Receiving Issue

[njemt7212]

Hello All,

So at my local EMS agency we have two Motorola VHF SM50's hooked up to various speakers around the building. One for Fire Channel and One for Police Channel. They are receiving only, no transmitting occurs. The recent issue we are having is since the police communications center which also houses fire dispatch has been renovated and switched to narrow-band operations, we are receiving what sounds to be bleed over from police onto fire. Basically the police talk and suddenly we hear clips of it over the fire sm50.

I initially thought of it as an issue with the communications center as infrastructure was replaced and antenna's as well , however I have a portable set to Fire when this occurs and no transmissions come through, not even on csq for the frequency. I have listened to the recordings as well and the clipping appears there.

Any Ideas so I can advise our radio repair appropriately ?

[tuckerm]

Have the radios in question been narrowbanded as well?

[Bill_G]

If all they did was narrowband, this shouldn't be happening. From a receivers perspective, the deviation went down, and the selectivity was increased. Nothing else changed. If anything, the FD rcvr should reject PD better than it did before. So, something else changed. Did the PD or the FD move to a new freq? Did dispatch move to a new location? Did they put up new base stations and antennas, etc etc? Who did the work, and have they been notified this is occurring?

OR - did someone just narrowband the FD rcvrs and never bother to check their alignment? (which I've seen far too much of) The quick and dirty way to see if it's an alignment problem is to set the FD rcvr back to wideband. If it suddenly quits hearing the PD, then someone needs to service and narrowband the FD rcvr, and probably the PD rcvr too, the right way.

[Will]

Maybe the receiver is being overloaded due to close spacing of the antennas.

Move the receiving antennas away from the PD transmit antenna. Use a attenuator in the receiver line.

[RFguy]

I doubt the SM50 was narrow-banded. it's not a programmable option. It requires a dozen or so component changes.

[SteveC0625]

I doubt the SM50 was narrow-banded. it's not a programmable option. It requires a dozen or so component changes.

100% agree. I have one here and NB is not a programmable option.

Best to replace the SM50's with something NB.

[Bill_G]

Then something else changed. It should have been transparent after the dispatch radios were narrowbanded. Somebody did something someplace.

[tuckerm]

Find some cheap analog (narrowband) scanners to replace your SM50s.

[njemt7212]

We are located in a separate building that does not house any type of PD / FD Transmitter. In fact we are located about a quarter mile away. I know the radio vendor did replace antenna's and base stations at the PD/FD building during narrow banding process as equipment was 30+ years old.

These SM50's are not connected to the repeater system in any way. They are simply installed with speakers placed around the building just for members to be able to listen. The radio's also haven't been touched since being installed.

[Bill_G]

So, they narrowbanded dispatch without touching your rcvrs - which means they didn't change freqs - and they replaced the base stations and antennas at dispatch (probably because they were not NB capable). Now you hear bits and pieces of PD traffic on your FD rcvr in the station. Either both channels are CSQ, or they share a PL. Some of the older systems date back to when multiple PL was an expensive option.

A solution might be to change the FD PL (if it uses one), but that would involve touching every radio. A simpler solution might be tightening the squelch on your FD rcvr. A third possible solution might be to have your radio serviced. If it hasn't been touched in a while, it may be out of alignment enough to hear adjacent channels. A fourth possibility is the new PD base station is running barefoot with no filtering on it's output. They biggie sized the transmitter (regardless of the license), threw a high gain antenna up, and you get clobbered with +500W 1200 feet away with direct power, or an intermod product that can only heard at elevation. Wouldn't be the first time I've seen that. That would fall on the vendor to correct.

I would tighten the squelch first. And maybe throw a 3db pad in the line. If that doesn't help, get the vendor involved.

[njemt7212]

Correct, they replaced equipment and did not touch our rcvr's. No need to. Also correct as both the FD Channel and PD channel utilize the same PL. Unfortunately cannot change the FD PL as it is not our department , EMS is separate entity. I will check on tightening the squelch, probably just replace them if I can. I have atleast one m1225 laying around not being used.

[Bill_G]

The suggestion to get scanners is looking pretty good right now.

[njemt7212]

Haha, Yes it is

[RFguy]

How far away, and how strong are the signals that you want to receive on the SM50's. If the signal is strong, you may be able to attenuate the antenna system to reduce the PD signal below a level that it's effecting your RX while maintaining and adequate level of the desired FD signal.

This might be as easy as changing to a mag mount inside the building. All depends on how strong your RX levels are.

[njemt7212]

We are about a quarter mile away. Our antenna tower is higher elevation than the PD and FD transmitters. I guesstimate that PD is transmitting out at 100w. I am going to try an m1225 when its removed from a vehicle within the next two weeks and replace the FD designated SM50 and see if that makes a difference.

Its not a major concern right now as the clipping occurs intermittent and at a tolerable level right now with membership but just an annoyance.

[jackhackett]

So nothing at your location has been changed, but the transmitters a quarter mile away have been narrowbanded and the antennas replaced. Your radios are still wideband.

Simply narrowbanding the transmitters shouldn't cause the problem. All you would notice is lower receive audio.
Changing the antennas could have several effects. They could have put in higher gain antennas, or they could have replaced bad antennas with good ones, either of which would raise the signal level on your end. Also, they may have done something that created some intermod (mixing of signals), which could cause interference.

Can you verify if the two frequencies use the same PL? Also how close are the frequencies?

[njemt7212]

One freq is 154.785 (PD) and the other is 153.98 (FD) both on 179.9

[njemt7212]

Another question, if I were to replace the sm50 with a CM300, is the pinout on the 16 pin connector the same ?

[jackhackett]

Yes, basically the same pinout.

[SteveC0625]

Another question, if I were to replace the sm50 with a CM300, is the pinout on the 16 pin connector the same ?

The pin outs for the SM50 are to be found on batlabs.com --> Model Specific Info --> SM50.

The pin outs for the CM200/300 radios are in the manual at: http://www.callmc.com/customer_service/ ... 0guide.pdf on page 6-5.