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SW Aviator Magazine
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3909 Central NE
Albuquerque, NM 87108
Phone: 505.256.7031
Fax: 505.256.3172 |
| editor@swaviator.com |
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| COMM RADIOS: Why 90 Channels Aren’t Enough |
| by Alisa Christenson, Santa Fe Avionics |
For those out there still flying around with 360 channel radios, this article is for you. On January 1, 1997 the FCC said each VHF aircraft radio used aboard a U.S. aircraft must be type accepted by the FCC as meeting a 30 parts-per-million (ppm) frequency tolerance. The radios accepted by the FCC utilize 25kHz spacing and have 720 or 760 channels.
Before I explain what this means to you, the pilot of an aircraft with a 360 channel radio, let’s understand the history of aircraft radios.
Some of the first aircraft radios were 90 channel spacing, which means you had 90 frequencies to choose from. These radios can still access many ATC frequencies, like unicom, flight service, flight watch, etc, however, these radios have not been manufactured since the early 1970’s. Then the FCC went to 360 channel radios. The FCC allotted only 90 spaces and when the spacing went from 90 to 360 more, spaces were added not by increasing the number of spaces, but by decreasing the kHz. With 90 channel radios each channel had 100 kHz. This was decreased to 50 kHz doubling the number of spaces creating 180 channels. However, more spaces were still needed so, the top end of the frequency band was expanded from 126.90 to 135.90, giving another 180 channels. This helped create the 360 channel radio.
The problem with older radios with tubes or crystals is that they are hard to keep stable, therefore causing your transmission to bleed over to other channels. This could cause you to talk to two different controllers at the same time. Not a safe idea.
As with the 90 channel radios, the additional channels still weren’t enough. More and more people were flying, more airports were being built, more controlled airspace and more services were being offered. The channels were getting over crowed again. The FCC recognized this and decided to split the frequency spaces again. They went from 50 kHz to 25 kHz plus an extra MHz which created another 40 channels, giving way to the 760 channel spacing. Currently there are enough channels to accommodate all aviation needs.
Back to those of you who still have 360 channel radios. The FCC has taken steps to minimize the impact of this rule on the aviation community. They have provided a decade for the transition to more efficient radio equipment. They are not requiring radios to be removed from aircraft in cases where pilots do not transmit radio signals. Finally, the FCC has allowed manufacturers the flexibility to manufacture “upgrade kits.” This applies to radios accepted prior to 1974.
So, if you are not going to transmit, it’s probably okay to have that old radio. However, with as much controlled airspace as there is out there, it is probably time to replace that 360 channel radio with a 720 or 760. There are many options to choose from, so contact your local avionics shop and they can help choose the right brand for you.
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The material in this publication is for advisory information only and should not be relied upon for navigation, maintenance or flight techniques. SW Regional Publishing, Inc. and the staff neither assume any responsibilty for the accuracy of this publication's content nor any liability arising out of it. Fly safe.
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