Picking a clear frequency for FM repeaters

Picking a clear frequency on VHF (150 MHz) on which to frequently transmit with a high-level station can be a daunting task, one likely to create strife if careful planning and modeling is not completed. We chose a frequency shared with a repeater in the Detroit area. Before going live, we first examined the likelihood of interference under repeatable worst-case scenarios.

The other stations used 15W ERP and 50 meters height, yielding a desired coverage area of maximum 15 miles radius to mobile stations with 80% reliability.

A reasonable starting point, especially for microwave links, is to use free-space loss to determine what the relevant signal levels would be under the worst physically possible conditions, conditions that would happen less than a few minutes per year. Since the transmitter sites are approximately 70 miles apart, the free-space loss is about 117 dB. Using Rayleigh “spot” conditions, and neglecting the effects of all ground cover (which significantly increases loss), 50% of the time there will be at least 154dB of loss over this path.

To look at the issue another way, this plot shows the area where for 50 meter high repeater antennas it is just possible to break squelch from W8MSU transmitter.

repeater coverage W8MSU

Thus, as verified by on air tests, it is not even possible to key up the repeater from W8MSU.

Another look is how W8MSU might transmit impact base stations 7 meters high who may want to hear the Detroit repeater. interference Longley-Rice

The green areas are areas effectively not impacted by interference. There are only some tiny red tinted areas where W8MSU might cause the slightest degradation, and those areas are on the order of 30 miles from the Detroit repeater, where there is likely not to be enough signal to even hear the repeater from a base station.