![]() Older comm transceivers (particularly those from the vacuum tube era) have a highfailure rate. If the signal is greater than three microvolts,then you hear whatever is there. If the received signal is less thanthree microvolts, then you hear nothing. Most auto-squelch circuits are set to open at a signal strength ofthree microvolts (just in case you wanted to know). Most of the newer comms have "auto-squelch" in which thesquelch level is pre-set on the bench and the pilot simply has a switch to turn thesquelch on or off. ![]() On most of the older comms, you manually adjusteda squelch knob until you heard the back ground noise, then backed it off slightly untilthe noise went away. The receiver portion of a comm radio is every bit as important as the transmitter.Receivers incorporate a "squelch" circuit to eliminate background noise whennobody is transmitting on the frequency. If you overmodulate the radio, your transmissions will begarbled, and may also interfere with adjacent channels. Newerradios have build-in protection against overmodulation, but most older radios do notincorporate this feature. This adjustment must be done by a shop with the proper testing equipment. Ifmodulation is too low, your voice will sound weak if too high, it will be badlydistorted. ![]() Most radio manufacturers call for 90% modulation of the carrier by voice. The receiverĪnother important thing that must be set properly is the modulation level of thetransceiver. Normally, at seventhousand feet, you should be able to receive and transmit a range of around fifty miles.This is of course is true only if it is line of sight and no big rocks like the Rockies orSierras in the way. When plugged in, the handheld uses theaircraft antenna for its antenna, providing greatly increased range. King makes a little gizmo that is installed between the antenna and the aircraft radioand permits you to plug in a handheld transceiver. Nothing worsethan paying $4,000.00 for a new radio, only to find out that it preforms no better thanthe old clunker you pulled out because the antenna is no good. I recommend that if you’reinstalling a new comm radio, you have the antenna system checked out also. As with all radios, the antenna is the heart of the system and a poor onewill do a poor job regardless of how good a radio you have. The best way to improve the range of an aircraft comm radio is by installing a goodantenna system. If you’re attempting to compare the power output rating of tworadios, make sure you’re comparing apples with apples (e.g., PEP with PEP). Some use phrases likePEP, RMS, average, etc. Anotherthing to look for is the way the manufacturer measures power output. I figure anything over ten watts is a waste and is added load on the radio. ![]() It can send a signalto a satellite thousands of miles away on just one watt of power, because there’s a clearline-of-sight. If Center can’t hear your 5-watt radio because there’s a hill in the way,100 watts wouldn’t do any better. VHF radios operate strictlyline-of-sight. Power, range, and modulationįrequently I hear a pilot say "I wish I had more power so I could talk to Centerwhile on the ramp." General aviation comm radios transmit at a power output of 2 to25 watts. Military aircraft use a different band in the "Ultra High Frequency"or UHF spectrum (300-3000 Mhz). These comm frequencies lie in theportion of the radio spectrum known as "Very High Frequency" or VHF, defined as30-300 Mhz. Modern aircraftcomm radios have 760 channels spaced 25 Khz apart. As more channels were needed, the channel spacing was reduced. Inthe old days our aircraft communication radios were limited to ninety channels spaced 200Khz apart.
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