quick question. i know changing the transmitter channel is bad bad bad.
but if it is so bad then why can i simply remove my tx crystal on my futaba 4yf. you would think they would at least solder it in the circuitry if they did not want you to change it.
so my 43 ch 72 mhz tx crystal can not be changed to a 44 ch 72 mhz (by me a unlicenced dude). 1. because it is illegal 2. because it could interfear with someone elses?
the next question is what would a manufacture do differently? crystals are inscribed with a "code" so doesnt a transmitter read the code and transmit it.? i understand how a change from a 72mhz to a 50mhz could mess up the signal but a 72mhz to a 72mhz conversion confuses me. someone please explain.
some say it is illegal ^ some not^ .which is it? i would like to do a 72 to 72 conversion (bought something from th scratch and dent)
What may not be obvious to users that are not necessarily "trained" in electronics is this: The crystal is the generator for a signal that is transmitted by the radio. But, it is not the direct generator. What happens is that the crystal generates a signal that is both amplified (increased in strength) and fed to a circuit within the radio that will multiply or divide the frequency, depending on the application, to the required CENTER frequency.
Crystals are "cut" to a specific frequency, however, it is not cut exactly to that frequency. So, electronic circuitry is in the radio transmitter that will "tune" that frequency to exactly where it has to be. That is now fixed for the crystal that was originally installed in the radio transmitter.
Changing the crystal to another channel caused this "center frequency" to be de-tuned because it, the radio, was never re-tuned to center up that frequency to where it has to be. When this happens, it will "bleed" over to another adjacent channel, or even worse, could generate sporatic harmonics (multiples of the original crystal frequency) that could be amplified and transmitted. This is where the problem comes in. Bleeding over on adjacent frequencies is problematic for reasons that I hope you understand. It is not good to have two transmitters operating on the same frequency (channel) at the same time at the same field. Bad.. very bad. The same goes for harmonics. And, as Opjose pointed out, spikes can also occur on other channel frequencies because of de-tuning caused by inadvertent changing of transmitter crystals.
Thus, the FCC has deemed this to be illegal when done at home. It must be done by a qualified radio technician that will install the crystal, tune it to the correct center frequency, then using a "spectrum analyzer", will examine the transmitter output to make sure that it does not: 1. Generate a signal that it is not supposed to (Harmonics), 2. Bleed over to an adjacent channel (could do that based on a mis-match of the crystal to the circuitry, even though it may be the right one, and 3. cause spikes elsewhere that could cause problems with other radio receivers in the local area.
So, take heed. Send them in to be done right. When you get it back, it will be operating at peak performance and will not cause problems with other pilots.
CGr.