It is an interesting story! Tuning Your Antenna By Ear Particularly look at the last section about “Unauthorized Use”, and the references cited. The wikipedia page on the airband channels is interesting. I'd complain, but it could well be my lab! This was collected in Packard EE, where there is a tremendous amount of background noise (the stripes in the waterfall plot). AM gives you a better idea of who else is out there. With FM you will just hear a buzzing if two users interfere, unless one station is very much stronger, and then you only hear stronger user. You will also be able to hear something of the weaker station. This is because when two users try to talk on the same channel the stronger channel will come through with AM, and this is usually the tower. Communications in these bands use AM modulation. Then from 118-137 MHz there are several bands used for communication between aircraft and the ground. There is a band from 108-118 MHz that mostly has navigation beacons that identify themselves by Morse code. The first part of the lab is to capture and decode AM signals in the air band, which is right above the commercial FM band we were decoding last week. When the signal is low, the signal varies more slowly, and the derivative is smaller. When the signal is loud, it varies more quickly, and the derivative is larger. The receiver just needs to differentiate the signal, which can be done many ways. The transmitter is a little more complicated, but still pretty simple. The frequency increases when the signal is high (loud), and decreases when it is low (quiet). Note that the signal goes up and down in amplitude as it goes along.įor FM, all we do is slightly change the frequency of the carrier. ![]() Then to receive, all we need to do is follow the envelope of the waveform. The frequencies here will only be a few kHz (the range of frequencies needed to convey voice).įor AM, all we do is add something to the message so that it is always positive, and then multiply it by the carrier. ![]() In this case it is shown as another sinusoid, which will be in the audio range. The sdr's only go down to about 30 MHz, and up to 1700 MHZ. For last week's signals the frequency was in the 100's of MHz. The carrier is the pure RF signal when it isn't modulated.
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