Mototrbo Repeater

[johnny1225]

I have just installed a UHF Mototrbo Repeater c/w Censcend C5 100 watt Power amp and sinclair cans instead of a duplexer. Although the hospital has great coverage there is a part of the hospital that gets very limited coverage which is called the tunnel which interconnects the 3 hospitals together. We put up 3 unity gain antenna's in this tunnel with approx 1100 feet of cable to make it back to the repeater. I was wondering what would be the best way to get coverage in this part of the hospital. It hits the repeater and you can talk but with alot of static. Is there something i could add? Should i change the antenna's to a 3db antenna? Could someone point me in the right direction.

Thanks

John

[Jim202]

First of all, you can correct me if I am wrong, but the Mototrbo was never designed for use by public safety agencies
and first responders.

With that said, your issue is in coverage. There are a couple of ways to solve the problem you have. It will boil
down to how much you want to spend to solve the problem. One way would be to run a leaky coax cable through
the tunnel. Connect this to the main repeater through a coupler that puts just a fraction of the power to the
tunnel and most of it to the main antenna. The intent is not to lower the normal coverage, only to give the tunnel
some coverage. This way the repeater receiver will have more of a direct ability to hear signals from the tunnel.

The second way would be to install a bi-directional amp system for the tunnel. This is a much more costly way
of resolving the problem. This would involve installing the leaky coax in the tunnel, having a box with the 2-way
amp system in it and a donor antenna to hit the repeater. The donor antenna needs to be far enough away from
the leaky coax, that it won't feedback into the system. In other words, you need some isolation to make it play.
The amp system normally has band pass filters to allow only the range of signals you want the system to pass.

This may seem like a bunch of work for what you get out of it. But you asked and here it is. There will be some
that will say to use a lower gain antenna on the repeater to get more signal below it. Remember that if you do
this, you will reduce the range out from the repeater that you have now. If it is only for one or two buildings,
then maybe you might want to try the lower gain antenna first. The results will vary depending on the building
construction.

Jim

I have just installed a UHF Mototrbo Repeater c/w Censcend C5 100 watt Power amp and sinclair cans instead of a duplexer. Although the hospital has great coverage there is a part of the hospital that gets very limited coverage which is called the tunnel which interconnects the 3 hospitals together. We put up 3 unity gain antenna's in this tunnel with approx 1100 feet of cable to make it back to the repeater. I was wondering what would be the best way to get coverage in this part of the hospital. It hits the repeater and you can talk but with alot of static. Is there something i could add? Should i change the antenna's to a 3db antenna? Could someone point me in the right direction.

Thanks

John

[WCHija]

Can you use a voting reciever? How about a second repeater and simulcast them.

[Jim202]

Simulcast requires a very stable frequency from the transmitter. Without a GPS controlled or an oven controlled
source, you will never be able to hold the transmitter to the required stability. Then you need to take into
account the audio delay and control it for the two transmitters. The repeater being used is not designed to
work under these conditions.

A voting receiver might be an option, but again you would have to find a way to interface a voter system into the
radio that wasn't designed to do this.

Jim

Can you use a voting reciever? How about a second repeater and simulcast them.

[FMROB]

Questions prior to answering your question. Are you using it in analog or digital mode. You say static, so I am assuming that you are analog?

- Rob

[johnny1225]

yes i am using the repeater in analoge mode. I have about 100 watts coming out of the Repeater and only about 40 watts coming through the filtering cans, does sinclairs filtering cans take that much power away from the system?

[Jim202]

This sounds like more loss in the cans (not a duplexer) than I would expect. However you haven't told us just what the
details are in the "cans" your using. Is this some cobbed together attempt at a duplexer? Why are you not using a
true duplxer designed for this application.


Most 100 watt repeaters require close to 100 db isolation between the TX and the RX. Have you measured what kind
of isolation you have?

Did you make your own cables between the "cans" that you used?

How did you determine the length of the coax cables used in this filter system?

What kind of coax cables are you using?

What type of connectors are you using on the coax cables?

Have you tried putting a dummy load on the antenna port and measuring how strong of a signal it takes to just
get into the receiver?

What type of coax cable are you using between the "cans" and the antenna?

Is the antenna a new one or used and has been removed from service elsewhere?

Have you looked at the RF output of the amp your using and see just how clean the sidebands look? Many solid
state amps are prone to raising up the noise floor and are a poor choice for repeater service.

How did you feed the 3 unity gain antennas in the tunnel and connect them back to the repeater? I hope your
not going to tell me that you just put a T on the coax at each antenna location.

Maybe you are getting the idea that there is more to trying to solve your problem that you are giving us information
to help you with. A good design goes a long way to a well operating radio system. A parts thrown together repeater
my end up fighting you for a long time before you get all the issues worked out of it. Some good test equipment is
in order here. Hope you have access to a good service monitor and a spectrum analyzer. You will need them to go
very far in making this repeater package play correctly. You might also want to spend a little time reading about
duplexers and how to tune them.

Right now, I am not sure where the issues lie. You need to provide some good details of exactly what you have and
what you have done to try and fix it all. Are you using tone squelch on the repeater and do you hear any strange
howls or screeching come out of the repeater on weak signals?

Jim

yes i am using the repeater in analoge mode. I have about 100 watts coming out of the Repeater and only about 40 watts coming through the filtering cans, does sinclairs filtering cans take that much power away from the system?

[johnny1225]

Jim, the filtering cans has always been there, we just unhooked the old CDM 700 repeaters and put new Mototrbo repeaters in. So the cans and all the cabling and connectors have always been there. The Hospital did get good coverage before, and they added this tunnel to connect the 3 hospitals together, that is why we bought 100 watt amps with the mototrbo because of the cable run to the tunnel. We did make a mistake though. The 3 unity gain antenna are T'd to one another. What would be a better way to attach the 3 antenna's? Thanks for your help. There is 2 repeaters (1) 452.3625/457.3625 and (2) 452.1750/457.1750 they are both TPL 123.0 and 12.5 spacing and i am using the repeater in Analoge mode.

John

[johnny1225]

At the tunnel we can open the Repeater up but just hear noise, no voice coming through the other radio.

[Jim202]

There are a number of ways to put antennas onto coax cable. But you just can't take a run of cable, cut it,
put a T on and slap an antenna on it. You need to sit back and think about what you just did to the impedance
of your antenna system. By any chance have you even tried to measure the reflected power on that hunk of
of coax.

If you do some homework on stacking antennas, you will find that one way of doing it is with phasing
harnesses. These are normally done for short spaced antennas to obtain gain or a directional pattern.
In your case, you need to have more than one antenna, but also use a large separation.

You might consider a 2 way power splitter on the antenna port of what your calling your cans. Use one
port to feed the normal antenna and the other to feed the tunnel. Place the tunnel antenna in about the
middle. Use something like a 1/4 wave whip hung from the ceiling. You haven't said if the tunnel is straight
or curvy. Even if the tunnel is curved, 500 feet is not a great distance to be from the antenna. Plus you
don't need the signal radiating out of the tunnel and giving you problems with coverage in other areas.

Bear in mind that in using the 2 way power splitter, 50 percent of your power will go to each of the antennas.
You will only get 50 percent of the received signal from each of the antennas. In other words, you cut your
power in half in both directions with the power splitter. They do work, but there is that loss to consider.

Jim

Jim, the filtering cans has always been there, we just unhooked the old CDM 700 repeaters and put new Mototrbo repeaters in. So the cans and all the cabling and connectors have always been there. The Hospital did get good coverage before, and they added this tunnel to connect the 3 hospitals together, that is why we bought 100 watt amps with the mototrbo because of the cable run to the tunnel. We did make a mistake though. The 3 unity gain antenna are T'd to one another. What would be a better way to attach the 3 antenna's? Thanks for your help. There is 2 repeaters (1) 452.3625/457.3625 and (2) 452.1750/457.1750 they are both TPL 123.0 and 12.5 spacing and i am using the repeater in Analoge mode.

John

[chartofmaryland]

Johnny,

There are many factors that are not being taken into consideration. Jim is hitting most of these and they are not ones that can be forgotten.

Your setup is lacking these things,

Isolation - Get those cans tuned properly or find where the loss is 100 in 40 out is not right.

Proper runs of transmission line - This jazz of 1100 feet of anything is CRAZY !!! On 450.XXX unless you found a way to make the customer buy LDF7 you are loosing alot of signal. Hopefully its not RG-8 or LMR-400.

If the original setup was a CDR then you are not seeing any change with your 3db of power increase using the cresend amplifier, as your cans output is only 1.5 db over the original setup, waste of time and money.

There is an easier option to try and is quiet inexpensive. Move the repeater until the coverage exists where you need it and its to a level the customer can use. This repeater is probably on the top floor next to the dirtiest equipment in a hospital and depending on the thoughts of some installers, it was put on top of a filing cabinet to try and get the signal alittle bit higher be cause the higher you go the better it will work. . . .

I would see if you can get a notch duplexer tuned up, set the Trbo up for ops off of the cresend and move the repeater and test coverage before doing anything else.

After that plan your attack.

CoM