There is tons of equipment out there from the old analog paging days that can
work on the 440 ham band and is simulcast capable. I worked on a 931Mhz
Glenayre system ears ago that was capable of analog voice simulcast and it
worked well. A great simulcast 440 ham system could be put together if you
could find this old paging equipment. Making all the transmitters simulcast
would not be a real difficult job, but your limitations would be receiver
sites. It would not be difficult to do a simulcast system with a single
receiver, but would be very complicated using multiple receivers.
1) Keeping all the transmitters frequency stable could be done with a TXCO.
Routine maintenance would be required, let's say twice a year, to adjust the
frequency. Interestingly, you would NOT adjust them all to be exactly the same
frequency! You would offset them, maybe 20-30HZ, instead of putting them all
on the exact frequency. This is known as "frequency offset" the paging
industry. This reduces deep phasing nulls in the overlap areas. It does cause
a 20-30HZ tone on the signal, but the average transceiver will not pass this
audio frequency.
2) You need some sort of audio delay circuit to adjust the multiple
transmitters launch time. For example, two transmitters are adjusted so that
the audio out of both transmitters leave at precisely the same instant. Then,
the signals that the mobile receiver hears will add to each other. Analog
voice paging stations have circuitry to create this delay.
3) Flat audio should be used in all the links and transmitters. The audio
should only be preemphasized once in the system so that the transmitters all
have the same audio characteristics. To keep things simple, all receiving and
transmitting equipment should be the same manufacturer and model.
4) Deviation needs to be tightly controlled in all the system.
5) The list goes on and on. Purchasing an old voice analog paging system in
the 450Mhz band, along with all the linking radios, would simplify the project.
The major problem I see is: How many receivers are you going to have? If it is
more than one, you will have to backhaul all the receiver audio to a central
location, select the active receiver, and send it back out to the transmitters.
- Steve Rodgers <[EMAIL PROTECTED]> wrote:
>
> Has anyone implemented a simulcast (multiple TX's on the same channel) system
> for a ham 440 repeater system? It looks like you have to have TXCO's capable
> of tracking within a few hertz of each other.
>
> You can't do this with standard TCXO's, so I'm wondering if anyone has tried
> modifying a TCXO to lock to a GPS source, or NTP source.
>
> Steve WA6ZFT
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> Yahoo! Groups Links
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