Batboard

I recently assumed responsibly for overseeing the care and maintenance of a 2-meter Micor repeater station, which is physically located in the same room as a combination analog/digital cell site owner by a wireline carrier. The repeater antenna is mounted on a rooftop, in the same horizontal plane as the cell site’s antennas.

Upon inspection of the Micor cabinet, I noticed that the original Micor receiver module had been replaced at some point with a GE Mastr II receiver module. A technician who’d previously worked on the station advised me that the original Micor receiver would not function properly after the analog cell site became operational. As soon as the GE Mastr II unit was installed, reception problems cleared up completely.

The time has now come to replace the Micor, but given the past receiver related problems, I would be concerned about selecting another Motorola product (e.g. MSF5000, MSR2000) as the replacement station. How the heck am I going to figure out if there’s going to be a problem before I spend any money on a new piece of Motorola equipment?

The cell site shouldn't be nuking your equipment that badly. However, before you throw money at things, you need to get ahold of a spectrum analyzer (and preferably someone well-versed in ones use) and poke around the site. See what's being tossed out... it may be that the cell site is generating spurs in places it shouldn't, in which case you can likely get the company to fix it (one way or another...)

If you don't have any other choice, there are a few things you need to do:

Look into placing a crystal filter on the receive side of the duplexer. Crystal filters are extremely narrow bandpass filters - a typical 4-pole will provide 70dB+ attenuation at 50KHz from the center frequency. Unfortunately, you incur about 6dB of insertion loss.
I use filters from MtronPTI - http://www.mtronpti.com/pdf/contentmgmt ... terweb.pdf
They'll run somewhere in the $500 range.

Case in point: I was handed a fire department repeater to maintain. The brilliant company who worked on it before I did installed two repeaters at this site. The antennas are DB224s, mounted on 18" standoffs on each side of a mast (top of water tower). The other repeater generates about 105 watts to the antenna, 315KHz from the receive frequency of my fire repeater. The proximity coupling is bad enough that you can see power coming back down the fire antenna with a wattmeter. Of course, when the other repeater went to transmit, it would totally wipe out the fire repeater. The installation of a 4-pole crystal filter (along with adjusting the output power of the other repeater a bit...) cured all of our problems.

The crystal filter will likely fix your receive issues, however you still have the potential for power to couple back down the antenna/coax and into your transmitter, which can cause nasty mixes in the PA and lead to you generating interference in random places. An isolator will address this issue. I'm using "iso-plexers" from EMR Corp. - essentially, a 6-can pass/notch with a dual-stage isolator built together. The isolator prevents power from being injected into the transmitter, thus giving the possibility for some nasty mixes. A 150-watt 6-can version will run about $2K from EMR.

If you've got the budget for an MTR2000 or a Quantar, good for you. However, you should give a serious look to the Kenwood repeaters, specifically the TKR-750. The newer units have integrated ID, DTMF control, and all sorts of other cool widgets that make hams happy. They're also cheap ($1K or so), extremely tight (great RF), and rock-solid radios. They are only 25 watts out, continuous duty, so you may want to look into a PA (check out Crescend), especially factoring in the 2.3dB of insertion loss.

Just a few thoughts...

Note that he said there are NO problems with the MASTR II receiver so there is no need to try to blame the cell company or put a lot of filters in.

I had a similar situation of a site with 4 simplex VHF Micor stations all in the 153MHz band. If any one was keyed it would desense the others. Motorola tried xtal filters, cavities etc ahead of the receivers with the result that the insertion loss of all than junk required about 2 microvolts of signal at the input to the filters to open the squelch and the interference was still present. The Micors and all the filters were replaced with MASTR II stations and the problem was totally solved. Receiver sensitity was better than 0.5 microvolts and no desense with the other stations keyed.

Unfortunately, I do not know for sure of any current receiver that has the interference immunity of a MASTR II. But, at the site I described, the MASTR IIs were eventually replaced with MASTR IIIs and everything was still fine. And, I have seen some other situations where a Micor was better than a MASTR II where intermod instead of a desense was the problem.

The best I can offer is to see if you can try some other receivers at the site and see how they work. And, yes, definitely look at Kenwood.

If you would like to stay with the Master II receiver since it works well adding a MasterII transmitter is a way to upgrade. There is some additional information on repeaters using Master II componets in the repeater builder.com site.

Keven is well versed on the Master II equipment.


Shielding is very important at a site that has a lot of RF as you describe. The factory shielding kits for both the Micor and the Master II are a must.

k2hz wrote:Note that he said there are NO problems with the MASTR II receiver so there is no need to try to blame the cell company or put a lot of filters in.

Bull. It may mean he's getting lucky, or that the Mastr2 receiver isn't tweaked properly and thus isn't very sensitive to start, but if a regular Micor receiver is getting nailed, there's a site problem. It's time to break out the test equipment and start tracking it down...

My two bits worth:
The MSF5000 has absolutely nothing in common with a Micor.

The MSF is designed to operate in an RF environment so intense that it would be unhealthy for you to be there.
That was the answer I got when I asked about the plastic control tray when it was introduced in '82.
As Customer Service Engineer, the Motorola sights belonged to me. Interference was always an issue.
I find that the MSF has excellent RF immunity.

I've never fooled with enough to have seen it myself, but I've heard from other people about the preselecter in Micors giving trouble. They are helical resonators, and normally give no trouble. However, they sometimes get a little corrosion inside them, and it detunes them quite badly. The corrosion, which is actually 'crystals' of a metal oxide, can also act as a 'mixer', potentialy causing the type of problem you describe. It may be that you could put a different Micor receiver in place of the Mastr II receiver and not have any problems with it.

MTR 2000 easy to program and tune, they work great.
I use them as marine repeaters co lacated with 2 cell systems, 3 FM broadcast stations and a 12 channel Smartzone site.

Unless of course you can baford a quantar