Receiver Preamps in Repeater Applications

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By Mark A. Malm NØFAZ
with some additional help from Kevin Custer W3KKC

In most situations adding a preamp to the receiver of a repeater will increase its receive range and should "clean up" some of those noisy signals. Common thinking is that you can never make up for any signals that you are not receiving due to coax loss, connectors, etc. Thinking along these lines, one would assume that the more preamp gain, the better. Preamp gain is great for making up for loss between the antenna and receiver in a receive only situation, but on a repeater if the cable between the receiver and preamp is just a few feet, we do not need a lot of gain!

More gain does not always mean more range. Gain is not really the determining factor in how well a receiver hears. Although sufficient gain is necessary to overcome losses, Noise Figure (NF) is the real determining factor. Receiver noise figure is determined by a few things. First is the quality of the very first active stage, second is the amount of incurred losses ahead of it, and third is the bandwidth of amplification.

Too much gain can reduce the intermodulation specification of your repeater's receiver. This can be a real problem if you are on a site with other transmitters in the same band. In GE's MASTR II UHF service manual, intermod rejection drops 5 dB in the units equipped with the optional preamp!

As stated earlier, the real reason a preamp helps is because it lowers the receivers RF noise figure. However any losses in the receive path, before the preamp, add directly to the noise figure. The closer we can get the preamp to the antenna, the more it will help our system. Since it is not possible to place the preamp inline with the duplexed transmit/receive signal, we have two paths to choose from.

The first, and most common, is to add the preamp in the receive path between the duplexer and the receiver itself. An option here would be to place the preamp between two of the filters in our duplexer. This would allow us to make up for any of the losses in the filters and connectors following the preamps output, and reduce the losses ahead of the preamp somewhat (by moving it closer to the antenna) allowing for better noise figure. Again, since we are not making up for a great deal of loss, we do not need a lot of gain. This example works great in UHF systems but likely shouldn't be tried on 2 meters.

Second, and most desirable, is to use separate antennas for transmit and receive, and place the preamp directly at the receive antenna. In this instance we would need to add some form of front end filtering to the preamp to protect it from being overloaded from out of band signals. This is the method the cellular folks use to get such fantastic range from those 0.6 watt portables (and some are half that)!

Most commercial two-way receivers have noise figures in the area of 10 dB. It is quite easy to find preamps with noise figures of 1 dB, or less. The goal is to realize the lower noise figure of the added preamp, without creating other problems. Different applications will require more or less gain, but a gain of 6 to 12 dB should be more than adequate for most installations. If you already have a preamp with more gain than you need, place a coaxial attenuator (pad) on its output port.

If you have the ability, you can probably construct a preamp that will more closely suit your needs than can be found commercially. Along these lines, you can find an article on building a good GaAs FET preamplifier here. Additionally, Masters Communications sells a UHF preamp that doesn't have a lot of gain to begin with, and it quite acceptable for many ham and GMRS installations. It's called the P440.

All the calculations in the world won't tell you how your repeater will behave with a preamp. When it comes right down to it, you won't know unless you try. Don't be afraid to experiment though. That's what this hobby is all about. If you do find something that works for you, let the rest of us know.

I've done some testing with the Ramsey PR-10 and PR-20 (bi-polar) preamp on converted MICOR receivers for 2 meters and the 222 MHz ham band. On 220 I get .2 uV (-121 dBm) for 12 dB SINAD, an actual sensitivity improvement of 5 to 6dB. On 2 meters I have seen .15 uV (-124 dBm), about 9 dB of improvement over the .35 uV factory spec. I believe the stated gain is 14 dB or so, but you won't improve the the actual sensitivity by the amount of stated gain (at least not with this preamp on these type of receivers, as you are making up the difference between the receivers actual barefoot NF [about 10 dB] and the preamps' NF [about 2 dB].)

And remember, as long as the preamp does not get overloaded, and the sensitivity of the receiver is above the noise floor of the site, and the NF of the preamp is less than that, the preamp will make an actual improvement on receiver sensitivity.

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