If the radio is a 8.33 kHz set, then in practise the difference will be that with channel 130.000 the reciever will have a wider frequency tolerance.
I don’t understand that. There is no “frequency tolerance” on a receiver. There is the option of changing the receiver bandwidth (see the threads I linked above) but I don’t see any evidence that say a KX165A or a GNS430 does that. Especially the KX165A on which 8.33 frequencies are simply tuned just as any others (you pull the knob out for the extra ones).
And since the TX bandwidth is still only equal to the audio bandwidth (3kHz or so), what do you gain? With 8.33 spacing, a 4kHz bandwidth is plenty good enough. 1kHz is ~8ppm on VHF so any 1990s+ radio should have the carrier accurate enough.
I will pop down to the plane and get a video of the KX165A frequency selection, to settle this 
Maybe someone can do the same with a GNS box?
Rwy20 wrote:
Ah now that you say it… no wait. Are we supposed to know these by heart, or am I the only one who doesn’t understand what is written there?
I don’t see why that would be necessary. Putting aside which frequencies 5. actually refers to, you can clearly tell whether any particular channel is in 8.33 or 25 kHz spacing (you can see an example in #55). You won’t be able to dial the correct 8.33 channel using a radio with 25 kHz spacing even though you could transmit on the actual frequency. In case it refers to only those listed in 4.(a), you’ll probably know for which one(s) you have a dedicated radio – which I imagine wouldn’t be a common practice in light GA.
Peter wrote:
I don’t understand that. There is no “frequency tolerance” on a receiver. There is the option of changing the receiver bandwidth (see the threads I linked above)Apologies for using the wrong terminology. ICAO Annex 10 uses the term “effective acceptance bandwidth” and that’s what I meant.
but I don’t see any evidence that say a KX165A or a GNS430 does that. Especially the KX165A on which 8.33 frequencies are simply tuned just as any others (you pull the knob out for the extra ones).
ICAO Annex 10 states that a 25 kHz receiver must accept a carrier which is offset by 8 kHz! Unless the KX165A and GNS430 do that when tuned to a 25 kHz channel, they’re not in compliance with ICAO standards.
8 kHz is almost the entire channel spacing for 8.33 kHz channels, so clearly the effective acceptance bandwidth can not be the same for a 8.33 kHz channel. According to Annex 10, for 8.33 kHz channels, the effective acceptance bandwidth should be ±2.5 kHz.
To sum up, when tuned to 130.000, the receiver should accept carriers in the range 130.000±0.008 MHz, while when tuned to 130.005, it should accept carriers in the range 130.000±0.0025 MHz.
ICAO Annex 10 states that a 25 kHz receiver must accept a carrier which is offset by 8 kHz! Unless the KX165A and GNS430 do that when tuned to a 25 kHz channel, they’re not in compliance with ICAO standards.
See the threads I linked to and the MM extracts. Clearly this is not how 25/8.33 has been implemented. And frankly if I was designing a radio and was aware
I would do the same.
Peter wrote:
I do have a spectrum analyser (10kHz to 2.4GHz) but it’s not a trivial job to bring it to the plane. However what would be the point?
I think the point would be to verify what your radio is actually emitting.
Good point. Let me think about how I can organise this…
Peter wrote:
See the threads I linked to and the MM extracts. Clearly this is not how 25/8.33 has been implemented. And frankly if I was designing a radio and was awarethat 8.33 is on the horizon, and
a ~1ppm TXCO is used by just about every other radio of relevance
I would do the same.
And in this post you yourself said that the GNS430 did it that way.
Here ya go… Note that that spectrum analyser deviation is 25kHz initially (i.e. from the middle to the RHS it is 25kHz, displayed as “50kHz span”) and 30kHz after 1:40. The radio is a KX165A/8.33.
There is also a ~1kHz error in that 130.000 is shown on the spectrum analyser as 130.001. I saw the same error at work this morning when testing with the Marconi 2024 signal generator. I believe this error is at least partly the result of the analyser setting, specifically the scan speed. However I was powering the analyser from the aircraft cigar lighter (via an inverter) and didn’t want to wait too long for the oven stabilised reference oscillator to warm up. The KX165A has its own TXCO warmup which is very visible on the spectrum analyser (not in this video) and takes about 30 secs.
The result is, ahem, surprising 
Very obviously, the radio transmits on 130.000, 130.00833, 130.01666, 130.025, etc.
What you didn’t expect is that 130.000 and 130.005 both transmit on 130.000.
you yourself said that the GNS430 did it that way.
Yes; you are right that the GNS should pick up a carrier which is 8kHz off, at -6db, so “something” should still come through. Whether it will be enough to break the squelch, especially as half of the 3kHz sideband will be beyond the 6db point, is another Q… IMHO the support for a 8kHz-off carrier is really marginal.
Peter wrote:
What you didn’t expect is that 130.000 and 130.005 both transmit on 130.000.At least three people have written exactly that in this thread recently so it should not come as a surprise…
Clearly, the problem with being a mod/admin, and having some sort of a life too, is that I don’t have the time to carefully read every post. Apologies for that.
The last figure is around 90% dross-free.
But at least I spent a couple of hours going to the airport with the gear and did the test to clear it all up
