09-26-2001, 11:53 AM
In addition to the primary frequency selection criteria - e.g. that there are no stations on, or adjacent to, your proposed frequency [remember, use a sensitive FM radio on high ground to determine this - park your car on the highest hill in town and check the whole dial - don't rely on seek / scan]there are some other factors affecting frequency choice. For example, if you live near an airport or under a major flight path, you may want to be careful of broadcasting on the high end of the FM dial - e.g. 106 MHZ and above. If you need to broadcast at these frequencies, use a scanner or an airband receiver to make sure your transmitter is not interfering with the airband. If you have any TV stations from Channel 7 to 13, check to make sure that your transmitter doesn't crank out harmonics that will interfere with TV reception. For example, recently I was broadcasting on 105.5 MHZ. A nearby TV station broadcasts on channel 13. Channel 13 is 210 to 216 MHZ [this span is to include both audio [FM modulated] and video [AM modulated]. Simple math shows that I had a harmonic at 211 MHZ. While my Ramsey low pass filter ensured that this didn't mess up my neighbour's reception, it messed up my own. By moving to 97.5 MHZ [last night's project] my harmonic was at 195 MHZ - in the middle of Channel 10 which is empty here. In addition, my Ramsey low pass filter gives maximum filtering at approximately 195 MHZ. Instant cure - flawless TV even though the TV's rabbit ears with built in 20 db booster is only 15 feet from the TM100 transmitting antenna. Use a low pass filter, keep it clean, and select your frequency with care. Note that the FM25A and FM100 do have built in low pass filtering, which is fine for the built in antenna and the low power setting. If you're going to try extending your range beyond this, such as a good external antenna mounted fairly high, I recommend the low pass filter FMLP-1.