BackPacker wrote:
So, in short, no?
If anyone is looking for the post, it’s now below http://www.euroga.org/forums/maintenance-avionics/1808-8-33-interference-on-25khz-radios-true-or-false-merged/post/117966#117966 . In short, only two-legged climax is possible on a channel with 8.33 spacing utilizing ±2.5 kHz offset. In other words, you either have one leg with f+0 or two legs with f-2.5 and f+2.5. E.g. redundancy in a terminal area is achievable with 8.33 (that is if you’re satisfied with dual and don’t need triple or more). The regulation about conversion to 8.33 doesn’t apply to channels utilizing offset carriers so they can keep them on 25 kHz if they want to.
same issue in the London FIR for the 2 info frequencies. Not sure in Scotland.
I see several option:
Build a very tall antenna and very powerfull transmitter = lots of money
Split in several sectors, more ATCO/FISO = money
Stay on a 25kHz channel. Possible due to the tricks discussed. loose 2 8.33 channels. Not much money
So, in short, no?
I wonder how Dutch Mil INFO (now 132.350, to become 132.355 later on) is going to handle this. They’ve got a bunch of transmitters as well, to provide low-level coverage across the country. Although I believe they use a slightly different system. As per the AIP we’re always asked to report our approximate position on first contact, so that they can use the transmitter that’s closest to us.
BackPacker, the human hearing range is much wider, typically 20 Hz to 20 kHz for young people, with the upper boundary decreasing with age. 3-4 kHz is the upper boundary of the frequency band for speech – or, more precisely, you won’t lose intelligibility if you cut off the overtones above that. To remove 1.67 kHz, you will need a highly selective notch filter, and even that is likely to affect intelligibility.
BackPacker wrote:
Can they do the same thing within the bandwidth of an 8.33 channel?
I wrote about that in the post that was moved back into the original thread. It’s the one below this: http://www.euroga.org/forums/hangar-talk/6491-threatened-7500-fine-over-8-33-capability/post/117885#117885
[I have moved it back here – Peter]
Interesting. Can they do the same thing within the bandwidth of an 8.33 channel? Offsets for a four-station channel would need to be something like +/- 2.5 kHz and +/- 0.83 kHz, leading to a 1.67 kHz heterodyne. Would that be cut off by the filter as well? I seem to remember that the human ear has an audible range from something like 0.5 kHz to 4 kHz so 1.67 kHz would be right in the middle of that.
25khz ‘channels’ are going to be with us for some time for the many ground stations which operate multiple simultaneous transmitters using the offset carrier method of operation also known as Climax. Extract from UK CAA doc below:
2. Within the United Kingdom, virtually all VHF RTF channels provided for en-route air traffic control and those used for emergency, flight information services and VOLMET are operated from between two and four radio stations using the off-set carrier technique in accordance with the standards set down in ICAO Annex 10. The off-sets employed are; ± 5 kHz for a two station system, ± 7.5 kHz and 0Hz for a three station system and ± 7.5 kHz and ± 2.5 kHz for a four station system.
3. When an aircraft is operating within the range of two or more stations, the individual transmissions combine in the airborne receiver to cause one or more audio heterodynes having a minimum frequency of approximately 5 kHz. These heterodynes appear above the normal audio pass-band of the receiver and are not heard by the flight crew.So does any onw know of a "slide in " replacement for the good old King KX155 radios?
Thought I’d try this. Switching the 430 is trivially easy – Aux, 4 clicks of the small knob, and there it is!
So I tried adjacent channels, on the ground, 50 yds from the tower. One up = perfectly clear, normal communication, slightly hollow sounding. One down, unreadable, buried in mush. So on that scientific test, a) the 430 ‘Comms Mode’ selection does do something b) the lower channel would not offer crosstalk at any practical range, the higher channel is still unproven but would probably vanish at any range.
The tower radio is still 25Khz.
The 430 will stay on 8.33 from now on.
LeSving wrote:
This is the kind of thing that only the EU bureaucracy could invent
It was unlikely to have been the EU who came up with this, given that it’s a world standard – more likely a world body like the ITU.
