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Topic Author
Posts: 545
Joined: Wed May 03, 2000 6:43 pm

VHF Radios-Standard For A Particular Model?

Wed Feb 21, 2001 12:29 am

I do a lot of monitoring of ATC frequencies from home which is maybe about 20 miles from the airport. There seems to be a large variation in the quality of transmissions from plane to plane. I can rarely hear planes once they are on the ground but on occasion I can hear an individual plane ask for pushback quite clearly. Some planes landing are much clearer than others even though they are following the same approach pattern for the same runway. I realize that the reception can vary from day to day but this is using the same scanner in the same location and within a 20 minute period so I'm assuming the variation in the signals received is due to the individual radio equipment in the aircraft. I'm wondering if the radio equipment that comes with each model of plane is standardized or does each customer specify particular equipment. For example does each 777 have the exact same radio equipment. Do the airline's technicians do a lot of tweaking on their own.

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Joined: Sat Oct 25, 2003 1:58 am

RE: VHF Radios-Standard For A Particular Model?

Wed Feb 21, 2001 1:21 pm

Newer large aircraft (757, 777, A320 etc) have very few options when it comes to VHF Communications. They typically use an ARINC 700 series unit (A standard for form and function) which are controlled digitally. More than one manufacturer including Rockwell/Collins and Honeywell/AlliedSignal and Sextant Avionique make these radios that are designed to be interchangeable.

Older aircraft (DC-9 727, L1011) use older ARINC 500 standard units which are also manufactured by many different companies.

All of these "airline" radios are designed to have similar performance but there are more factors involved.

For instance, all airliners have 2 or 3 (or more) units installed. The primary (#1) unit usually has it's antenna on the top of the airplane and the secondary unit has it's antenna on the bottom. VHF is transmitted in the line of site so if a 747 is transmitting on the ground from #2, it will have longer range than #1 because its several feet higher.

Also, the quality of the transmission is only as good as the pilot's microphone. Some of these microphones are regularily plugged with everything from cigarette tar to coffee (and other stuff...ewww). The microphones are specially constructed with air vents on either side so that sound which is allowed to enter both sides "cancels" and sound entering only one side (the guy's voice) does not cancel. When the air vent on the back of the microphone is clogged, the noise cancelling feature is defeated and you get a poor transmission.

The only other thing I have to offer, is that VHF communications is AM (amplitude modulated) and is susceptible to man made interference like electric motors, transmission lines, etc., and not so man made interference like lightning. You may find that reception is better when the neighborhood is quieter (less electrical interference). An outdoor antenna may help if you don't already have one.

Topic Author
Posts: 545
Joined: Wed May 03, 2000 6:43 pm

RE: VHF Radios-Standard For A Particular Model?

Wed Feb 21, 2001 11:22 pm

Thanks for the response. I was worried that nobody was going to bother responding.

Posts: 1989
Joined: Mon Jul 10, 2000 4:51 am

RE: VHF Radios-Standard For A Particular Model?

Sat Feb 24, 2001 2:49 am

Airplay, you know your stuff! Also, some a/c did come with a choice of suppliers. For example, the Dash 8 could be ordered with Bendix/King or Collins radio packages. The autoflight package was Honeywell on all a/c. The Collins was far better, showing their heritage in airline avionics. As for ARINC equipment, again, I`d say Collins produced some of the best. A bit over engineered, but good quality. On the point of "tweaking", that isn't done ,as it just screws things up. A lot of engineering went into making them work, and the bench tech isn't going to improve things by tweaking.

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