A good rule of thumb is that no matter what the frequency of operation is that at a spacing of 1 wavelength, at any frequency, the attenuation will be ~ 22 db with antennas in the same plane and in the near field.
Then each time you double the distance from this initial 1 wavelength separation the power decreases by another 1/4 or 6 dB.
As an example lets assume 150 MHz or 2 meters wavelength(6.56 feet):
At a separation of 2 meters (one wavelength at 150MHz) the power is down 22 dB from the transmitters output power at the antenna.
At 2 wavelengths spacing, 4 meters or 13.12 feet, between antennas of the same band (150MHz) with lets assume 0 db gain antennas, in a perfect environment, the attenuation would be approximately 28 dB.
So the signal would be 28 dB down from the transmitter output power into a 0 dB gain antenna at 13 feet.
So if you are transmitting with 50 watts (47dBm) at 150MHz and receiving in the same band, with 0dB gain antennas at each end, that are only 6.5 feet spaced apart on the roof of your vehicle then the power into your scanner could be around (47-22)dBm or 25 dBm or 317 milliwatts, which may cause damage to the scanner.
At ~ 13 feet spacing it would be a further 6 db down or around 79 milliwatts which the scanner may be able to handle.
Now whether or not the scanner has a narrow front end tuned filter that further attenuates an in band transmitter signal at the scanners antenna is a different story but these would be the worst case levels.
Mike
