Batboard

I was at a site the other day servicing one of our units. This is a crowded site with a dozen other systems in the one room that we're in. I saw a 2 channel 900 MHZ MOTOTRBO system there. It caught my eye is the TX freqs. were only 12.5 kHz apart.

TX #1 was 937.7000 and TX 2 was 937.7125. I confirmed both transmitters were operating in digital.

I wonder how well this would work. I would have thought that the TX's would have needed to be at least 25 kHz apart. I'm not worried about the TX combining. That can be taken care of by a hybrid combiner. I'm thinking of a subscriber unit in the field and the radio discriminator and selectivity between channels that have no separation at all.

Anyone have experience with adjacent transmitters like this?

Even a hybrid would have trouble with that. Insertion loss will be horrid. Like you, I think the rcvrs will have a tough time with it too. I think selectivity is around 65db for Trbo. Both channels power will always be about equal relative to each other. Could turn into a mess in the field if they get too close to the site. Uhhhgly.

Spacing that close to each other will place two or more subscribers at a disadvantage if they are near each other. The close spacing of these two transmitters will wipe each other out, the desense has to be pretty severe.
I suspect they are unaware of the potential trouble this will cause for subscribers being in the same vicinity, but on differing channels.
One will wipe out the other, both might never know of the real reason either.
I wonder what the channel spacing on the site's receivers are configured like?
12.5 on both RXs and two moderate powered mobiles on at the same time, and using the same site will have nasty results.
I see cross talk, IMD for starters....Not to mention severe desense of co-channel users. Forget long distance coverage if the system is heavily loaded.

Really guys? Read the OP again. It's not as much of a problem at 900MHz as it is at VHF/UHF. We have a 10 channel system in the area with all 10 channels 12.5kHz apart. Keep the 39MHz offset between transmit and receive frequencies in mind. With that kind of TX/RX spacing, two subscribers next to each other will not have any problems, unless they are immediately next to each other, and on the EXTREME fringe of coverage.

Remember that 900MHz has always been narrowband - 2.5kHz deviation. Unless the radio is AT the site and extremely overloaded, there's a marked difference between 937.7000 and 937.7125. It wouldn't cause any problems at all. That's what selectivity is about, and part of the reason that commercial radios cost more and have better specifications than consumer radios.

One thing nice about living in a large metro area that HAS 900 systems that are active, but here in the bush country, good luck.
We are not graced with such issues, and are lucky to have to deal with highband or UHF troubles from GMRS and commercial ops running 452/463 and so on.
If any 900 issues do occur, they will be of the repeated variety and rarely trunked.
I know of two in my area I worked on, and it was a repeater and a base run in talkaround only. Portables were on repeater-that's the way they wanted it...so they got it.
I miss the large systems, the mass of cable routings, the tuning of individual isolators and cavities...made my day pass by so fast...I was busy!
Now, it's retune a BP cavity, renotch and realign a duplexer and MAYBE run a sweep for cable loss and you are out the door....Nobody wants to do preventive maintenance, but complain loudly when they have a failure they could have prevented.

d119 wrote:Really guys? Read the OP again. It's not as much of a problem at 900MHz as it is at VHF/UHF. We have a 10 channel system in the area with all 10 channels 12.5kHz apart. Keep the 39MHz offset between transmit and receive frequencies in mind. With that kind of TX/RX spacing, two subscribers next to each other will not have any problems, unless they are immediately next to each other, and on the EXTREME fringe of coverage.

Remember that 900MHz has always been narrowband - 2.5kHz deviation. Unless the radio is AT the site and extremely overloaded, there's a marked difference between 937.7000 and 937.7125. It wouldn't cause any problems at all. That's what selectivity is about, and part of the reason that commercial radios cost more and have better specifications than consumer radios.

Good insight. Thanks.

Guys... read it again... TX #1, TX #2... transmitters don't receive. As far as the radios themselves, they are probably plumbed to two different antennas.

You're going to be looking at the differences between 12.5/15/20/25 KHz channel separation and the width of the IF filters. And even then, you can turn the deviation down a hair on both for better (apparent) RX isolation.

http://www.repeater-builder.com/tech-in ... -20khz.pdf

Historians will enjoy the above comparison of the UHF Syntor X against other radios...