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Not much field strength is lost looking at 88 vs 108MHz for a 11” antenna.

To be complaint, if the power is not adjusted for frequency, the TPO needs to be set so the field at the top of the FM band meets part 15 limits. Then the field at the bottom of the FM band can be calculated as:

E_88=E_108 * sqrt (G_88 / G_108)

Where:

E_88 is the field strength at 88MHz

E_108 is the field strength at 108MHz

G_88 is the antenna gain at 88MHz

G_108 is the antenna gain at 108MHz

I do not have the gain for an 11” antenna over the FM band, but at 88MHz an 11” antenna is 0.08 wave lengths and at 108MHz an 11” antenna is 0.1 wave lengths. (I am sure someone will have better numbers than what I have next, but it is the best I have on hand). Based on a regression for gain vs vertical antenna over ground for HF, a 0.08 wavelength antenna has a gain of -2.1dB or a power gain of 0.61 and a 0.1 wavelength antenna has a gain of -1.7dB or a power gain of 0.671. So roughly if a transmitter with a 11” antenna puts out a field of 250uV/m @3m for 108MHz, I would expect it to put out a field of 238uV/m @3m for 88MHz, or a 5% loss in field strength.