Batboard

Here is my dilema. In order to get townwide talk-out coverage, I need two transmitters... one north and one south, due to the oblong shape of the town and the terrain. There is a high point just north of the middle where I could put a single transmitter but that one still won't cover good enough. The PD has one there but they are running a lot more wattage than we will ever be licensed for.

So, the most logical thing is two transmitters. I have two choices...

1. Simulcast system costing big $$. Syncronized north and south transmitters, all the fancy stuff. REALLY expensive and I am still not sold on the reliability of the overlap zones. The overlap would be substantial and that concerns me.

2. Poor mans simulcast costing not a lot of $$. Two seperate transmitters with a common input. Obviosuly there would be a voter controlling the receivers and the audio would be piped out on a north output frequency and a south output frequency. So, all radio traffic will go out on both ends of town. The only difference is you would need to select north or south on your mobile/portable to hear it best.

I can envision it working, I am just solicating opinions of those who might have tried this and how it played out with the end users.

While I haven't tried this, nor do I have any opinions on its success, I would like to point out that you'll be using up two frequencies for the same traffic, which the FCC may not want to let you have. Receiving at multiple sites with a voter is certainly the way to go.

Of course you'll definitely have a user problem when they don't remember to switch freqs as they cruise from north to south.

Bob M.

kcbooboo wrote:...you'll be using up two frequencies for the same traffic, which the FCC may not want to let you have.

Well, would they like to give me a few hundred thousand dollars so I can do it with one frequency?

Common inputs but different outputs, with all traffic simulcast over both outputs? What happens when (as will usually be the case), BOTH inputs hear the same thing at the same time? I would think it more practice to have a single input, a main transmitter, and infill transmitters situated (and antenna'd) to cover the north and south fringes will little area of overlap.

Multicast is easy enough, I do it at work on a regional basis with Astro, and am part of a system on the 440 ham band with conventional FM and 12 TX freqs across 2 states. As long as you have the output of your voter routed to all transmitters, it will work fine. If you have good TX coverage, you can set your mobile squelch high enough that it takes a near full quieting signal to open and let the mobiles function as a "poor man's voter" for mobile RX. The FCC had no problem with our statewide multicast system, I don't think you will have a problem with getting 2 TX freqs.

RKG wrote:Common inputs but different outputs, with all traffic simulcast over both outputs? What happens when (as will usually be the case), BOTH inputs hear the same thing at the same time? I would think it more practice to have a single input, a main transmitter, and infill transmitters situated (and antenna'd) to cover the north and south fringes will little area of overlap.

I think your missing how I was going about this.

The receivers are all on the same input frequency and tied to a voter. The voter output would go to BOTH transmitters which are on different frequencies.

BTW, On the Astro system, we hide the individual channel names on RX from our end users, and the mobiles are set to automatically scan. From the user standpoint, it appears as a single channel. You can do the same.

On 440, we have 3 satellite repeaters linked to a hub. The hub has multiple voted RX sites. You can use the hub input frequency and let the mobile scan all 4 TX freqs, just as you describe. It works really well. The system to our south does about the same thing on a much larger scale, I think they have about 8 voted RX sites, and an equal number of discrete TX freqs.

multi-site TX isn't that difficult; you don't need more than a crystal oven or a high-stab crystal to make it work.. just an audio delay for the difference between the two sites and both on the same frequency...

I've used that configuration...works fine as long as the end user understands that they have to change channels.

nmfire10 wrote:... the audio would be piped out on a north output frequency and a south output frequency. So, all radio traffic will go out on both ends of town. The only difference is you would need to select north or south on your mobile/portable to hear it best.

I can envision it working, I am just solicating opinions of those who might have tried this and how it played out with the end users.

We had a Parish (county) Sheriff's Office use same freq for Tx, but different PL's..worked for them, even when they had to change channels

I was considering a similar idea for my area. What about this?

Two main transmitters located at different ends of your service area. Both will have the same output frequency. The input frequency would be the same, but with different PL's. Then, you choose say the North or South Tower channel (PL). If they forget to change channels, at least the have a chance of still hitting a tower. It would require two repeater set-ups. The talk-in power need not be great, especially if you use portables. The mobiles could be used at a relatively low power since they are more efficient than portables.

That will not work here. People on both ends of town need to hear all the radio traffic and you need both transmitters going at the same time for that to work. Also, there will be at least 3 more receiver only sites connected to the the voter.

Why don't you try and talk with one or more antenna manufacturing company? See if they can build you a custom antenna with gain in the directions you require (high gain N/S mid/no gain E/W) 5 or 6000 dollars worth of antenna engineering to save building an expensive simulcast system or a (possible) hodge-podge multicast system seems worth it to me.

A custom central antenna and voted receivers would function extremely well (especially if the FCC'll give you even a couple of watts power bump)
...Z

A local next door county along with several others I know of use two sites with identical freqs but the input PLs are different. Either transmitter can be heard from anywhere in the county, ya just have to remember to switch to the appropriate one to transmit. BUT, the simple solution is the one previously suggested of one site with directional antenna(s) and two receiver sites fed to a voter. That isn't such a costly endeavor if done without the vendors trying to rob ya.

Here's some food for thought:

Are you creating an opportunity for any of the "units" to get lost? By that I mean would the design create the ability for a mobile or portable to wander into the "other" coverage area and become unreachable?

I see you may have some some sort of Fire backgound from your screen name so I need to ask if there is paging on the channel? If so how would you handle paging in a two output frequency multicast enviroment?

I too would be reluctant to design a simulcast ovelap zone in the heart of town. That said, is there another solution if 3 sites were used? (Yeah its expensive but we have to ask the question, right?) Could it be a 3 dimensional problem and not two dimensional?

If we assume that talk out range is far greater than talk in range (portables) I'm left with the question that if two sites will work for inbound why do we need two sites outbound? Can you EM me a 1:100K jpg of the topog?

Anyway, not knowing the terrain or the application I'm shooting in the dark here but its all just food for thought. Good Luck, Bob

Give me a little bit to format some maps and I'll post them.

It will require more than those two sites for talk back. There would be at least 2 other remote receivers on the voter.

We page on low band right now though I would like to be able to phase paging onto UHF.

There is a north fire department (me) and a south fire department. There is also an ambulance that covers the whole town. So, people in the south end of town can have their pager on the south transmitter. People in the north end of town can have their pager on the north frequency. I wish I didn't have to do that, but it seems like the only option other than a few hundred grand for simulcasting.

I guess the other way we can do this is to have two seperate repeaters on seperate frequencies all together. North and south would have their own system each. It makes it a little more complicated for the ambulance, but that might be something t concider. It would cost more too.

A local SO did this out here for years.

Single TX output with countywide coverage using two individual RX input channels

1 North

1 South

Both fed same output using same PL.

There wasn't an issue of getting lost since you were manually selecting TX based upon where you were but th e RX was the same county wide.

Worked quite well.

apco25 wrote:Single TX output with countywide coverage using two individual RX input channels

see, nmfire's trouble is the central spot isn't gonna give him outskirts coverage as well as he needs...he needs more than one TX site in the center of his "egg" hahah.

The one county that dispatchs us for ambulance has this set-up on there Main channel and it works very good.

The only two problems that have been mentioned is that when you have limited channels in your radio (which no one really should anymore) and the other one is if the units are not zoned for patrol some people complain as the are trying to listen off of the east tower and should be on west.

This way, IMHO, is the best way to solve the issue of coverage. We have done some things will site transmitter steering with a JPS voter. However, in our case here the two transmitter sites covered the city pretty well but it gave the little bump we needed every once in awhile for better portable coverage.

Good luck but it sounds like it will work out for you very well.