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Message from AirScan at Friday, 06-Mar-15 17:04:30 GMT

There seems to be some confusion with how 8.33 kHZ spacing works in Europe.

8.33 kHz spacing means the frequencies are spaced 8.33 kHz apart. This is derived by taking 100 kHz of frequency bandwidth and dividing it into 12 parts. 100 divided by 12 equals 8.333333.

Looking at the 100 kHz of bandwidth between 134.000 and 134.100, there would be 12 frequencies available. Starting at 134.000 then incrementally adding 8.33 kHz. 134.000 mHz + 8.33 kHz = 134.00833, 134.00833 Mhz + 8.33 kHz = 134.01667, etc. etc.

Giving the following 12 frequencies.

134.00000
134.00833
134.01667
134.02500
134.03333
134.04167
134.05000
134.05833
134.06667
134.07500
134.08333
134.09167

Now because it would be cumbersome for controllers and pilots to use the actual frequency when talking on the radio they came up with a "channel" numbering system to simplify communications. Each "channel" corresponds with a specific frequency. The "channels" always end with the numbers 05,10,15,30,35,40,55,60,65,80,85,90. It is these "channels" and not the actual frequency you hear the controllers and pilots using.

Using the example above, the corresponding "channels" would transmit on the following frequencies.

"134.005" = 134.00000 mHz
"134.010" = 134.00833 mHz
"134.015" = 134.01667 mHz
"134.030" = 134.02500 mHz
"134.035" = 134.03333 mHz
"134.040" = 134.04167 mHz
"134.055" = 134.05000 mHz
"134.060" = 134.05833 mHz
"134.065" = 134.06667 mHz
"134.080" = 134.07500 mHz
"134.085" = 134.08333 mHz
"134.090" = 134.09167 mHz

This will work for any "channel", for example a channel that ends with 15 always equals a frequency that ends with 1667. eg: "132.715" = 132.71667 mHz.

As a quick tuning reference.

.x05 = .x0000
.x10 = .x8333
.x15 = .x1667
.x30 = .x2500
.x35 = .x3333
.x40 = .x4167
.x55 = .x5000
.x60 = .x5833
.x65 = .x6667
.x80 = .x7500
.x85 = .x8333
.x90 = .x9167

When a pilot selects the "channel" on the control panel the aircrafts radios automatically tune the correct 8.33 frequency as above. The typical scanner or radio here on Global Tuners does not do this.

Now with most (8 of the 12) 8.33 "channels" there is only a difference of 1.666 kHz between the channel and the actual frequency. However there are four 8.33 channels that share the old 25 Khz spacing frequencies were the difference is 5 kHz. These are the channels that end with 05,30,55,80.

Being off frequency by 1.666 kHz is usually not significant and you won't notice a difference, unless you are trying to pull in very weak signals. However being off frequency by 5 kHz can be noticeable in some cases, to the point where you may not not be able to hear the transmissions.

For example when listening to London Control you tune the radio to 127.430 instead of the correct frequency of 127.425. Off frequency by 5 kHz. This is no different when listening to 25 kHz spaced frequencies and tuning to say 135.670 instead of the correct 135.675 because you only heard the pilot say "135.67".

Actual results vary depending on the radio, some are more sensitive and accurate than others. It also depends on the bandwidth you have selected (15k,6k,3k). The wider the bandwidth, the chunk of frequency you are listening to, the less noticeable it will be. As a starting reference to get best results it seems logical to tune the most accurate frequency possible.

More info. here ...

http://www.lightaircraftassociation.co.uk/2013/Magazine/May/radio.pdf

AirScan

Last edited by AirScan at Thursday, 12-Mar-15 19:27:54 UTC
Message from Tango5 at Monday, 18-May-15 21:02:55 GMT
Very good, but note that frequencies spoken as the exact 25kHz multiples refer to 25kHz bandwidth transmissions.
So:
x00 means .x00 (25kHz bandwidth)
x25 means .x25 (25kHz bandwidth)
x50 means .x50 (25kHz bandwidth)
x75 means .x75 (25kHz bandwidth)

So in each 100k band there are in fact 16 channels, 12 at 8.33 bandwidth and 4 at 25k bandwidth. Obviously careful planning is needed to avoid mixing the two in any given area.
Message from AirScan at Thursday, 28-May-15 11:17:44 GMT
>> So in each 100k band there are in fact 16 channels <<

Technically only 8.33's are referred to as "channels".

There are still only 12 frequencies available because the 4 25k's share the same frequency with the respective 8.33k "channel".

eg: 132.050 (25k) and 132.055 (8.33k) both use frequency 132.050 mHz.

As you point out you can't use both in the same area, so it's either 4 25k "frequencies" or 12 8.33k "channels".

Regardless of what you want to call them there is still only 12 actual frequencies available.


Last edited by AirScan at Friday, 29-May-15 05:58:52 UTC

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