Primarily for general aviation discussion, but other aviation topics are also welcome.
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User avatar
By kanga
#1566698
Simplistically, AIUI, a channel is a nominal exact radio frequency plus the band of adjacent exact frequencies deemed to 'belong exclusively' to that frequency. Comprehensible human speech requires about 3.8 KHz of protected bandwidth, so an adjacent nominal frequency should be at least that far away, preferably a bit more. Significantly less bandwidth is needed for Morse or single channel printer. For data, bandwidth requirements rise proportionately with data rates.

Welcome better explanation by the many greater experts among Forumites
flybymike, rikur_, Joe Dell liked this
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By Marvin
#1566709
condor17 wrote:PS. Does anyone else remember 90 channel radios ? Then we went super at 180 , sooper dooper 360 ......760 et al .

condor .


Yes. My father flew a mini cab with a radio set with at most 4 crystals, they were expensive, and a radio powered by a motorcycle battery fastened to the floor.

Enjoyed my first trip to France in that aircraft via the water guard and customers in Southend and then to Boulonge. All without LARS, Transponders or GPS just a stopwatch, compass and 500,000 chart.

Those were the days.
User avatar
By CloudHound
#1566733
Anyone remember ASH Radios? We still have one at the strip to listen in on Safety Com.

My first in the Nipper was a 360 that performed flawlessly. A friend had a 9 crystal ASH in his plane. He could buy new crystal units from Mazel Radio in Ardwick.

Happier days :(
User avatar
By Dave W
FLYER Club Member  FLYER Club Member
#1566734
Marvin wrote:All without LARS, Transponders or GPS just a stopwatch, compass and 500,000 chart.

Those were the days.


You could still do that if you wanted; some French Customs airfields (e.g. Calais) are outside Class D so no requirement for transponder.

You could even do it non-radio, with some pre-flight faffing.
By scottish_ppl
#1566740
flybymike wrote:As a radio thicko, can someone tell me what the difference is between a channel and a frequency?


Normally a channel is just a simple number to make selecting a frequency easier.

So if you are into boating your marine transceiver will use simple channel numbers 1,2,3,4 etc instead of needing to enter a frequency. You would select channel 16 to make a distress call, whereas in aviation we would directly enter the frequency, 121.5 We could just as easily be entering a number of 1 to 720 instead of frequencies if a different decision had been made in the past.

Now in the crazy committee camel that is 8.33kHz rollout, when you enter a long multi-digit number that looks just like a frequency, the radio is transmitting on a frequency slightly different to what you have entered. So people are calling this number a channel instead of a frequency....
flybymike, UpThere liked this
By chevvron
#1566809
riverrock wrote:Seems like a positive thing to me - means we can hear the broadcasts rather than them being hidden on UHF, although I tend to fly at weekends when the military have days off, so haven't heard many broadcasts.
SafetyCom isn't congested in Scotland so isn't an issue.

On safety grounds alone, I can't object to the RAF transmitting blind when they are operating low level in the Highlands, it's just their use of Safetycom I object to, even if it isn't congested in Scotland, some place across the border might get blocked by their transmissions.
I know in the London FIR there is a UHF FIS frequency used by pilots to transmit blind when operating (for instance) along the south coast at low altitude so why do they not do the same on a discrete VHF frequency in Scotland? They must have several frequencies held in reserve and at least one of them should be suitable for use without interfering with other users, plus civil aircraft would be able to monitor it.
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By Dave W
FLYER Club Member  FLYER Club Member
#1566814
It's for the military to use below 2000ft. Would not South of 56N blocking have been commented on during the trial and by now if it was a particular problem?
Last edited by Dave W on Sat Oct 21, 2017 11:40 pm, edited 1 time in total.
By riverrock
FLYER Club Member  FLYER Club Member
#1566858
chevvron wrote: it's just their use of Safetycom I object to

GA also broadcast our position - we don't just monitor.
I think you've picked the wrong fight here.
Is it not a good thing that the military are also monitoring when you are in the circuit at a grass strip, which is almost certainly not on their map, so they can avoid it too? It only operates North of Glasgow / Edinburgh (North of 56degN). I suspect that it would also be fine if it operated North of 55deg N (Newcastle / Carlisle).

If the frequency was congested - it would be different.

In Scotland - the UHF frequency was used until SAFETYCOM started to be.
T67M liked this
#1566890
kanga wrote:Simplistically, AIUI, a channel is a nominal exact radio frequency plus the band of adjacent exact frequencies deemed to 'belong exclusively' to that frequency. Comprehensible human speech requires about 3.8 KHz of protected bandwidth, so an adjacent nominal frequency should be at least that far away, preferably a bit more.

Minor clarifications: human speech is generally considered, for communications quality systems, to be in the range 300Hz to 3000Hz. We use amplitude modulation (AM) in the air band* and AM has two sidebands, each equal to the maximum modulating frequency. For example, a transmission on 123.600MHz occupies the frequency range 123.597MHz to 123.603MHz.

Thus, the actual bandwidth of an AM transmission is twice the highest modulation frequency. This means that the adjacent "channels" must be at least 6kHz away, not 3kHz. Now think about 8.33kHz. The sidebands of an adjacent pair of channels extend 3kHz towards each other, leaving a guard space between channels of just 2.33kHz. What this tells us is that 8.33kHz is the end of the road - there can be no further channel subdivisions after 8.33kHz using AM.

* Forumites may be curious to know why we still use AM for air band when it is comparatively inefficient both spectrally and in signal-to-noise ratio terms. Better voice modulation techniques have existed for 50+ years. There IS a perfectly valid reason!
kanga liked this
#1566893
PB wrote:Amplitude modulation avoids the 'capture effect" that ocurs with frequency modulation and it's obvous when two stations are transmitting at once (even if the result is a horrible screaching in your headset.

Well done that man, 10/10. It sounds like an inconsequential matter but it has significant safety benefits. Now, for bonus points: what causes that screech?
By chevvron
#1566943
The Westmorland Flyer wrote:
kanga wrote:Simplistically, AIUI, a channel is a nominal exact radio frequency plus the band of adjacent exact frequencies deemed to 'belong exclusively' to that frequency. Comprehensible human speech requires about 3.8 KHz of protected bandwidth, so an adjacent nominal frequency should be at least that far away, preferably a bit more.

Minor clarifications: human speech is generally considered, for communications quality systems, to be in the range 300Hz to 3000Hz. We use amplitude modulation (AM) in the air band* and AM has two sidebands, each equal to the maximum modulating frequency. For example, a transmission on 123.600MHz occupies the frequency range 123.597MHz to 123.603MHz.

Thus, the actual bandwidth of an AM transmission is twice the highest modulation frequency. This means that the adjacent "channels" must be at least 6kHz away, not 3kHz. Now think about 8.33kHz. The sidebands of an adjacent pair of channels extend 3kHz towards each other, leaving a guard space between channels of just 2.33kHz. What this tells us is that 8.33kHz is the end of the road - there can be no further channel subdivisions after 8.33kHz using AM.

* Forumites may be curious to know why we still use AM for air band when it is comparatively inefficient both spectrally and in signal-to-noise ratio terms. Better voice modulation techniques have existed for 50+ years. There IS a perfectly valid reason!

Farnborough East 123.225 uses two Tx/Rx stations, one at Riegate Hill and the other a few miles east at Warlingham. Obviously they must use offset carrier waves with 25khz spacing to avoid interfering with each other, but will this still be possible with 8.33 spacing?
#1566987
chevvron wrote:Farnborough East 123.225 uses two Tx/Rx stations, one at Riegate Hill and the other a few miles east at Warlingham. Obviously they must use offset carrier waves with 25khz spacing to avoid interfering with each other, but will this still be possible with 8.33 spacing?

Yes, in theory an 8.33kHz channel can support two offset carrier transmissions at 2.5kHz either side of centre frequency. It's right on the limits of what is technically feasible though and it only works because it steals the guard space from the adjacent channels. Area systems such as London/Scottish Info will be amongst the many services that have to stay on a 25kHz channel.
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