In Building Portable Coverage

[sc800]

I'm wondering if anyone can help me with this question:

My town operates on a VHF Conventional radio system for both Police and EMS.

The police are operating on a repeated frequency pair of INPUT: 151.355 OUTPUT: 158.280
EMS is operating on repeated frequency pair INPUT: 156.2325 OUTPUT:151.3925, using Motorola HT750 portables.

The coverage in buildings by EMS radios is spotty to say the least, yet the police have much better in building coverage for transmitting. Anyone have any ideas why?

Is it the fact the police input frequency ends in a 50 as opposed to the EMS input frequency ending in a 25?

EDIT: Looking at the database, the police appears to be licensed for two repeaters, EMS only one, but even still, outside coverage is still fine, the only real difference in performance is in buildings. Also, the EMS repeater is on the same tower at same height as one of the Police repeaters

 

[GTR8000]

Well for starters, the PD is licensed for 50w (70w ERP) vs EMS at 25w (25w ERP). Also, the PD's antenna is mounted 20 feet higher on the tower than the EMS antenna.

The PD antenna could be a higher gain model than the EMS antenna. The PD's repeater could also be a different brand/model than the EMS repeater, and/or maintained better. The antennas could be mounted on different sides of the tower, which would give them different characteristics even while standing in the same spot in the middle of the town, depending on any nulls that are introduced.

You also have the fact that the PD input frequency is 5 MHz lower than the EMS input. Lower frequencies tend to have better propagation, although 5 MHz is not terribly significant and shouldn't really come into play in this case.

There are a lot of factors that might come into play, but the difference in output power, ERP and antenna height are the most significant.

No, it has nothing to do with what numbers each frequency ends with. We're assuming that both frequencies were narrowbanded, so any loss resulting from that process would affect them both fairly evenly.

PS - There is only one PD repeater, the other location is the backup simplex transmitter at the police station.

 

[N4DES]

If you say that they (PD) have better talk-back (radio to dispatcher) there is a very good chance that they are utilizing satellite receivers to enhance that. They are not licensed devices and you will not see them on the FCC website.

If there are talk-out problems there could be issues such as tower and/or building shielding that is causing issues or something as simple as poor maintenance. When was the last time that someone put a VSWR meter on the transmission line or looked at the antenna up close?

 

[rfradioconsult]

Are you sure of the frequencies involved, especially if both repeaters are in close proximity to each other. The input frequency of the PD repeater is really close to the output of the EMS repeater, close enough to wipe out the PD receive whenever the EMS repeater keys up.

 

[sc800]

Yes, those frequencies are really close to each other. So close that if a police officer is transmitting close to an EMS member, you will hear the transmission over the EMS radios.

 

[beachmark]

The difference in the end digits in the frequencies has nothing to do with it as noted.

The voting receiver note is a good one and would explain it well.

The only way that the 20' antnena height difference would be significant is if both antennas are really low, like 20' and 40' or 30' and 50'. 100' and 120' heights should not show any regularly noticable difference.

SC8 your orignal symptom description is not totally clear to me; you use the term 'for transmitting'. Do you mean that the talkback (portable-to-dispatch) for EMS is weaker than for the PD, and that talk out (dispatch-to-portable) is equal for both? Does this in-building coverage probem occur when the EMS radio is being used solo (i.e., not police radios around or active)?

For talkback only problems, the maintenance of the EMS reciever and duplexer at the repeater is a good question; poor uplink sensitivity of the EMS receiver would cause exactly this problem. The casue can be desense for a variety of reasons. The outside coverage might be OK but in-building coverage is more marginal to begin with and this problem would show up more readily than outside. However, fringe area coverage should also suffer IF the EMS radios are used over a wide area. Is the EMS radio coverage radius limited to the town/city only, or is it used county wide?

The small talkout vs talkback frequency differences indoors is not a factor. My company does in-builidng systems for our main business, and at VHF, you will get peaks and nulls in response all over any building due to the significant inbuilding reflections and all frequencies will be effected at different points, VHF, UHF, 700, or 800.

As to this comment "Yes, those frequencies are really close to each other. So close that if a police officer is transmitting close to an EMS member, you will hear the transmission over the EMS radios. ". This sounds like a different problem: direct portable-to-portable bleedover.

 

[SCPD]

One thing I noticed is this is common in several places pd repeater being higher, more power yet a ems one not as good. Most cases owned by the same entity and maintained by the same. I heard ems switch to county fire repeaters just to get signals in various areas.

 

[jim202]

Coverage with any radio is dependent on a number of issues. To start with, just how old is the installation of each of the repeaters?

If you have 2 repeaters that are of different ages, just the weathering of the antenna system now comes into play. You could have poor RF connections at the antenna. The antenna could be weathered to the point that the internal connections have started to break and the gain of the antenna will drop when this happens.

You could have old coax cable where water has started to migrate into the cable. You could have water getting into the outdoor connections and causing problems.

Your also trying to compare 2 different radio systems that don't even have the same RF output power. Just the difference between 25 watts and 50 watts is 3 db. That is a doubling of the signal on the receiving end. To put it in a different light, if the lower powered radio is noisy, the higher powered radio would probably be 100 % copyable. With the limited information you have provided, there is no way any comparison can be made between the two radio systems.

If nothing else, the age of the antenna system needs to be known. The type and gain of both antennas need to be know. What type of coax is being used on both repeaters? What the performance of the duplexer used on each of the repeaters is. (how much loss they both have in the TX and RX path) What the receiver sensitivity is on each of the repeaters. How much desense there is on each repeater. Is the police system using any external voter receivers?

As someone else has already mentioned, the two pair of frequencies is going to cause you problems. If nothing else, I would try to flip one of them around so that both repeaters were using the same segment to receive on and the same segment to transmit on. This way your not going to cause each other to jam each other.

This whole package is like a disaster waiting to happen. You really need to sit down and come up with a game plan on what to do next. It is going to take some major leg work just to collect all the facts. It will take the efforts of the radio shop to do a full test on both repeaters. It may come down to trying to change the license of one of the repeaters to flip the pair around. This will at least allow both of them to survive on the same tower and play well out in the field.

Let us know what you find out and what the plans for down the road will be.

 

[beachmark]

As someone else has already mentioned, the two pair of frequencies is going to cause you problems. If nothing else, I would try to flip one of them around so that both repeaters were using the same segment to receive on and the same segment to transmit on. This way your not going to cause each other to jam each other.

Good point; the PD portables transmitting on a channel very close to the EMS portable's RX channel is probably a major factor in the bleedover of PD transmissions being heard in the EMS portables. But the frequency pairing does not have nay obvious connection to poor overall EMS IB reception, unless the EMS system desense only occurs when the PD repeater is active.....Sounds like an appropriate project for a good RF systems troubleshooter, armed with plenty of test gear and experience....

 

[mformby]

Maybe the repeaters.

I would have them check the repeaters and make sure the receiver sensitivity on the PD repeater is still OK. The location of the antennas could make a difference and the coaxial cable could be different in quality. A two way tech needs to hook up a service monitor and sweep the coax all the way to the antenna and make sure there is no water in the line, or possibly a bullet hole (that happens) which would deteriorate the signal. The frequencies are so close that I can't see that being a factor. The age of the duplexer could be a factor if it has oxidized over the years internally. Sounds like it just needs a good check up if it hasn't had one lately.

I'm wondering if anyone can help me with this question:

My town operates on a VHF Conventional radio system for both Police and EMS.

The police are operating on a repeated frequency pair of INPUT: 151.355 OUTPUT: 158.280
EMS is operating on repeated frequency pair INPUT: 156.2325 OUTPUT:151.3925, using Motorola HT750 portables.

The coverage in buildings by EMS radios is spotty to say the least, yet the police have much better in building coverage for transmitting. Anyone have any ideas why?

Is it the fact the police input frequency ends in a 50 as opposed to the EMS input frequency ending in a 25?

EDIT: Looking at the database, the police appears to be licensed for two repeaters, EMS only one, but even still, outside coverage is still fine, the only real difference in performance is in buildings. Also, the EMS repeater is on the same tower at same height as one of the Police repeaters

 

[mformby]

Radio interference

A notch filter can be installed at the repeater that can be tuned to block "notch" out the PD frequency from getting in the EMS receiver. That's easy to do and I am surprised it hasn't been done already.

Yes, those frequencies are really close to each other. So close that if a police officer is transmitting close to an EMS member, you will hear the transmission over the EMS radios.

 

[mkewman]

A notch filter can be installed at the repeater that can be tuned to block "notch" out the PD frequency from getting in the EMS receiver. That's easy to do and I am surprised it hasn't been done already.

Perhaps a certain Motorola chain of shops installed the repeater? One whose name starts with an animal, and ends with a com. :lol:


Seriously though, perhaps your municipality should start requiring Bi-Directional amplifiers in the building code for large building/dwellings. Many areas are doing that to prevent issues in high-rises when emergencies arise.