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By Daniel P. Kaegebein
President, TX RX Systems

Prediction of a "specific system improvement" requires considerable knowledge of "specific equipment/component specifications", along with the details of their interconnection. Additionally, the ambient RF noise level will also limit the effectiveness of a tower-mounted preamp if all that is accomplished is the amplifying of a "noisy environment".

A particularly "noisy site" is usually already known by reputation, and if suspect, can be evaluated for tower mount preamp location by determining the noise floor of the site with a spectrum analyzer, preceded by a preselector of the design used in the tower mount preamp. This test requires a high performance spectrum analyzer, and the test procedure is beyond the scope of this article. Practically, more sites suffer from intermodulation due to high level input from nearby mobiles, and sometimes, peculiar interaction with UHF site frequencies, where system antenna isolations may be too low.

A system with improved sensitivity will also allow you to hear IM products you may not have been aware of before, and transmitter sideband noise may need to be suppressed to a greater degree in the transmit multicoupler.

Assuming we have a "clean environment" to work in, we will make a projection of system sensitivity under two different multicoupler configurations, one using a tower mounted preamp.

System "reference points" must be established for making a meaningful comparison. We will assume the same antenna is used for both systems, so the input of each system will be the transmission line immediately below the antenna. The end point of each system will be the receiver.

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