Receiver Preamplifier

[taco6513]

I am wondering if a a receiver preamplifier would increase the coverage of my repeater?

Thanks

WRCW870

KI5GXD

 

[Radioguy7268]

Receiver pre-amps are NOT magic. In many cases, they'll actually make a system worse.

 

Signal needs to be greater than noise. The benefit of a pre-amp is in raising the strength of the desired signal above the noise floor. If you raise the noise floor at the same time that you're raising the signal, you've done nothing to improve the Signal to Noise ratio.

 

You need to have proper front end filtering in place before you even consider a pre-amp. If you're using a 'flat pack' compact mobile duplexer, you're not doing yourself any good to add a pre-amp. If you've already spent the money to hang a quality antenna that's up above the tree line, and you're running a high quality bandpass/band reject duplexer, you might see some improvement by adding a pre-amp. Might. We have no idea what your system is currently, or how it's performing.

 

Do you have the tools and knowledge to measure receive sensitivity of your repeater? Do you have the tools and knowledge to measure de-sense?

[taco6513]

My setup: I have a Icom Fr4000 repeater with an internal duplexer. ( I beleave it is a mobile duplexer)

The antenna is a DB420 on top of a 200ft tower.

The heliax is 1 5/8" length is 210'. 

I am the only repeater in the tower in the UHF band. There is a repeater in the public service VHF band 100' below my antenna.

 

I have a DB4076A duplexer from a repeater I had long ago. I would need to have it tuned.

Would this be a better duplexer if I want to use a pre-amp?

Also I have found 8 cavity mobile duplexers on E-bay. Would that be any better?

 

Thanks

WRCW870

[Radioguy7268]

Sounds like you're in a good position to move forward. It's definitely worth it to get that DB4076 tuned up to your frequencies, and just add a tuned bandpass cavity to the receive side - effectively giving you 3 cans on the receive side, which should get you something north of 85dB for isolation and tamp down any noise before heading into the Preamp.

 

I like the Advanced Receiver Research P460VDG GaAs-FET pre-amp for a budget conscious GMRS build. The specs are great, and the price isn't bad. Make sure you get the unit that covers the 460 MHz band. Noise floor is below 1dB - and effective gain is +12dB. You might even need to pad that down a little depending on your repeater's own internals. I've never messed with any of the Icom series of Repeaters, so no idea how good that FR-4000 is. I'll just about guarantee you that a "hot" pre-amp with 20dB or more gain will overload the repeater's front end.

 

I've heard of some guys liking the Mini-Circuits pre-amps for amateur stuff - but the advertised noise floor of 2.5 dB scares me off. I've never actually had one on the bench to try it out. Maybe someone else here has.

 

The best pre-amps I ever touched were made by Angle Linear - but apparently they closed up shop around 2017 or so. Sinclair used to use the Angle Linear inside all of their high end Multicouplers - which I took as pretty high praise, since apparently Sinclair didn't think they could make one on their own any better.

 

Lightning is the #1 killer of pre-amps. Make sure you're bonded and grounded. Read up on Motorola's R56 standards if you haven't studied them before.

 

Good luck with it.

[berkinet]

I strongly suspect that with a good antenna 200 feet in the air attached to a decent radio, and extra isolation on the input, you will not see any improvement at all from a pre-amplifier. In fact, the most visible impact of the preamp will likely be to your electric bill.

 

I would suggest you install the repeater with the extra cam and then start driving around and testing to make sure that your reception is balanced. Then, if you find problems, possibly you might consider adding amplification on the input or output.