Confused on setting up a repeater

[mjz55]

I want to set up a GMRS repeater. What is the most power I can output? What is the max antenna height located on a building?

Any suggestions on a system?

Can a repeater run two separate set of frequencies, IE a GMRS and a ham set of frequencies?

 

[redneckcellphone]

Gmrs cannot work with ham frequencies. Channels 1-7 on frs are the repeater inputs


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[mjz55]

I understand that gmrs and ham are two different entities. So if a repeater cannot run two separate set of frequencies, can one antenna share two repeaters?

 

[kayn1n32008]

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I understand that gmrs and ham are two different entities. So if a repeater cannot run two separate set of frequencies, can one antenna share two repeaters?

Yes, but it is not cheap to do.

 

[mjz55]

Ok, thanks. So, what is my antenna height limit from roof of building and what is power limit for a repeater?

 

[trumpetman]

I think your best option is to sit down and review the rules over a few times to get a good grasp on what you're about to tackle. Setting up a repeater (even a quick and cheap setup) is no easy task. Do it right the first time, spend the money where it needs to be spent for a proper setup, and follow all the rules to avoid fines, and you should be dandy.

 

[gewecke]

Ok, thanks. So, what is my antenna height limit from roof of building and what is power limit for a repeater?

This link should answer your questions,

GMRS Information

73,
n9zas

 

[RKG]

1) Part 95 limits the repeater to 50W at the amp output; there is no ERP limit.

2) There is no FCC limit on antenna height. Depending on where you are, there may be a requirement of FCC registration.

3) Any repeater programmed with multiple channels will only work if the several channels (both input and output) are within the bandwidth of both (a) the repeater receiver's pre-selector and (b) the duplexer's cavity tuning. As a practical matter, this would preclude combining 462 GMRS channels and 440 HAM channels.

4) The only system I'm familiar with that would enable two repeaters to share a single antenna is the T-Pass system made by Tx-Rx. $10,000 and counting.

 

[jeatock]

Repeater 101:

The receiver of the repeater cannot 'hear' any energy coming from the tranmitter of the repeater. This includes both the transmitter's center frequency, AND the off-frequency energy- there is a lot of that coming from the repeater's transmitter. A small amount of off-frequency energy will blank out the 'uplink' signal coming from the mobile. A lot of signal can trash the receiver's 'front end' components.

RF filters are used to pass only the desired frequency and ground out all other frequencies. Each set of filters is mechanically tuned to create a 'notch' in the spectrum. Any substantial change in frequencies requires re-tuning of the filters.The further the transmitter's frequency is from the receiver's frequency, the less off-frequency energy there is to deal with.

Duplexers are filters arranged in a row; the antenna is connected in the middle, the transmitter to one end and the receiver to the other end. The more seperation between the uplink and downlink frequencies, the less filtering is required. Mobile 'uplink' energy from the antenna hits the center of the duplexer and is allowed to pass through the notch to the receiver on one end. Transmitter 'downlink' energy hits the other end and is allowed to pass through the notch to the antenna. The transmitter's energy cannot pass pass on to the antenna port due to the receiver-side notch tuning.

Multiple fransmitters/receivers can be connected to the same antenna, but require very precise (expensive) filtering: transmitters cannot be allowed to send their energy into the finals of other transmitters, or to the receivers.To get a nice clean 50 watt signal at the antenna port, you have to create 100+ watts- the rest dies in the filters. A transmitter combiner connects multiple transmitters to a single coax lead. Again, the more frequency seperation you have between frequencies, the better off you are. A multi-multi-system may loose 80+% in the filtering.

Receiver filtering operates in the same precise way, but much good signal is caught in the filtering so active pre-amps are required to boost the uplink signal from the mobile. The pre-amp has to be protected from the transmitter just like the receivers, or you will let the smoke out.

Receivers transmit a small amount of energy also, so multi-couplers that further isolate each receiver from the other(s) are required. Pre-select filters on each receiver also filter out off-frequency energy.

This is often the case in public safety systems and can be done, but is not easy or inexpensive.