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Directional FM and Non Directional FM on same Antenna

Yes, it is possible and legal as long as that one directional pattern was developed/measured by the OEM at manufacture, and is reproduced by that antenna after installation on the customer's tower.

Almost certainly the patterns of the other two non-D FM stations also will somewhat directional. However the ERP of those stations will be achieved at the RMS value of whatever that pattern is, based on calculations - not measurement. Therefore their real ERP can exceed their licensed ERP in some azimuth sectors, and be less than their licensed ERP in other sectors.

The ERP of the directional station will achieved only at the peak gain of their licensed pattern, rather than at its RMS value, and can never exceed the licensed ERP even at the peak gain of the pattern.

RF
 
Richard is correct. I have done several licensed directional FMs simply by playing with the mounting on the tower (no parasitics).
These antennas could also have easily been used for non-d stations as well. I have also seen the FCC allow parasitics in non-directional FM stations to improve the circularity of the signal.
 
Fieldtech1 said:
Im still soaking up you guys knowledge but how is this possible/legal in the eyes of the FCC? I present example A All 3 stations sharing the same antenna, only KBIU is directional:

I have seen this tower in person and observed there are parasitic radiators on the 10 Bay Dieletric antenna.
Click to expand...

Here's the pattern test report:

https://licensing.fcc.gov/cdbs/CDBS...?appn=101054933&qnum=5080&copynum=1&exhcnum=3

As you can see, the vertical component is 15 dB down at 50 degrees, and no vertical parasitics are shown. The antenna is mounted on a 42 inch face tower, so the steel itself has plenty of effect. The other two stations using this antenna probably have similar patterns in vertical polarization -- but fortunately, the site is only a few miles from town and population density out to the northeast isn't high.
 
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