Sep 1, 2008 12:00 PM, By Harold Kinley

Choosing the right one will depend on cost, future site planning and co-tenant attitudes

The RF output from TX-A is 100 W. All antennas have a gain of 6 dBd. All transmission lines have a loss of 2 dB. The path loss between TX-B and TX-A is 60 dB. The path loss between TX-A and RX-C is 60 dB. The conversion loss in the RF output stage of TX-A is 20 dB. The sensitivity of RX-C is 0.3 µV, or -117 dBm. It is desirable to suppress the intermodulation signal (2A - B at the receiver input) to a level that is 10 dB below the receiver's sensitivity, or -117 dBm minus 10 dB equals -127 dBm. With no filtering installed, the intermodulation level is equal to -74 dBm at the input to RX-C. (See calculations in Table 1.) This is 53 dB too high. Some type of filtering or other isolation must be provided to reduce the intermodulation signal by at least 53 dB.

By installing the filters (on TX-A) — represented by graph D (green trace) in Figure 1 — with the center frequency at 159 MHz, an additional loss or isolation of approximately 30 dB could be obtained. The B signal would be attenuated by approximately 15 dB and the intermodulation signal (C) also would be attenuated by approximately 15 dB, for a total reduction in the intermod signal of 30 dB.

This would still leave the intermodulation signal approximately 23 dB above the desired suppression level. An alternative would be to install an isolator at the output of TX-A. A dual isolator could give a 60 dB attenuation to the B signal before it entered the output stage of TX-A. This 60 dB reduction would be more than enough to suppress the intermodulation signal well below the -127 dBm level at the input to RX-C. The isolator should be followed by at least one bandpass cavity filter to suppress any second harmonics that might be generated in the isolator.

There almost always is more than one way to mitigate an interference problem. The choice depends on cost, attitudes of RF neighbors and site planning for the future. The attitudes of your RF neighbors can sometimes be less than desirable. Some aren't willing to tolerate even 0.5 dB of insertion loss to their transmitter power even though a filter might improve the situation for many of the RF tenants on the site. Of all the criteria with which one must deal, these attitudes can be the most difficult to overcome. Education and diplomacy are the keys to getting it done.

Until next time — stay tuned!

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