Aviation breakthrough on Band II? - UK Vintage Radio Repair and Restoration Discussion Forum

Croozer · 28th Oct 2018, 7:27 pm

Times have changed and i thought the days of picking up emergency service transmissions on VHF vanished when the band was cleared above 100MHz 30 years ago. Maybe you wise and experienced can clear up the following anomoly.

The police helicopter has been circling the city on account of the football today. Listening to local radio on 97.3MHz was interupted as if by a point to point communication; a quick snatch of inteligable speach, an unmistakable 'kilo alpha' but the rest was to brief to copy.

What was it? Assuming an IF of 10.7MHz would it possible to pick up 118.7MHz? Is that a frequency still used for aircraft? I thought aviation used AM, and the limiter in the receiver would make AM reception impossible.

I assume it couldnt be police radio as i thought this went digital some time ago.

This is the second time ive heard such a breakthrough, both times on 97.3MHz, both occassions wihen the police chopper was direct overhead, both times on a roberts transistor with whip aerial.

Any suggestions?

G6Tanuki · 28th Oct 2018, 7:54 pm

Aviation still uses classic double-sideband-and-full-carrier AM, yes. That's the global standard - 8.33KHz channel-spacing these days.

Given a seriously-strong signal, 'image' interference is still a potential issue - I guess your receiver has the local oscillator running 10.7MHz above the frequency it's tuned to, so a big signal 2xIF (21.4MHz) above the tuned frequency can be expected to cause issues.

And yes, because the limiter's not 100% effective 'AM breakthrough' on a FM receiver is a fact.

AC/HL · 28th Oct 2018, 9:22 pm

Yes, 118.7 is the Edinburgh tower frequency.

Croozer · 28th Oct 2018, 9:43 pm

Mystery resolved! Many thanks gents.

I didn't realise that an FM receiver was capeable of AM reception. Good to be able to identify the frequency.

Radio Wrangler · 28th Oct 2018, 10:59 pm

Edinburgh tower's transmitters are on Corstorphine hill.

Police helicopters have to use airband. Few other radio services are licensed for use in the air, though they may use airwave for police business communications. Most importantly they have to be in normal contact with ATC, airport tower, airport ground as needed.

Aviation isn't just still using AM, there is an update underway to 8.33kHz channel-spacing and just about every radio in every plane is required to be replaced.

Normal power requirement is >16W carrier and >70% mod, so PEPs in the 70W region are usual.

Discriminators are often amplitude sensitive and limiters don't limit perfectly, also there is a problem called AM to PM conversion in limiters.

David

paulsherwin · 29th Oct 2018, 9:50 am

Some cheap radios which claim airband coverage just extend the FM coverage above 108. They rely on the discriminator producing some sort of intelligible sound from the AM signals. This does work after a fashion.

28th Oct 2018, 7:27 pm	#1
Croozer Pentode Join Date: Jul 2016 Location: Edinburgh, Scotland, UK. Posts: 110	Aviation breakthrough on Band II? Times have changed and i thought the days of picking up emergency service transmissions on VHF vanished when the band was cleared above 100MHz 30 years ago. Maybe you wise and experienced can clear up the following anomoly. The police helicopter has been circling the city on account of the football today. Listening to local radio on 97.3MHz was interupted as if by a point to point communication; a quick snatch of inteligable speach, an unmistakable 'kilo alpha' but the rest was to brief to copy. What was it? Assuming an IF of 10.7MHz would it possible to pick up 118.7MHz? Is that a frequency still used for aircraft? I thought aviation used AM, and the limiter in the receiver would make AM reception impossible. I assume it couldnt be police radio as i thought this went digital some time ago. This is the second time ive heard such a breakthrough, both times on 97.3MHz, both occassions wihen the police chopper was direct overhead, both times on a roberts transistor with whip aerial. Any suggestions?

28th Oct 2018, 7:54 pm	#2
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,007	Re: Aviation breakthrough on Band II? Aviation still uses classic double-sideband-and-full-carrier AM, yes. That's the global standard - 8.33KHz channel-spacing these days. Given a seriously-strong signal, 'image' interference is still a potential issue - I guess your receiver has the local oscillator running 10.7MHz above the frequency it's tuned to, so a big signal 2xIF (21.4MHz) above the tuned frequency can be expected to cause issues. And yes, because the limiter's not 100% effective 'AM breakthrough' on a FM receiver is a fact.

28th Oct 2018, 9:22 pm	#3
AC/HL Dekatron Join Date: Nov 2003 Location: Heckmondwike, West Yorkshire, UK. Posts: 9,642	Re: Aviation breakthrough on Band II? Yes, 118.7 is the Edinburgh tower frequency.

28th Oct 2018, 9:43 pm	#4
Croozer Pentode Join Date: Jul 2016 Location: Edinburgh, Scotland, UK. Posts: 110	Re: Aviation breakthrough on Band II? Mystery resolved! Many thanks gents. I didn't realise that an FM receiver was capeable of AM reception. Good to be able to identify the frequency.

28th Oct 2018, 10:59 pm	#5
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,899	Re: Aviation breakthrough on Band II? Edinburgh tower's transmitters are on Corstorphine hill. Police helicopters have to use airband. Few other radio services are licensed for use in the air, though they may use airwave for police business communications. Most importantly they have to be in normal contact with ATC, airport tower, airport ground as needed. Aviation isn't just still using AM, there is an update underway to 8.33kHz channel-spacing and just about every radio in every plane is required to be replaced. Normal power requirement is >16W carrier and >70% mod, so PEPs in the 70W region are usual. Discriminators are often amplitude sensitive and limiters don't limit perfectly, also there is a problem called AM to PM conversion in limiters. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

29th Oct 2018, 9:50 am	#6
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,966	Re: Aviation breakthrough on Band II? Some cheap radios which claim airband coverage just extend the FM coverage above 108. They rely on the discriminator producing some sort of intelligible sound from the AM signals. This does work after a fashion.