Changing Your Radio Frequency By James Goss      As you probably already know you can change your radio channel by simply changing your crystal in the receiver and transmitter. With Futaba radios the receivers are center tuned for either the low band or the high band. Channel 11 - 35 make up the low band and channel 36 - 60 make up the high band. So the low band frequency is 72.010 - 72.490 Mhz. and the high band is 72.510 - 72.990 Mhz. Center tuned refers to the receiver being tuned and peaked out to the center frequency of the low or high band. In the low band the center frequency would be 72.250 Mhz. and in the high band the center tuned frequency will be 72.750 Mhz. There is no doubt that the receiver will work best at or near the center frequency and produce the greatest range because that is where the receiver is peaked out. It will be difficult to have a flat response curve all the way through the low or high band so that channel 11 will have the very same amplitude as channel 35 when the signal reaches the IF amplifiers in the receiver, or that channel 36 will have the same amplitude as channel 60. This is why they tell you that if you change your crystal you must be sure to range check before you fly, you may not have the same range characteristics.      Knowing this I have always desired to use a channel at or near the center frequency of the low or high band. Channel 23 has the exact same frequency as that of the center tuned low band and channel 48 carries the same frequency as the center tuned high band. So if you are using these channels the receiver and transmitters are tuned for them. This is not to say that the equipment is not tuned for all our channels, but these may develop the greatest amplitude. When I say the receiver is peaked out for the center frequency of the low or high band, this means at those frequencies the oscillators have been tuned by adjusting coils, and in some cases capacitance, to oscillate at those frequencies. By changing the crystals the oscillators are forced to change their frequency also, but the farther we get from center frequency, the more difficult it is for the oscillators to sustain oscillations, and their output will tend drop off. The same theory takes place in the transmitter and receiver so either can be altered. Both systems use phase locked loops and these circuits are very accurate in maintaining an oscillator on frequency, but nothing in electronics is perfect.      You might wonder why they just don't have one band that reaches from channel 11 to channel 60 and center tune to channel 35 or 36. This would be trying to get too broad a bandwidth and the lower and upper channel amplitude would drop off radically. Basically the cutoff point is known as the half power point and this is reached when the signal amplitude reaches 70.7 percent of what it was at the center frequency. If the voltage amplitude drops off to 70.7 percent of center frequency max, the current capability will also drop and this produces half the power in watts as compared to the developed power at center frequency. The 70.7 percent reference is used to determine if the amplitude of a signal is useable or not. If its amplitude falls below this reference it is not useable.      I just feel better knowing that I have a receiver near the center-tuned frequency instead of one near the 70.7 mark. I always use channel 22 and 48 for that reason. If you use channel 20 - 26 or 46 - 50 you are close to the center frequency of the high and low band.