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Receiver Preamps in Repeater Applications
By Mark A. Malm NØFAZ
with some text additions by 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 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 allowing for better noise figure. Again, since we are not making up for a great deal of loss, we do not need a great deal 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 FM 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 10 to 12 dB should be more than adequate for most installations. If you already have a preamp with more gain than you desire, place a coaxial attenuator 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.
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, a 5 dB actual sensitivity improvement. 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 16 dB or so, but you won't get the stated gain improvement on actual sensitivity (at least not with this style preamp on this receiver as you are making up the difference between the receivers actual bare NF [about 10 dB] and the devices NF [about 1 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 still make an improvement.
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This article is © Copyright November 19 1999 M. Malm N0FAZ
Hand-coded HTML © Copyright February 18, 2000 Kevin K. Custer W3KKC, all rights reserved.
This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.