Total posts : 45366
6600 is not an integer multiple of 1200. Harmonics are integer multiples, but you may have actually been tuned to 6000 or 7200. If your receiver is very close to the antenna, you can get all sorts of spurious signals at weird frequencies, usually due to receiver overload at the transmitter frequency.
If you are using a tuned antenna (some sort of loading coil and whip antenna) it will have a fairly sharp tuning peak. In radio jargon, the antenna will have a “high Q”. This is a very good thing to have for rejecting harmonic radiation. Your antenna will radiate very well at the tuned frequency and very poorly at harmonic frequencies.
Make sure your antenna is tuned precisely to your frequency for best performance. Even with a high Q antenna, you will still get some very low level harmonics that you will hear on a nearby receiver. My bet is you are too close to the antenna, and moving the receiver away a couple dozen feet will make the 6600 signal all but disappear.
There are “low pass filters” that will greatly attenuate harmonics, but they are typically designed to go in-line in a 50 ohm coax feed. Even if you are using coax, I don’t think such a filter is a good idea because it will add some loss to an already anemic 100mw signal.