How They Know What You're Listening To On Your Radio! 

Imagine working for TV licencing and going to all the effort of triangulating an unlicenced local oscillator, knocking on their door and saying "Hello, I'm from TV licencing, can I come in?" just to be told "No."

I knew they were monitoring me as they kept giving it away by announcing "You are listening to Radio 4 ........" - now I've had it confirmed.

Back when I was 14/15 I discovered the local oscillator leaking RF at the 1st harmonic 10.7mhz above the tuned frequency on "VHF", so a radio tuned to say 90mhz would transmit at 100.7 a blank carrier. By exploiting this feature I was able to set up an FM pirate station by opening up the radio, locating the oscillator, attach an antenna and modulate the carrier with music from my 3 in 1 stereo, all the neighbours would tune in...those were the days my friend we thought they'd never end.

In the early 2000s two detector Vans used to park up overnight at the fire station I served at. We got quite friendly with the operators who used to pop in for a cuppa before starting work, they confided in us that the Vans were all bluff ! Not believing them they showed us that the Vans were indeed empty of anything !

It still BLOWS MY MIND that there are places where listening to a scanner is unlawful. I have had a scanner on in the background almost constantly since the late 1970s. (I actually have TWO running right now, one for local police and fire, the other sweeping CB, GMRS, FRS and MURS for entertainment, LOL) in Pennsylvania (USA) Scanners are as "regulated" as any other radio RECEIVER. In other words, using a scanner here is the same as using a "normal" AM (MW)/FM or SW radio! Also of course our over the air (Broadcast) TV and Radio is ad supported and thus is free to watch or listen to without a license, In fact the local TV and radio stations want AS MANY listeners/viewers as possible. Back in the 1970s I got a pocket AM radio for FREE while attending a baseball game, The radios were being handed out by the radio station that carried the baseball games!

I saw the inside of a detector van in the early nineties. Except for a crisp packet, it was completely empty.

The main culprit in modern electronic TV are the computer (an ARM chip) clock circuit, and the backlit LED driver. But those signals are drowning in a plethora of clock signals and switching power supplies present by the dozen in any house nowadays. I have problems listening LW band for this very reason. The Powerline adapters, which send Ethernet over the main power line, are a work of the devil, as they radiate powerful harmonics up to the 10 meters band! Thank you for the video, was a trip down the memory lane...

In South Africa we once made an oscillator that ran off a 9v battery. We tossed it into various trees, the signal being so strong it would swamp their equipment. We seldom had much trouble with them after that.

German WW2 uboats were equipped with a radar detector operating in the VHF band used by earlier Allied anitsubmarine radars. The detector was called Metox and it was a superheterodyne receiver. "Metox also emitted a weak signal, a property common to many radio receivers, especially superheterodyne receivers. In an indirect way, this had serious consequences. In the spring of 1943, the U-boats suffered badly because of the introduction by the British of a 10cm ASV radar. But a captured British officer told the Germans that their misfortunes were caused by the transmission of Metox, which were detected by Coastal Command aircraft. After verifying that this was technically possible, the Germans believed the story. This delayed the introduction of Naxos by some months, during which the U-boats suffered heavy losses." Naxos was a newer radar detector built to detect signals with wavelengths between 8cm and 12cm.

Brilliantly researched, as usual, Lewis. Truly the Mark Felton of radio geeks! The German WW2 submarine service was absolutely paranoid about local oscillator radiation - by mid 1942, they'd issued directive after directive on radio procedure and even banned the use of the military issue 'civilian' type broadcast receiver that was issued to U-boats to amuse the submariners (the output could be interfaced with the boat's PA system so the crew could all be entertained) except when the vessel was in port. The Kriegsmarine SIGINT people believed that the LO on this non-military designed radio was particularly leaky. Could D/f be used on a specific radiating LO signal from an operational U-boat? I think it could. The principal sub command net was hosted on a powerful permanent LF/VLF transmitter at Lorient, France, on Brittany's Atlantic coast, which operated on a fixed frequency. Furthermore, U-boats carried a standard complement of radio transmitters receivers (some with their own D/f capability) so, 99% of the time, listeners to Lorient (of the naval rating, Oberfunkmat, U-boat type, anyway) were using exactly the SAME type of receiver. - and therefore, identical LOs. The Allies by this time had captured enough U-boats to understand this. Knowing the receiver IF and also the frequency of the incoming transmission, means that the likely frequency of the radiated LO signal is exactly known. Sure, the power of the radiated LO signal will be extremely low. And, in fact, the U-boat receivers DID use an RF stage ahead of the mixer. But, those valve mixers (triode/hexode or pentagrid) are unbalanced and provide nil in/out port isolation, so SOME of the LO will be radiated regardless of the RF amp stage. Remember that this is a LF/VLF signal which will propagate via ground wave - and over a highly conductive (seawater) surface will get a considerable distance. The idea that an aircraft within a few kms could accurately detect this easily identifiable signal (and home in on it) isn't so fanciful. However, LO radiation wasn't really the U-boaters headache. It was a red herring - but a herring that also caused them to abandon the R600A 'METOX' 200MHz aircraft radar detection receiver device on the same basis, 'vulnerable emissions'. Had they known about British HF/DF detection and most importantly, Bletchley Park's penetration of the naval Enigma, they would have lost interest in local oscillators completely.

Fun fact: I do believe that even though you had/have to supply a name and address when buying a TV, for licence purposes, neither you nor the retailer were obliged to prove that address was correct. Cue thousands of TVs being watched in 10 Downing Street, by Mickey Mouse.

I have only recently come across this Manchester Ringway site. As an ex Eccles stonian now living in Australia and a very long time ago a JET (Junior engineer in Training) at Winter Hill ITA transmitter. I found the article on Winter Hill first on a nostalgia search most interesting and surprisingly accurate. I was there during the effects of the Emley Moor mast collapse and the implications on the Winter Hill mast. I was on my basic training course at Marconi Chelmsford when Emley More actually collapsed. Exciting times as colour for ITV and BBC1 was being implemented. How technology had moved on in the last two decades particularly.

There was a similar system used, in the States, in the 1950s. At prime time, a detector van would drive around a given neighborhood w/ a system to detect which households had their sets on. But not only that, they were also able to detect which channel the sets were tuned to (presumably via the harmonics). These were tallied up for the TV ratings.

Our local cable company (USA) would drive a windowless van through neighborhoods during TV prime time...the rumor was that they were looking for people watching scrambled "premium" channels that weren't paying for them. Showtime could be descrambled on an old Atlanta Scientific push-button cable tuner by depressing the "3" and "7" buttons together 😂 (note-it required taping the buttons down, as the spring mechanism would release when two buttons were simultaneously pressed!).

Local oscillators send a carrier wave further than you would think. Back in the 80s a friend used the oscillator from a grundig transistor radio and just by attaching an fm dipole at the coil and feeding in an audio output, managed to broadcast over half a mile.

Honestly appreciate the ‘plain language’ and analogies. Not a radio geek since the 60s. You brought together your research, visuals and knowledge in an explanation a layperson could comprehend. Really enjoyed it.

Although I understand why licensing of radios and television sets was implemented in the UK, the whole idea of pay radio sounds strange to those of use who grew up in America. The rule of thumb here is that if it's coming through your airspace, then you're free to listen... unless, of course, you're using equipment designed to illicitly decode secure government, military or financial communications. You can get in a lot of trouble for doing that and rightfully so.

This channel just gets better and better. I'd like to see a video about the first Freebanders.

I was in the police when these vans were in use. They were simply transport to take the TV licensing officers to the next batch of addresses and frighten the locals into paying. Detection was 99.9% people confessing on the doorstep. We used to reluctantly enforce the warrants.

I’ve heard that the Cat Detector Van from the Ministry of Housange could pinpoint a purr at 400 yards.

Great video. I was about to mention 'Spycatcher' but you beat me to it! It reminds me of the time I discovered - completely by chance - that by mixing the radiated LO carrier (or a harmonic of it) from a nearby valve mediumwave radio, to my portable, Marine Band radio, I could resolve SSB and CW transmissions from radio amateurs on 160 and 80m. I was chuffed (I was 10!)

As an American I cannot even imagine getting in trouble for having an unlicensed TV. While in the US Navy I knew that during the Vietnam war, we could tell when a receiver was turned on within 50 miles, we could even tell the brand of the radio.

I think it's hilarious how in North America, and many other places where no idiot tax is collected that electronic technicians call bullshit on the whole tv reception detection equipment and propaganda spewed by their respective local authorities. I asked my dad who is a certified electrician and electronic technician who used to service radar equipment (of all things RF). We spent several hours with RF detection equipment trying to catch the famous "European LO leak" and get this YOU CAN'T DETECT ANY RF LEAK FROM THE MAJORITY OF TV SETS OF THE 80'S AND 90'S. We tried for hours on 8 different tv models. The only one the leaked any RF was an old Hitachi from 1977. It could only be detected up to 6 feet away max. So, detection vans are only propaganda to justify stealing money from people so they can keep doing bad things to children.

All our TV here in the states was supported entirely by advertising with the exception of PBS which was mainly supported by viewer donations. Beyond that, any commercial free TV that was available was on cable TV like HBO which we had to pay extra for.

I never saw the point of Post Office detector vans. Post Offices aren't that hard to find!

Back in 1994 at work a FM table radio had a local oscillator signal strong enough to appear on the screen of a spectrum analyzer what was testing a FM broadcast interface for a rear window antenna intended to address high interference in Detroit's Greenfield Road test course due to the many radio stations there. Broadband field strength at Greenfield was over 500,000 uV/M! Two FM stations even created cross band suppression of a AM station at 590Khz!! (The table radio at work was 200 Ft away from the lab! Finally, a shielded room was installed.)

So what you’re saying is that in the 50s, one of the major ways you could detect the presence of a TV or radio inside a home was the electromagnetic radiation coming from its antenna, but in the visible spectrum. Worse yet, the leakage was present even when the device was fully turned off!

Halfway through writing the spycatcher comment when you mentioned it 🤣 amazing that they had planes fitted with the RAFTER system, never mind vans.... but it's also the start of the 'white van outside listening to me' trope that started in TV and movies, All the early vans were white as they needed to be made out of fibreglass, and white was the only colour they could do it in and still have it look like a metal bodied van afterwards. Weird how a little detail like that goes on to spawn a trope that's still being used today

the detector van thing was a case of "they could detect" but in reality it was all a visual threat and as it worked so well in the early days they had no need to actually do any detecting.. the old 405 line aerials were huge too.

No need for a TV licence anymore 🙌

They used a directional microphone aimed at your living room window. Listened to what you were watching/listening to to use against you on the doorstep and get you to pay up.

A friend retired as a cop and got a job driving the detector vans. This was around 2007 if I recall correctly. He told me the van was in fact empty by then and his job was to drive round areas and park up for 10 minutes at a time to be sure to be seen.

When I was in the Army training at Fort Devens, MA, the barracks I lived in had people who would tune in a local AM country music station and blast music in the barracks. I had to use earphones with my radio so I could hear the station I wanted to listen to. One day the country music was very loud. I was tuning through the AM band to check out what AM stations I could hear, when I heard a faint whistle grow louder in the country music until it was almost all that could be heard on the other guy's radio. The whistle grew quieter until it disappeared as I continued tuning through the AM band. When I turned the knob and tuned back, the whistle returned to the other radio. The guy changed to a different AM station and turned the volume up even louder. I tuned back and fourth across the band until the whistle appeared on the new AM frequency. When the other guy found he could not make the whistle go away, by changing radio stations, he turned his radio off and put it away. It was years later that I found out that the local oscillator of my radio was transmitting a signal that interfered with the other radio, causing a heterodyne noise, which was the whistle. Even though I did not know what it was or how it was happening at the time, I realized that I was jamming the other radio and used it to good effect. Whenever someone turned up the volume of their radio to an annoying level on an AM radio station frequency, I would get my radio out, put earphones on, and tune around the AM band until the heterodyne jammed the other radio. In a short period of time the other radio would be turned off. I also found out that it only worked with AM radio stations and not FM.

I’m an amateur radio operator and I had no idea about this!

I am surprised to hear that people have been prosecuted for listening to transmissions. Here in New Zealand, there is no law against listening to anything. Basically if you can hear it, good for you. The only thing you are not allowed to do is act on any information you hear. For example, if you hear that the police are about to raid your neighbour's place, you're not allowed to call them and tip them off.

Genuine thing: just outside a supermarket in a carpark in Oxford back in the late 90s when I was a teenager, I spotted a rather conspicuously parked "TV detector van" (fairly near the shop's entrance, but not too close). I remember one mother commenting to her child not to go close to the van with the blacked-out windows. I on the other hand was both far too curious and was prepared for any consequences. Sure enough as I suspected just with a bit of life knowledge from the shape of this vehicle, when I cupped my hands on the side window to block the outside light to see into the van it was just a regular minibus. A classic bit of propaganda. Of course TV licence "detection" is mostly done by having to register your details with the retailer at the point of sale and of course the rare visit from the licensing people.

really informative and an enjoyable watch, cheers M7SPI

This video makes all of the Monty Python sketches about detector vans make a LOT more sense now..

Redifusion set up a large cable undertaking here in the south east essex uk in 1964 it closed the antiquated 5 channel system in 1997 due to competition from the superiority of the the multichannel vrigin media system ...in late 80s though they introduced Premier movie channel .£10 per month ..but my friend noticed that all they encrypted it with was a simple signal that they removed with an inline notch filter at your end ...so a few electronic buffs soon copied these filters / pcb .....lol.....and sold them in local pubs ...for £20 sovs Or £30 sovs if you wanted the fitting service x

SWIM modified a Walkman in the 90s to modulate the IF and would play their own music over restaurant radio PAs that they patronized. Of course now everything is Bluetooth. The good old days.

Fortunatly TV licences were abolished in Australia in the early ‘70s I think by Whitlam, I remember a TV ad which showed a man furtively watching a TV and then he gets a knock on the door. I recall it was $12 a year and included all radios not just TV and you paid at the post office. (PMG).

Across the pond in the 'States', there was something called "The Tempest Standard", also known as Van Eck phreaking or radiation. Specialized equipment was used to detect leaky signals from CRT Displays, such as TV's or Computer Monitors, & display them on a remote display. With the right equipment, this was successfully accomplished from a respectable distance, unbeknownst to the surveilled individual.

hi lewis i can remember the old 405 line tv set,s from the 70,s they used 6 meters back then i lived high ground near bath we could get atv southern westward and at times thames tv many moons ago now thanks m3hnl warminster cheers lewis

Fascinating!

I missed this first time around & just found it. So many people think detector vans never worked but I had a run down of how the detection vans worked & got to see inside in 1984. Then they got decommissioned in 84 to be replaced with fake vans looking for just the masts. I was told at the time that the newer TV could not be detected so days was numbered This also gave me an insight into why you could not share a TV aerial as before the filter design change they would interfere. Like the end of the detection vans so did my apprenticeship but the first few weeks I learned so much before leaving. I was NOT going to be part of the detection team but the same company.

Thank you. I've often wondered if that were possible - yes it is! Looking at the intermediate frequencies generated by amateur radio equipment I think it might also be possible to identify which make and model (in some cases) is being used.

As you know Tempest proofing both buildings and kit was big during the Cold War. A principal driver for these efforts was the knowledge of just how much information was being gleaned from Soviet bloc kit. Nothing was covert in central London, the I.F. receiving aerials were just impossible to hide. The easiest to spot were between the buildings on Church Street and Kensington Palace Green/Gardens;with the obvious data sources being the various Soviet bloc buildings lining the private road. Top left side of any southbound bus gave a grandstand view if you were looking - very few were. These were very large vertically polarised aerials with limited swing. After the Cold War ended the site became a large housing block with eye watering service charges. Regarding information in the public domain:- Wikipedia has a Tempest category which covers a wide range of topics. Remember to check the edits log for some of the information submitted by the public.

Great video,thanks,I never thought about the IF stage leaking. None of the techniques you described would work these days as the noise floor or EMI in residential areas is about S6 to S9 due to cheap electronic appliances and solar panels.

We never had to license our TV's. We wouldn't have stood for it. I realize that conditions were much different during the war in Britain, but even our power grid now emits "broadband - over - power - line" interferences because the power companies just got lazy and didn't want to pay a meter reader and so set things up to read meters remotely. A Ham Operator has to endure so many illegal signals that sometimes we receive nothing but illegal interferences. So, one day I had had enough. I put a low frequency bandstop filter on the incoming mains, and then searched out every switching power supply there was, and replaced them with transformer operated power supplies. Did it get rid of all the noise on my receivers? No, but after shooting out a security light often enough with my airgun, they finally gave up on it. No noise anymore. I imagine that I had pretty much the same equipment that your "post office" officials had to search out all the offending noisemakers. In America, Ham Operators police ourselves - and others. Those who do not cooperate are taken care of.

Bloody hell i have a lot to learn about radio! Lost in lingo in just the first minute :) Just got my first handheld, the uv5r. Cheers, subbed :)

Imagine being required to have a license to watch television. The only thing harder to imagine is an "official" visit because you're watching television.

I had no idea there was a TV mafia in the UK

Thanks Lewis

i think there are flaws in the circuitry of the receivers that the police can detect such as the local oscillator and the oscillator used to set up the cancellation to extract the signal. there was a device back in ww2 that was supposed to go between the antenna and receiver that would block the stray signals from coming back out to the antenna so you could listen in secret.

Thankfully in my country it is still legal to listen to radio.

Using a very primitive set up with just a short wire antenna and an RSPduo SDR, I can pick up the LO from a small handheld MW radio about 5metres away - so not much. Has anyone any idea of the range with a more optimised set up (good directional antenna, LNA and a professional spectrum analyser)? I imagine that tube receivers with an attached outdoor antenna were also much better radiators.

The UK detector vans didn't actually have any working equipment in them - only tea and coffee making equipment and a chemical toilet.

I rememeber this forn the ham school study, i almost forgot this. thanks.

The technique was called Rafter (Wright), I've used it before some years ago. But have had little success with modern kit. In the modern urban RF environment without knowing what the target is monitoring and what kit they're using, Rafter is very difficult.

Very well played my friend a video time of 555!

That's a Hammarlund HQ-120X receiver in the center at 2:50. It had an RF stage which isolated the oscillator from the antenna.

Who was the comedian that said "watch out for the vans with the spinning roof racks" ? lol

Imagine back in the day the lengths people would go to to avoid detection and now we just give governments and corporations our most intimate personal data, our thoughts, feelings , our contacts list, our tastes in hobbies and pastimes, our finances and purchases and our political and sexual persuasions .....all through our phones. We give it up freely without a second thought.

My nan had a visit from a van , she was watching Culumbo

interesting about the tv vans and how they "worked" but im not sure as there has been no court cases (as far as i know) where evidence gathered by a van has been used and theres anecdotal evidence from ppl that used to drive the vans saying they didnt work it was all fake equipment , they had a list of houses that didnt have a license and they just drove down the street looking for at first an aerial and then the flickering light from a tv and reportedly turned a tv on inside the van and just went through the 2 or 3 channels until they matched the flickering seen through curtains when matched they knocked on the door of the target house and i was told by a salesman that the uk in the 70s and 80s had to report the addresses from tv sales to the relevant body at the time.

Always thought TV/Radio detector vans were just a ruse - thanks for explaining!

I’ve got this book and read about this! There is a additional book to this one.

I can't believe in this day and age, Britain still has TV and radio licences. We got rid of ours down under ages ago.

By accident I found this out, I was listening to npr on my radio and I found a weak npr station in the kHz range. Couldn't figure any harmonics for 81mhz then I tuned the radio and realized it was the intermediate frequency stage in my receiver. You could drive around listening to that most radio use the same IF

WOW,I DID NOT KNOW RECEIVERS EMITTED A EMISSION..THANK YOU

came here to comment about spycatcher, but you got that. 10/10 there technique of listening to the LO and broadcasting on the frequency of interest and hearing the LO "squak" ment the reciever must have been listening to that frequency. super cool

We had a similar scanner van thing apparently here in Sweden, though when my mom worked there (Radio & TV controller) some 10-15 years ago it was more of a prop then anything. Nowadays we don’t have any fee though, after they changed so it became a tax instead and every single person has to pay since a few years ago.

To be honest they can tell what your radio is tuned to when it is turned on, but they can't tell if you are actually listening to it.😊

Peter Wright: Spycatcher. They had/have it in the MI5 too, they called it RAFTER.

This reminded me of the World War 2 "Foxhole radios" sets built American soldiers. These were essentially homemade primitive radio receivers using a pencil lead instead of a crystal as the diode ("cat whisker") and didn't use electric components to give away its location like regular radio receivers would. These radios were the only way American soldiers could hear radio stations for news and entertainment.

Here in the US the radar detectors detectors do the same thing for the police looking for people using radar detectors on the interstate highways or any roads to drive above the speed limits.

THANK YOU ,NICE WORK,,LOVE THE HISTORY,,, DES CREAN,BELFAST,IRELAND

I thought you were Jason Manford when you started speaking :) Thanks for the video!

Thats a shame they went after scanners. I'm in a rural area in the US and they are essential for us farmers who have to fight fires too. Super useful to be able to hear the FD about how a fire is progressing or to hear what they want to do with air support.

Very interesting yes.

I used to know what TV channel my neighbor was watching on his CRT-based TV since I could pick up the oscillators from his TV on my low frequency receivers.

As a "entitled American" im shocked the tv feds would come looking for unlicensed tv sets! WHAT THE ACTUAL FK!?!? WOW

We are watching them too 👁️👁️

Back in the 1980’s when Australia has CB licenses the local radio inspector would drive around DF’ing 10.7MHz and 455kHz IF’s.

Very interesting. Had no idea a license was needed for television back then.

It is possible to determine the frequency a radio is tuned to by picking up the local oscillator provided you know the intermediate frequency. Standard intermediate frequencies are 455KHz, 10.7MHz etc. If a receiver was constructed with a really unusual IF then the local oscillator could still be picked up but adding or subtracting the usual offsets will give the wrong answer.

The Post Office detector van was a tool used to frighten people but of little actual use

My antenna is outside, however my receiver is inside a pole barn. Would they pick up my receiver that is shielded by a metal pole barn?

2m26s Lovely to know you needed a car radio licence for £1 and 5s. Didn't know that. Sherpa Van - A-reg. I used to drive one for the railway. Lucky they got theirs started each morning with all the kit inside! And the bloke in passenger seat just has a list of addresses without a current licence. Let's just knock on Mrs Jones' door at number 29 while her husband at work and she'll spill the beans.

@4.20, 'We have definitive proof that they were watching TV, they had an Antenna!'...🤣🤣🤣🤣 Ya'll Take Care, John

Another very informative and interesting video. 73 M7TUD

Here in Germany it's even worse (or easier for the government). Everyone has to pay for a license if he owns a radio or TV or even any internet capable device. So every household has to pay that fee and they can opt out if they can proove they don't own any of those devices. But as everyone owns ar least a phone, it's easy to say everyone has to pay.

Thats interesting .... I heard on the radio the other day that TV Licencing admitted that the detector vans were actually fake and were used as a kind of deterrent because they couldn't actually tell if a tv set was on in any particular house 🙂

Hello: In your country you needed a licence to watch TV? (!!!). it is unusual today but in Argentina you just put an antena and watch some channels, analog and digitals. I still have them . Vans are very usual, filled pickig illegal FM Radios and use >100 m high antenae to investigate them. Cheers from frozen Patagonia.

Foh kin hell 😂 all news to me! Hehehe.. Excellent video 👍

Police in Australia used modified seized radar detectors to pick up the local oscillator of other radar detectors. No point having one your going to get caught and add more detectors to the pool. I did consider modifying one to detect the detectors.

What were the radio telescope dishes used for?

The TV detector vans from what you have said appear to work the way I thought. From various FOI requests, they still say they still work but I have a sneaking suspicion it all fell apart when PC CRT monitors, etc became common. Are you aware of any way they can still determine or should I say find out if you are using live TV? I ask this because in no court cases has tech evidence been submitted as far as researchers can ascertain. Thanks for the video as usual great content.

I built a one valve regenerative radio kit, H.A.C., in 1969 and confirm they put out a signal. Spy radio operators in WW2 were located by the regenerative receiver as well as by the transmitted signal, the most well known was the Paraset. When I tell people a t.v. could be detected by a van they don't believe it, yes, around 10 and 16 kHz with harmonics into the 160m and 80 m bands causing a lot of noise to radio amateurs. I knew when my neighbours switched on a t.v. so a detector van was very possible. G4GHB.

I don't know it it is true but in the days of TV detector vans it was rumoured that they could detect which programme you were watching by checking the synchronisation of the line scan stray field they were detecting with the transmitted line rate for that station. Thus eliminating false positives e.g. from early computer monitors made from tuner-less TV sets.