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

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

Last month, we discussed various types of RF-resonant cavities. This month the discussion focuses on the actual use of these cavities to mitigate RF interference. Due to RF congestion at popular transmitter sites, such as high points near a populated area, RF interference has become the norm rather than the exception. The types of interference include transmitter noise, receiver desensitization and intermodulation interference.

Transmitter noise is the broadband noise that is radiated along with the desired signal from a transmitter. The amplitude of this noise decreases with frequency away from the carrier frequency. Much of this noise is generated in the exciter stage of the transmitter and amplified by each succeeding stage all the way to the antenna output port. All transmitters are not created equal — some transmitters generate more broadband noise than others. The specifications of the RF filter required to mitigate the noise from a particular transmitter will depend, in part, on the transmitter model — all other factors being equal.

Transmitter noise graphs, as supplied by the equipment manufacturer, usually show the level of transmitter noise referenced to the carrier level. Because the noise is on the receiver's frequency, transmitter noise can't be mitigated at the receiver — it must be done at the transmitter. The total amount of isolation required to suppress the transmitter noise to a non-interfering level depends on the transmitter power, sideband noise level and receiver sensitivity.

For example, suppose the transmitter power is 100 W (50 dBm), the operating frequency is 160 MHz, the receiver sensitivity is 0.25 µV (-119 dBm) and the receiver frequency 159 MHz. At a frequency 1 MHz away from the carrier, the sideband noise is 100 dB below the carrier level (or 50 dBm minus 100 dB equals -50 dBm). To avoid interference, the noise should be suppressed to a level that is 10 dB below the receiver's sensitivity figure (or -119 dBm minus 10 dB equals -129 dBm). So, the noise level must be attenuated by 79 dB (-50 dBm minus -129 dBm).

The first step is to determine the path loss between the two antennas. If the antennas are in line-of-sight, free space path loss can be calculated or obtained from a chart or graph. Suppose that the antennas are 0.25 miles apart. The free space path loss would be approximately 68 dB, assuming isotropic radiators for the transmitter and receiver antennas. But in this case both receiver and transmitter antennas have a gain of 6 dBd, or 6 dB referenced to a half-wave dipole, which has a gain of 2.15 dB over an isotropic radiator. Taking these antennas into account, the path loss is now 68 dB minus 16.3 dB — the sum of the antenna gains referenced to an isotropic radiator — or 51.7 dB. An isolation of 79 dB is needed to mitigate this interference, but the path loss provides only 51.7 dB, leaving an excess of approximately 27 dB. This extra 27 dB of isolation can be handled with a filter.

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