VHF FM in Aircraft messing up Land Mobile?

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doogymon

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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.

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".

I already understand why AM is used in the aviation industry.

Just wondering.

Doog
 

ff-medic

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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 !!!
 
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radioman2001

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Aircraft radios normally are very low power, on the order of 1 watt, this is all part of FAA and FCC regs. Our shop did often receive interference which I believe was a portable on our channel that I tracked down to an aircraft that was spotting for a commercial fishing fleet a few hundred miles from us (Westchester Co N.Y. from somewhere offshore from Cape Cod area).
 

ff-medic

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Aircraft radios normally are very low power, on the order of 1 watt, this is all part of FAA and FCC regs. Our shop did often receive interference which I believe was a portable on our channel that I tracked down to an aircraft that was spotting for a commercial fishing fleet a few hundred miles from us (Westchester Co N.Y. from somewhere offshore from Cape Cod area).
I was thinking 5 watts output. I don't believe I was far off.


Onboard comm radios are in the 25W class, meaning 25W pep output (peak envelope power), which translates into 6 to 7W carrier output.
http://www.airliners.net/aviation-forums/tech_ops/read.main/100331/

I don't see how you loose that much wattage ( radio to antenna ) but I am not an "aviation radio" expert either.



FF - Medic !!!
 

WB4CS

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I don't see how you loose that much wattage ( radio to antenna ) but I am not an "aviation radio" expert either.
If we're still talking about AM, it's not a "power loss" but rather how AM works. Your carrier output power is a constant value, and any audio that is modulated changes the peak envelope power output.

Example: AM PEP Assuming linear, perfectly symmetrical, 100% modulation of a carrier, PEP output of an AM transmitter is four times its carrier PEP; in other words, a typical modern 100-watt amateur transceiver is usually rated for no more than, and often less than, 25 watts carrier output when operating in AM.

Peak envelope power - Wikipedia, the free encyclopedia

So, 6W carrier would mean at 100% modulation the PEP would be 24W, or 7W carrier would be 28W PEP.
 
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902

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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.

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".

I already understand why AM is used in the aviation industry.

Just wondering.

Doog
The short answer is: Yes, it does.

The full list of requirements and prohibitions is in 47 CFR 90.423. Bullet by bullet, operations are limited to less than 1 mile altitude (5,280 ft), no greater than 10 W output, SECONDARY to land mobile use (that requires non-interference), and possibly other impositions that may be required for better co-existence with land-based operations.

A 10 W portable repeater operating at 5,000 ft. with a unity gain antenna beneath an airplane can cover a footprint of tens of thousands of square miles (I know first-hand).

But I think you might be asking something different. There is a clear line between aviation radio (AM) and its use and public safety radio (generally FM or some common digital format) and its use. It's not the modulation technique more than who is using the frequency and how aggressively it has to be reused within the potential footprint. Those limitations are on the public safety FREQUENCY, not because they are using FM, but because they are using those categories of frequencies.
 
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