[PaulB]

As ever, I’m just curious because I don’t know how these things work.

The other day I was driving happily listening to BBC R2 on my car’s FM radio when it started crackling as if it was losing reception. (With RDS this doesn’t happen much nowaday.)

Them a GPS voice told me to take the second exit at the next roundabout! This was surprising as my car does not have a GPS. The radio then searched and locked onto some random station.

I re-selected R2 and about 2 minutes later the same thing happened with GPS instructions for the next junction.

I suspect that some close by vehicle was erroneously transmitting on 88.3MHz. Any other explanations? What sort of system would it be?

[Paul_Sengupta]

Just an FM transmitter so that the person in whatever car it is gets the announcements or music or whatever over the radio.

Something like this.

https://www.amazon.co.uk/dp/B0756WZ2V3/ref=asc_df_B0756WZ2V357951769

[riverrock]

Indeed. They have very short ranges so probably the car in front or behind you.

[seanxair]

On a similar theme - why are AM radio stations prone to leakage from neighbouring frequencies and particulary at night? I occasionally try to listen to some sports stuff on 5 Live and the later it gets the more chance there is of having a significant amount of signal coming and going and hearing some warbling from a what I suspect is a French radio station dedicated to home grown 'music'

[Paul_Sengupta]

That's not neighbouring frequencies, that's the same frequency being re-used. Why is it worse at night?

https://www.fcc.gov/media/radio/am-stations-at-night

On top of this, the way amplitude modulation works means the signals mix together so you hear both. With FM (frequency modulation), you get what's known at the capture effect. But VHF signals aren't prone to bouncing off the ionosphere (except in "sporadic E" events).

[Nick]

I seem to remember from distant physics some 50+ years ago that this is caused by the Appleton–Barnett layer which is in the stratosphere and comes lower during night time reflecting radio waves.

I am sure @Paul_Sengupta Will be along soon to put us all right

Nick

[Nick]

I said he would.

Thanks Paul.

Nick

[PaulB]

Just an FM transmitter so that the person in whatever car it is gets the announcements or music or whatever over the radio.

Something like this.

https://www.amazon.co.uk/dp/B0756WZ2V3/ref=asc_df_B0756WZ2V357951769

Thx. So someone around may have been listening to R2 on their radio, but got a device like that to transmit their phone GPS instructions on the same frequency?

Why did my radio lose reception then and search for (& tune to) another station?

[lobstaboy]

I think he already did!

Night time medium wave (AM) propagation was something that fascinated me in my early ham radio days. You could build a big tunable loop antenna that worked like an aircraft df loop to null interference out on the same frequency (co-channel interference). Using that it was possible to hear stuff from huge distances.
North America uses different frequency spacing on medium wave to Europe: 10khz rather than 9khz. That means there are places on the dial where North American stations are between European stations and so they can be heard relatively easily. There was a station on 930khz in Newfoundland that was very regularly heard in the UK.

[Paul_Sengupta]

More information on the ionosphere here...

https://en.wikipedia.org/wiki/Ionosphere

Thx. So someone around may have been listening to R2 on their radio, but got a device like that to transmit their phone GPS instructions on the same frequency?

Maybe, or maybe 88.3 was a blank frequency for where they started their journey or where they normally live. In my experience these transmitters transmit constantly so they wouldn't get Radio 2. This may have changed these days to just switch on the transmitter if there's audio being fed to it, I don't know.

Why did my radio lose reception then and search for (& tune to) another station?

It probably screwed up the RDS system, i.e. the interfering signal didn't have any, so when it lost the signal your radio went searching for another station. RDS has a neighbouring frequency list, a bit like the neighbouring cell list on the cellular network. That's how it knows where to look to switch frequency as you drive to keep you on the same station (or some radio allow you to switch to the same type of station if the same station isn't available).

I wish SAM FM would sort out their transmission delays to the different transmitters. Driving on the M4 from Swindon to the Severn Bridge, it switches back and fore between different frequencies, all of which are timed differently, and you get either a skip or a repeat of half a second or so. It's like listening to a scratched CD!

[PaulB]

Thx. So someone around may have been listening to R2 on their radio, but got a device like that to transmit their phone GPS instructions on the same frequency?

Maybe, or maybe 88.3 was a blank frequency for where they started their journey or where they normally live

As I was typing my previous response, I no that they may have been listening to their MP3s interspersed by the GPS instructions, but if that was the case, I’d have heard their music too.... but I didn’t.

So, I was wondering if that may have been listening to R2 and trying to “superimposed the GPS from the phone onto the same frequency. This clearly didn’t work for me, so I assume it wasn’t successful for them either!

[Paul_Sengupta]

No, the capture effect (see above!) would mean that only the strongest signal is heard, there would be no mixing. Maybe they didn't have any MP3s playing, maybe they just had the GPS on and weren't listening to anything else. Or maybe the transmitter was switching on and off. But it would have been unreliable for them if they were actually listening to Radio 2 with the RDS switched on, as their car radio could have been switching frequency away from their transmitter. They could have switched the RDS AF off of course.

[Cessna57]

I used to have one of those transmitters (£10 eBay)

I’d hazard a guess that maybe they had it tuned to 87MHz or some such, and it was of such poor quality that it also blatted all over R2, which they weren’t listening to anyway.

[riverrock]

There are some old phones (mainly Nokia is my memory) which included a built in FM Transmitter. Perhaps they only transmitted when there was audio to transmit?

[seanxair]

That's not neighbouring frequencies, that's the same frequency being re-used. Why is it worse at night?

https://www.fcc.gov/media/radio/am-stations-at-night

On top of this, the way amplitude modulation works means the signals mix together so you hear both. With FM (frequency modulation), you get what's known at the capture effect. But VHF signals aren't prone to bouncing off the ionosphere (except in "sporadic E" events).

Thanks @Paul_Sengupta