Does VHF FM at altitude mess up the assigned frequencies in Land Mobile Radio?
Example: Police operating out of a plane at 3000 ft AGL.
I have never heard of it occuring. Its not like one radio next to another radio transmitting and when the one radio "Keys up" , you get "Bleedover" on the AM/FM radio.....or old analog Televisions. Aircraft radio transmisions are low power.
Seems like some none aircraft type authorities, like the police, do not generally like AM aircraft QSO's and seem to whine about it asking "why is FM not used".
The designated radio frequencies and bands are allocated and assigned by the Federal Communications Commission.
FCC Frequency Allocations
Example = You have a business. You have a UHF radio. You cannot transmit from 400 to 420 Mhz, because that band is allocated to the Federal Government AND certain UHF frequencys ( And VHF-High Frequencies ) are dedicated....An example woudl be = 406.025 ; is a satellite beacon frequency. You have to transmit between 460.900 - 462.175 , 463.200-465.000, 466.025-467.175 ( business & alarms ). I have seen exceptions to Tow trucks and County school buses, in that they are on the "Public Safety" band becuse when they wrote the paperwork up for the radio license, the license was under the "County" or the "City" entity. So they got a UHF or a VHF-High frequency allocation to transmit on. But with licenses, that means that within a certain distance....no one else can "legally" transmit on that assigned radio frequency...REGARDLESS of the PL or CTCSS.
So if you want to scan the "bands' to listen ot an agency or "entity" that you do not have the frequency for, you just place your scanner in "Search" mode and scan the allocated frequency range. I have done it many times at work.
I already understand why AM is used in the aviation industry.
Doog
AM radio signals travel farther than FM radio signals - most especially at night. Alot of AM radio startions tranmsit at a higher power of a day, than of a night. But like digital radio signals....AM radio signals are more subjective to interference and terrain problems than FM radio signals. When an aircraft is at 20,000 feet, they are essentially broadcasting with a 20,000 foot antenna. The reason you have trouble hearing aircraft at a distance, or have low modulation is that they broadcast at very low wattage. I live in Appalachia, and at times I can hear Langley Air Force aircraft. I can't hear the tower, but I can hear the aircraft. I also do well with the Air Force Aircraft in Ohio. Sometimes the modualtion is low, but I an hear them.
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406 MHz Distress Beacon Channels and Radio Frequency Spectrum:
a. The radio frequency spectrum band for 406 MHz Distress Beacons comprises 19 channels between 406.022 MHz and 406.076 MHz.
http://www.capmembers.com/media/cms/Prosecuting_406_MHz_Distress_Beacon_DAE24A42EC69E.pdf
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A 5 watt signal ( UHF - Ultra High Frequency ) reaches to a satellite at about 625 miles above the earth, pretty impressive. A simple five watt signal - to a satellite.
COSPAS satellites characteristically orbit at 1000 km altitude
International Cospas-Sarsat Programme - Wikipedia, the free encyclopedia
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Cospas-Sarsat (satellite) compatible beacon frequencies[edit]406 MHz UHF- carrier wave at 406.025 MHz ± 0.005 MHz
Distress radiobeacon - Wikipedia, the free encyclopedia
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FF - Medic !!!
