In-band VHF mobile repeater(/extender)

[karlanke]

Hey Everyone,
I've found a lot of good info on this topic, but I have a couple questions. I work for an ambulance company that's looking into in-band mobile repeaters in our rigs. We're a small agency (3 ambulances), so pretty much everyone already either owns a VHF portable or has one assigned by the company. Therefore, we're not really interested in going with cross-band.

What I'm thinking of doing is putting a hood mount antenna opposite the stereo antenna - I know there's no ground plane there, but I'm hoping with receive-only while on scene it'll work. I'm thinking I should avoid the roof where the current NMO VHF antenna is because our 110 watt XTLs will drown out the receiving radio - is that true / is the vertical separation gained from going with a hood mount enough of a benefit to out-weigh the lack of ground plane?

On the radio side, I'm thinking with our limited budget it's best to get a cheap mobile (CM or CDM series) that has a COR output, and then just do a straight cable connection between them (I've seen schematics kicking around with just a resistor, I believe). It'll be a simple, unidirectional link with no ability to channel select, but I think that'll be fine for our purposes. Thoughts? Am I way off base?

Thanks!

 

[kayn1n32008]

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With a 100w Vhf radio it is going to be rather difficult to run a mobile repeater on Vhf

 

[popnokick]

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Difficult AND costly. Trying to keep the 100W xmtr from desensing the front end of the mobile extender receiver will be very difficult... Which is why crossband extenders are common. Much simpler to prevent mutual interference.
If your vehicles are about the length of a semi-trailer (60+ feet) you MIGHT be able to achieve some isolation by putting one antenna at each end with 60 foot separation.

 

[WA0CBW]

And don't forget that the mobile repeater transmitter would need to be licensed as an MO3. As well as the VHF portable transmit frequency to the mobile repeater.

BB

 

[nd5y]

...On the radio side, I'm thinking with our limited budget it's best to get a cheap mobile (CM or CDM series) that has a COR output, and then just do a straight cable connection between them (I've seen schematics kicking around with just a resistor, I believe)...

A setup like that is a very bad idea for public safety use. You have no control over the repeater. An interfering signal can lock up everything and if there is more than one repeater active it won't work. That's why real vehicular repeaters are expensive. They are either full duplex with two frequencies or they use a simplex frequency with carrier detect windows. They can also control eachother so that only one is active if another unit arrives at a scene.

 

[box23]

I believe some of you mis-read the original post.

What karlanke wants to do is put a receiver in the ambulances that is in turn hooked to the mobile radio. Sounds like they can hear but just can't talk out on the portables.

This can be done. The hard part, though, is creating enough isolation between the receiver and the mobile transmitter. However, this is done in a lot of places everyday right alongside other in-band transmitters/receivers.

In your case you would need to license an additional mobile frequency for the portables, unless you have one available already that isn't being used. An MO3 frequency on the license *should* not be needed because there is no transmitter. (The FCC does not license receivers)

The easiest way I can think of would be to use 3 different tones on the link: Tone 1 for Ambulance 1, 2 for 2, etc. Then add 3 channels to your portables each with the dispatch channel receive frequency, the link frequency for transmit, and the proper transmit tone for each ambulance.

Since you are only receiving, you can get away with the worst possible antenna that you can find that will still reliably hear your portables (think a paper clip hidden from the mobile antenna by as much metal as possible). This will help by adding some isolation.

You will end up putting in filters, anyway, but the more isolation you can add beforehand the less you will need in the filters (less money). Using a band pass/band reject cavity system (basically the receive side of a duplexer) you can pass the link frequency and reject the mobile transmit frequency, hopefully then gaining enough isolation.

This assumes your radio system uses a single mobile/portable receive frequency and a single mobile/portable transmit frequency. Even with simplex this can work. You would want the link frequency as far away from the mobile transmit frequency as possible in all cases. If you need to transmit on different frequencies from the mobile radio then it can get tricky, but still possible.

As I described each ambulance would have a different channel (with different tone) in the portables, and even if at the same location will not cause more than one mobile to transmit. There are no extender transmitters so no possibility of interference as nd5y was referring to. Also the way it is set up there is control of any transmitters at all times, the same as before.

You said you have a limited budget. I think doing it this way is about the cheapest it can be while still having a reliable system. The problem is that cavity filters are not inexpensive. Good quality calculations by someone RF aware that knows more about your system would be invaluble in your situation. Remember that you don't need to go overboard on isolation, just enough to eliminate or severely decrease any desense, and a little creativity can go a long way.

Hope that helps.

 

[karlanke]

Yeah, I actually thought of box23's idea of different PLs for each rig to cover the multiple transmitter issues. Additionally, we will need a new frequency for this project, so we're looking into that. We're thinking we may try to go for a whole new pair (ideally, one around 150.0 and one around 159.0 to maximize separation). Then we'd keep our existing channel as a talkaround. I'll look into filters as well. Thanks everyone!

 

[rfradioconsult]

Don't reinvent the wheel, there are commercial products that wioo do what you want to do, they will interface with your mobiles and include filters for in-band use.

 

[902]

In-band MO3 operation has been a thorny issue for a while.

One particular manufacturer has been a proponent of opening 170 MHz frequencies that are usually used for forest fire operation and 173 MHz telemetry frequencies for use with their devices. This particular device has filtering that requires wide separation (>10 MHz) from the main radio. That's not normally do-able with regular 90.20 frequencies.

Several waivers have been applied-for, but the FCC has largely tried to stay away from the issue. In some instances, vendors have (inappropriately) used the telemetry frequencies with excessive emission, and in some instances, agencies have been able to license one of the four former Motion Picture and Relay Press radio services (173.225,275,325,and 375) through interservice sharing. You might be able to make a 5 MHz separation work IF you use helical resonators as a reject filter and stay with only one frequency on the XTL and one frequency on your MO3, but that's been impractical for most users.

 

[phillmobile]

why 100watts? in the uk all radios are allowed to use 25 watts only thats repeaters and mobiles and we can cover 100 miles so how far are you trying to go

 

[karlanke]

We have pretty tall mountains, and lots of vegetation to push through. Also, what frequencies are you using? Lower frequencies will penetrate better.

 

[kayn1n32008]

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why 100watts? in the uk all radios are allowed to use 25 watts only thats repeaters and mobiles and we can cover 100 miles so how far are you trying to go

Well put. 100w is only 3db more than a 50w mobile, you are not going to a whole lot more talk distance than a 50w mobile

 

[phillmobile]

this is a video i posted to youtube showing my vehicle repeater (vhf main/uhf repeat)

in car crossband repeater vhf uhf - YouTube

hope you guys understand my yorkshire accent haha

also just a thought instead of running 100watts why not use high gain antennas and mount them in the middle of the roof, i see some installs on here where they are using a 100watt radio and have it running in to one of those small coil antennas on the front wing.

 

[zerg901]

WQCG859 - Town of Jay Police in Franklin County Maine - 6 MO3 on 173.2625 plus 42 portables on 173.3125

pretty sure it is for inband linkage - 154.725 might be their FB2 output channel

If you plug those 173 Mhz freqs in the FCC ULS you might find someone near you with an inband MO3 setup

 

[wd9cms]

Another possibility is a audio store forward repeater. You transmit to the mobile radio on one frequency, the store forward repeater receives the handheld transmission and stores the audio. When the handheld unkeys the repeater keys the high power mobile and transmits the stored audio. I remember that Zetron used to make a box that does this, it was intended to be used to extend fire two tone alert paging ( I think it was the model 19). This would eliminate the requirement for special filters and antenna spacing. You could also hear your message go out on the high power radio. Hope this helps.....

Bill