Have I spotted an airband radio problem or am I just confused?

[G7RUX]

Afternoon all.

I have spotted something in VHF airband which I do not understand so I was wondering if anyone else here in the UK (SE) has noticed this too, or can confirm what I am seeing.

First let me explain that here in the UK, and across much of Europe there is a rearrangement of VHF airband into an 8.33 kHz channel bandwidth and raster to increase the capacity available in a congested band with busy airspace.

This has led to a lot of confusion since the "channel names" look like frequencies:
Controller says: Actual frequency to set on receiver:
118.005 118.0000
118.010 118.0083
118.015 118.0166
118.030 118.0250
118.035 118.0333
118.040 118.0416
118.055 118.0500
118.060 118.0583
118.065 118.0666
118.080 118.0750
118.085 118.0833
118.090 118.0916

This means that if the controller says "118.025" then they mean the old channel on that frequency with a 25 kHz spacing and chaannel width whereas if they say "118.030" then this means a *frequency* of 118.025 MHz with a width and spacing of 8.33 kHz.

On and off for a number of years I have been listening to VHF airband radio. Where I am usually found we are fortunate to have quite busy airspace and a number of rather busy commercial airports as well as a good number of smaller airfields to choose from. Among these are the famously small airports of Gatwick and Heathrow.

I believe I may have spotted an issue with one of the transmitters at Gatwick. In particular, Gatwick Director on 118.950 MHz. When listening to this I noticed it seemed to be off-frequency (whislting) and using a spectrum analyser I noted that it appeared to be pretty much spot on 118.955 MHz but this does not fit with any of the listings I can find. Indeed when aircraft transmissions are received they are on 118.950 MHz and the responses come 5 kHz high.

Doubting myself, as all decent engineers would, I decided to look closer to see if it seemed that I was hearing a *channel* of 118.960 which would be on a *frequency* of 118.9583 MHz but it appears not to be the case...it is on .955 as close as I can measure with a (calibrated, high-quality professional unit) spectrum anayser.

My suspicion is that someone has been given a configuration sheet saying a *channel* of 118.955 which would be an 8.33 kHz channel width centred on a frequency of 118.950 MHz; what I am seeing definitely looks like an 8.33 kHz channel width although NATS EAD documentation does NOT indicate that it should be an 8.33 kHz channel.

Interestingly I do also see 118.950 MHz signals but there are only three locations in the allocations: EGKK and EGTT, both associated with Gatwick and EGOQ associated with Mona in North Wales (Anglesey) so I am really quite confused.

It is my intention to spend a little while this evening listening to eaxch to see if I can identify their sources from what they give as their calls.

Tra confusedly,

Piggly

I am perplexed so hopefully someone can help me.

 

[Minus1]

Your mention of this frequency being used by Gatwick AND London is the clue.
Many frequencies that are served by two transmitters offset each transmission by +/- 5 kHz to avoid mutual interference.
If you explore the area you may find one site is +5 kHz and the other site is -5 kHz.
Possible sites might include Gatwick itself, Reigate, Warlingham, Greenford (London).

 

[G7RUX]

Ok, that’s an interesting idea but I wasn’t aware that this technique was used with any UK airband stations. I would expect carrier hetrodyning would cause beat frequencies to intrude into the audio recovered.

I also find it interesting that you seem to suggest that I referred to Gatwick and London separately; I don’t think I did.

All that said, none of the NATS documentation I can find mention this technique being used so if you have any references that I could look at then that would be lovely thanks.

 

[G7RUX]

Oh my goodness, I’ve just received a reply from NATS and you are spot on! I’ve never heard of this technique in this service context and it makes a lot of sense when I read the explanation.

It’s nice to learn something new each day.

 

[Minus1]

If there are three transmitters, they use offsets of -7.5 kHz, 0 kHz, and +7.5 kHz.
The London Information frequencies of 124.6 and 124.75 are examples.

 

[G7RUX]

This has all been rather interesting but now I come back to how I tripped over this technique in the first place; hetrodyning in my R30!
I have the IC-R6 and the R30 as handheld scanners and the R30 seems to have a surprisingly wide passband for VHF AM airband use, leading to audible 5kHz hetrodyne products in the audio output where this technique is in use; I haven’t noticed this with the R6. Am I missing a setting somewhere to reduce the high audio frequency response with the R30 on AM?

 

[Minus1]

You need to change the receive mode to AM-N for 8.33 kHz channels, though not for 25 kHz channels.

 

[G7RUX]

You need to change the receive mode to AM-N for 8.33 kHz channels, though not for 25 kHz channels.

I know.
The system in use is for 25 kHz channels but relies on a proper audio filter removing the 5 kHz hetrodyne. The IC-R6 has a suitable audio filter but the R30 appears to be too wide.

 

[Ubbe]

Most scanners use too wide filters in AM, if they have no bandwidth settings for them. A Uniden UBC780 use the 25KHz FM filter for AM when there is a more suitable NFM filter to use. For a SDS100E I can measure close to a 9KHz wide AM filter being used, as it is programmable in the DSP, that suits the transmissions perfectly and the result are the best AM sounding scanner I have, and as it is AM it has to use AGC that it unfortunately are lacking in FM audio. If it hadn't those intermod issues it would had been a terrific airband scanner.

/Ubbe

 

[G7RUX]

You need to change the receive mode to AM-N for 8.33 kHz channels, though not for 25 kHz channels.

Whilst the emissions from the ground stations in this system are around 9-10 kHz wide, selecting the 8.33 kHz step introduces the narrow AM filter...tuning to the actual output of 118.955 MHz means that the aircraft responses are just too far off-centre and are severely distorted or the radio mutes.