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Old 16th Aug 2021, 11:28 am   #1
1955APREN
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Default 87.9 MHz FM Car Radio receives phone calls. Explanation?

At 11.40 today sitting parked in doctors surgery car park . ON 87.9 Fm
ch seemed as if a carrier was present (no Hiss). I then heard a telephone conversation , a receptionist booking a doctors appointment but it did not
seem to be our local doctors . Then ch muted after a few sec 's I heard
ringing tone this time it was answered by a local solicitors in town about
a small distance from were I was parked . After a few sec radio returned to normal hiss between ch's. My question how did telephone calls get on to 87.9
Any explanation
Derrick
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Old 16th Aug 2021, 11:37 am   #2
G6Tanuki
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Default Re: 87.9 Fm Car radio explain

Could this be the second-harmonic of the old analog courdless-phones that operate between 46 and 49MHz
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Old 16th Aug 2021, 12:21 pm   #3
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Default Re: 87.9 Fm Car radio explain

Very odd.

The analogue cordless phone we had in the late 1980s was audible on MW, and we could listen-in on various neighbours' calls too.

But most of these would have been dumped about 20 years ago when DECT cordless phones became so cheap that almost nobody wanted the old, bulky, hissy ones any more. I certainly haven't seen one in use for 15+ years, let alone in a busy commercial environment like a solicitors or GP practice
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Old 16th Aug 2021, 12:21 pm   #4
Lloyd 1985
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Default Re: 87.9 Fm Car radio explain

There are in car FM transmitters available that also double up as Bluetooth hands free kits, I bet you were receiving one of those nearby! Certainly wouldn’t want to be talking to your bank on the phone using one of those!!

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Old 16th Aug 2021, 12:31 pm   #5
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Default Re: 87.9 Fm Car radio explain

Ah, that's more likely, well done Lloyd.

Several of the Bluetooth modules I've bought from eBay seem to have inputs for microphones, which ties up with this.
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Old 16th Aug 2021, 12:39 pm   #6
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Default Re: 87.9 Fm Car radio explain

Many cheap car FM transmitters default to that frequency, but that doesn't explain why you were hearing a doctor's receptionist. They are normally used to transmit MP3 music files from an SD card or pendrive, or to relay music from a phone via Bluetooth. I guess someone may have left a phone connected to BT while phoning the doctor from the car park.
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Old 16th Aug 2021, 2:04 pm   #7
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Default Re: 87.9 Fm Car radio explain

I'm inclined to go with the 2nd harmonic theory.

The non-approved just-above medium wave naughty cordless phones had a very noticeable output in the 80m band. Being illegal, there was no oversight of spurious emissions.

One local entertainment for radio amateurs was a woman in the Dunfermline area who regularly made assignations to meet her 'fancy man' at the phone box at the bottom of Garvock Hill. There was enough info for those with local knowledge to identify them, though those knowing, had the good grace to not reveal the names. It went on for quite some time. I suppose spilling the beans would have ended the entertainment, and we could always take cover behind the clause of not revealing the content of anything intercepted not specifically allowed by our licences

The nature of what was said made it quite clear that there wasn't the faintest inkling that what was being spoken into was a radio transmitter. I guess the same is still true.

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Old 16th Aug 2021, 2:38 pm   #8
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Default Re: 87.9 Fm Car radio explain

I can remember when I got my first VHF radio in 1980. It was a Binatone 'Royal' alarm clock radio. I was ten years old.

I'd never had VHF (FM) before, having been brought up through the 1970s listening to my Mother's battered old Medium Wave transistor radio (wrapped around with yellowing Sellotape to keep the battery compartment cover in place!) always tuned to BBC Radio Blackburn (on 351 metres) every single morning. Radio Blackburn eventually became BBC Radio Lancashire in 1988, and gained three FM frequencies, to cover the whole county.

I remember tuning very slowly up and down the VHF waveband on my new radio to see what I could find.

I definitely heard police radio communications very regularly. But the best fun was listening to the local council house repair men, talking to their base, and each other, from their vans.

Sometimes it was too easy to recognise who they were saying uncomplimentary things about! Such as when one of them had gone to such and such's house on a certain road and the occupant was 'too idle to get out of bed' to let them in to do the repair work.

It was fascinating, but gradually stopped happening. I suppose they eventually realised they could be overheard and changed the technology for privacy reasons.
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Old 16th Aug 2021, 2:47 pm   #9
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Default Re: 87.9 Fm Car radio explain

At one time, with respect to PMR frequencies, there was something called 'M' band where the TX frequencies were in the region of 109 MHz upwards and the RX frequencies were around 139MHz - so there were a lot of utility companies and yes, police transmissions which could be received in the upper part of the broadcast FM band for quite some time before the majority moved to 68-88MHz or 158-170MHz (roughly). Another peculiarity of 'M' band was that both AM and FM modulation appeared to be allowed on that band, some of the synthesised Dymar PMR radios for that band could be programmed to 'be' either AM or FM according to user requirements. So some of the transmissions you might have heard on 'M' band may have been difficult to tune in properly. Even FM transmisisons would have been rather quiet, having only a fraction of the deviation that a broadcast station would have.
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Old 16th Aug 2021, 2:47 pm   #10
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Default Re: 87.9 Fm Car radio explain

Frequencies around 102MHz were used by emergency services until the late 80s, especially police area controls. They could indeed be heard using an ordinary domestic radio. The Home Office had been trying to clear the VHF broadcast band from the early 70s, but there were many delays getting the replacement systems in operation.

That is unrelated to the subject under discussion though. Please stay on topic.
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Old 16th Aug 2021, 3:31 pm   #11
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Default Re: 87.9 Fm Car radio explain

Quote:
Originally Posted by paulsherwin View Post
They are normally used to transmit MP3 music files from an SD card or pendrive, or to relay music from a phone via Bluetooth. I guess someone may have left a phone connected to BT while phoning the doctor from the car park.
Normally yes, but some are certainly sold as a handsfree solution for making/receiving calls too.
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Old 16th Aug 2021, 3:56 pm   #12
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Default Re: 87.9 MHz FM Car Radio receives phone calls. Explanation?

I can confirm that some of the FM/bluetooth/microphone things do indeed transmit the audio on the FM channel when used with a 'phone. I bought a couple for expeeriments, ended up using one of these https://learn.adafruit.com/adafruit-...usage?view=all much more fun.
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Old 16th Aug 2021, 9:36 pm   #13
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Default Re: 87.9 MHz FM Car Radio receives phone calls. Explanation?

Yes people had these phones that worked handset to base on around 49MHz and base to handset just above MW, about 1700kHz both NBFM and many an unmodified MW radio would tune in with no problem, and recover sufficient audio by slope detection.

You effectively tapped your own line and broadcasted it to your neighbours.

I even heard a BT spokesman interviewed on the radio claim he had no idea that these phones were so insecure, BT sold them in their showrooms (remember them).
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Old 16th Aug 2021, 10:21 pm   #14
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Default Re: 87.9 MHz FM Car Radio receives phone calls. Explanation?

I still think the explanation in #6 is the most likely. People using Bluetooth connected mobile phones are very unlikely to go to the trouble of unpairing the connection if they want to make a phone call - they'll just turn the volume down on the car radio. I used a very similar cheap microtransmitter in the car a few years ago, and never bothered turning it off.
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Old 17th Aug 2021, 8:44 am   #15
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Default Re: 87.9 MHz FM Car Radio receives phone calls. Explanation?

We often hear customers cars in the workshop burst into harsh telephone conversation right in the middle of music on the radio.
I've often wondered if I'm being listened-in on.
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Old 17th Aug 2021, 10:20 am   #16
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Default Re: 87.9 MHz FM Car Radio receives phone calls. Explanation?

The range on some of these Bluetooth modules is quite impressive too, along with any FM section.

As an example, earlier last year our mode of conveyance failed to proceed & the RAC came to the rescue. Once he was inside the car & I was a good distance away ... I heard him shout 'your phones ringing ...!'

Eh? Is it?, looks at the device, sure enough, it was, so I answered, couldn't here a thing, yet the other half (as I later found out) could her someone muttering about 'that should see them home, rather odd mind you' & the poor chap could hear the other half 'what's broken, who's odd'

Oh the possibilities these days

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Old 17th Aug 2021, 11:29 am   #17
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Default Re: 87.9 MHz FM Car Radio receives phone calls. Explanation?

I heard something that sounded like a 2 tone marker around 88 Megacycles during a lift years ago then a woman came on arranging transport for someone who she called sir several times. I could hear her but not the other person. A few summers back I had my micro FM tx on and accidently changed its frequency right in the middle of my TV program I tuned the FM band looking for it and heard aircraft comms. Obviously a mixing product between my little tx and the plane flying overhead at the time all good stuff
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Old 17th Aug 2021, 3:54 pm   #18
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Default Re: 87.9 MHz FM Car Radio receives phone calls. Explanation?

I was surprised when these micro power 88-108 MHz Tx's were legalised in the UK, I don't exactly remember people marching in the street demanding this facility.
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Old 17th Aug 2021, 5:06 pm   #19
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Default Re: 87.9 MHz FM Car Radio receives phone calls. Explanation?

I think it was down to an EU directive to introduce common standards. They were already legal in some EU countries.

The legal restrictions are still pretty tight, but given the relationship between 'CE' and 'China Export' the market soon became a free for all. The strictly legal transmitters are very low powered indeed.
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