It has been claimed that this problem exists, but I have never heard anything myself.
With 8.33, you get two new channels in between every consecutive pair of 25k channels. The end result is that all channels remain equally spaced. That also means that every 25k channel has an 8.33 channel either side of it (logical!).
So we should be getting loads of interference on 25k radios (close to 100% of light GA). But it doesn’t seem to happen…
With AM, the frequency deviation is equal to the audio bandwidth, which in aviation radios is pretty severely limited. I don’t know the spec (the KX155A is specced to 2.5kHz or so) but it will be lowpass filtered to something like 2-3kHz. So with 8.33 channel spacing (which – in the simple non-SSB scenario we have in aviation – gives you 4.16kHz to play with) one would not expect to get interference unless the receiving 25k radio has a really crap selectivity.
Interestingly enough the KX155A (see above link) has a receiver spec of “6dB bandwidth +- 8.4kHz” which does look like it’s cutting it a bit close. Yet the facts (zero interference noticed over 12 years with two of them) doesn’t support this. Most likely any 8.33 activity is below the squelch level – that must be the answer! If you pull out the squelch you hear all kinds of crap…
The KX165A/8.33 radio (PDF, page 1-3) has essentially the same spec on the receiver selectivity of -6db at 8.0kHz, so the answer must be that the squelch is used to suppress adjacent 8.33 channel activity.
I also heard people saying this, but a fact is that 8.33KHz spacing is in use since a couple of years in the upper airspace > FL195.
This could not have been allowed with aircraft communicating on 25KHz in the airspace below, e.g. one flight at FL190 (25k) and the other in FL200 (8.33k) if there
actually was a chance of interference.
This is just a matter of radio spectrum management, isn’t it?
As long as you don’t mix 25K channels with 8.33K channels, all should be fine.
You mean geographically separated? But you easily get 200NM range around an airborne transmitter.
This thread seems based on an incorrect premise. 25k and 8.33k already operate together. What is the ‘problem’ we are trying to discuss? There is no known issue with the two operating together.
As long as you don’t mix 25K channels with 8.33K channels, all should be fine.
They are mixed.
They are equally spaced now and they will remain equally spaced when the 8.33 frequencies are added.
In the old days you had e.g. 125.000 125.025 125.050 – 25kHz spacing.
Now you will have 125.000 125.0083333333 125.016666666 125.025 125.033333333 125.0416666666 125.050 – 8.33333333kHz spacing
Due to limitations of avionics displays, and radio phraseology, the frequencies displayed and their “names” used on the radio, approach plates, etc, are modified from those actually tuned by the radio gear, so that e.,g. 125.016666666666666666 will be called “125.015”.
Peter, your right, I think that it is a common mistake to read channel spacing as channel bandwith in such discussions. It is not the same as you indicated.
Although bandwidth and channel spacing are different things, if one were designing a receiver for a radio with 25KHz spacing, one would not design in the same selectivity as one would for a tighter spaced receiver.
In the former case it is necessary to ensure the receiver only receives the intended channel, the designer would not have considered that there might be a transmission only 8.3KHz away. Therefore I would expect the adjacent 8.33 channels to break through on the receiver of 25KHz sets, if the signal is strong.
I’ve just found this in the GNS430 IM
So, when switched into the 8.33 mode, it tightens up the receiver selectivity.
Yet, King (KX165A/8.33) don’t do that. And I have been flying with that one for a few years and have never heard anything resembling any adjacent channel interference.
I think the reason both of these products work is that the transmitted carrier is modulated by only about 2.5kHz either side, so any old 25kHz receiver works fine even with an 8.33kHz transmitter running either side of it.
Therefore I would expect the adjacent 8.33 channels to break through on the receiver of 25KHz sets, if the signal is strong.
Yet, they don’t seem to. The modulation is not enough to do that.
Of course to get the break through you would need another aircraft to be in close range, and transmitting on an immediately adjacent 8.33 freq, maybe that hasn’t happened yet. Longer range and weaker transmissions perhaps get trodden on by squelch. One could experiment by selecting a .25 freq that was immediately beside a 8.33 and listen for break through.
