, you're done. The "click clack" you hear is going to be some combination of on channel and wideband noise. You said you only notice it on weaker RX signals into the repeater so use the highest gain antennas you can on the mobiles and portables (no "stubbies" and quarter waves..). My noise got worse over time so this approach might not work for long. Oh, DON'T put a hi gain preamp on the front end of the repeater. If you have noise problems now, you will make it 15-20 dB worse. Best thing to do if you want to try other things is find 1 or 2 cavities, tune to your RX freq and run with that. You "may" want to put a preamp between the cavities and the RX front end but make sure the gain is padded to just equate the loss in the filters. The preselector in the MSF is pretty good and the cavities will give you another ~20 dB each reject outside of 1 MHz or so to help get rid of wideband noise. Again, you can't do ANYTHING to filter noise that falls ON your RX frequency.

Good Luck with it....

Dave


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