| Radios work by simple mathematics. For example, most tune to a frequency by mixing that frequency with another (local oscillator) frequency which is slightly different. This mixing process primarily gives us the two original frequencies, their sum, and their difference. The radio's Intermediate Frequency (IF) filter normally passes either the sum or difference frequency, and this is then processed into the sound we hear. Because nothing is perfect, certain "harmonics" will also get through if they are strong enough. For example, if a radio's IF is 10.7 MHz, we might be able to tune to a frequency 21.4 MHz (2 x IF) above (or below, depending on the radio's design) a strong signal and hear it! This is more evident in a dual-conversion radio than a triple-conversion radio, because the triple-conversion radio's first intermediate frequency is quite high. This causes the image to be so far off frequency that it is easy to effectively filter it out.
Just because a radio doesn't receive something which another does is not necessarily an indication of a problem. The one radio may simply not be "tricked" into picking up an image! This rejection of undesired signals is one reason that a triple-conversion receiver costs more than a similar dual-conversion model. If you are more interested in finding more out about radios and radio operation, a good location to start looking is your local public library. You might also wish to contact the ARRL, as they are an excellent source of informative texts on the subject. |
