Car fob RF protection

After many months of deliberation I have finally changed my vehicle from a 2005 vintage one to a 2020 vintage one. The new vehicle is ‘keyless’, accessed by a fob which is then required to be within the vehicle to start it by pressing a start/stop button. All very nice and modern, but without any actual key, the system worries me somewhat.

With my old vehicle I had a few incidents where activating with the car key fob in my pocket actually corrupted the fob. I suspect using a 40 watt linear on 70cm was the cause, but I cannot be sure. Returning to the car I was unable to gain access using the central locking system, but having a physical lock and key I was able to gain access and then re-match the fob to the vehicle. With a keyless system I would have been in trouble.

To avoid any issues activating with my keyless fob in my pocket I am going to purchase a ‘Faraday bag / pouch’ so RF should not be an issue. I wonder whether other activators take such steps to avoid potential problems, or am I worrying too much?

73, Gerald

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I also have such a system in my car (Subaru Outback) that I bought new 3 years ago. Problems with the key, which I always have in my pocket, I have not yet noticed … no matter how close I was to the antenna.
However, it probably makes sense for security reasons to have a shielding cover for the key.

73 Armin

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Good Morning Gerald,

I’m sure your new motor is lovely. I have had two keyless cars now. I like the convenience and the other associated gadgetry that comes with a modern car. The current car a Ford Kuga ST PHEV. It has a physical key secreted within the keyless fob. I’m not sure if this is common to Fords. I have needed to use the emergency key on two occasions in 18 months, both times at home to get into the car when the 12v battery was only providing 4 volts, so there was insufficiient power to operate the keyless door locks or anything else. Maybe your model does not have the emergency pull out key built in to the fob? Worth mentioning in case you have one but wasn’t aware of it. This physical key won’t start the car though. The electronics in the fob needs to be working and in the car to start it up.

I don’t operate 70cms so I don’t foresee any problem with wiping the keyless function out with HF radio.

73 Phil


Hi Gerald @G4OIG

I’ve had a few vehicles with keyless fobs but this year I was prompted to buy a Faraday pouch and Faraday box for the house in response to posts on a Facebook user group for my car. These posts were in relation to stealing a car after remotely copying the coding, like you I thought was I worrying too much?

Anyway, the Faraday pouch works extremely well and the car and fob will not talk to each other next to the car so it could well reject your RF. Like Phil OBK I have a key secreted in the fob which will at least open the car.

73 Allan GW4VPX


Hi Gerard,

Some folk have trouble opening their car doors in the car park at our local radio club. I understand the interference (receiver blocking) comes from a nearby warehouse’s security system. The ACMA regulator isn’t interested, mumbling something about no protection to LIPIDs. No money in it, no big corporate customers, so go away.

I bet this isn’t unique, either the interference or the fobbing off of complaints.

I have a Japanese vehicle with a wireless link for cabin entry but haven’t experienced the problem.



Probably. I’d worry more about having proximity unlocking where you just walk up to the car and it unlocks when you reach for the handles. I’d have that disabled so you need to press the fob unlock/lock button instead.


Just became the owner of a new Subaru Crosstrek. While I don’t have an answer to your specific question, “will a 70cm signal interfere with the keyless FOB,” I can offer you some related information.

Your key has two ways to turn it OFF. One way, the simpler, puts it “to sleep.” Hold the lock button down, then press the Subaru emblem button rapidly twice. The FOB will respond with the tinsy LED blinking several times. Press any key to wake it up.

Doing this also protects your car from theft. A thief approach is done with two people, a man-in-the-middle attack. After you walk away from your car, one man stands by the car while another follows you and can be sixty feet back. The man at the car intercepts the car’s signal searching for the FOB. He relays this signal to the man following you, who rebroadcasts that stronger, and picks up your FOB’s response which is relayed back to the guy at the car who then opens the car and because he has the FOB’s signal, he can start your car and drive away.

At least, that is what I have read on the web is how it is done.

Your car also has a “secret” PIN coded button on the tailgate. You can program it with a pin, then put your FOB into “sleep” mode, and leave your FOB in the car. You can open the car with the PIN on the hatch.

73 fred kt5x


I use a pouch for my bridge & toll road 900 Mhz transponder when not needed.

Elliott, K6EL


Hi All,

Many thanks for the responses. Food for thought there. I think I will invest in a couple of pouches for use at home and on the hills. It makes sense, even if I might be over-reacting. The pouches are not expensive, so why not?

A few years ago when Paul G4MD and myself were staying in a Travelodge we had an issue with our access cards that got us onto our rooms. That was definitely down to RF and in that particular instance probably not down to 70cm. Thankfully the staff on reception were happy to reprogram the cards.

Some years ago I met Nobby LX1NO on Wendover Woods G/CE-005 and while he operated HF I made a few contacts on 70cm. I well recall my car central locking system not liking it.

Indeed Armin. However I do wonder whether the whole issue is over-exaggerated. I can’t see techno-thiefs sitting at the parking spots for SOTA summits waiting to copy the fob data of unsuspecting activators.

It does have a key Phil, but as you say it gets you into the car, but won’t start it. It could be an expensive exercise to have to call out someone to get the car going, probably involving the nearest main dealership. These things are made complicated for a reason… it keeps it all “in house”. :hushed:

That’s good to know Allan. All this is new to me. The new vehicle is like going from a biplane to a jet fighter! :grinning:

That’s more efficiently produced than mine Ron which is manufactured in Germany and apparently assembled in Mexico… really, what’s that about?

Precisely my thoughts… the idea is to assist with getting the shopping into the car for those that can’t manage it using the fob. Absolute madness.

As I said earlier Fred, I’m not too worried about thieves lurking around remote parking spots in Scotland. I’m working my way through the 400+ page user manual and I’ve not yet seen mention of pin code access, but it might be specific to your make. I’m more concerned about the potential for my activation zapping the key fob when on a summit. It looks like the pouch idea will give me the comfort I need.

That’s interesting Elliott. I can see that becoming the norm over here. There is already a move to more and more congestion charging in cities, not as though I go into many cities. :grinning:

73, Gerald

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Simply putting the FOB into “SLEEP” mode serves the same purpose as a pouch, … without a pouch. If it is OFF, then RF isn’t going to bother it. Press and hold the “lock” button, the double tap the “Subaru” labelled button. Any button to wake it up.- fred

All of our contest group have keyless entry and keyless ignition cars. We don’t take precautions(*) with fobs when contesting at QRO levels on 6/4/2/70/23cms.

(*) I leave my keys in the cupholder in the car when contesting now simply so I know where they are!

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Around Newcastle and the North East, Faraday pouches or boxes have become essential for overnight car security. There are small groups of thieves who have obtained car fob scanners and zappers and they use these to open up cars parked on drives and take what they want. Three people in this small street of ours have had things stolen. We have caught sight of them twice on our security cameras but not usable images. The police are fairly sure they know who these people are and are frustrated that so far they have been unable to catch them in the act or obtain good evidence.


With high gain antennas at height, maybe there is less RF floating around at ground level than when sitting directly under a small yagi, just a couple of metres away (must do the calculation sometime). Anyway, whatever the situation, I’m all for taking precautions wherever possible. At less than £4 for a Defender pouch, it makes sense. I’d rather use one than arrive back at my car and the fob not work.


Some years ago i was sitting in the small car park at Win Green G/SC-008 (a short walk from the grassy summit) talking to a contact on 70cm. After a while I suggested that I might go quiet for a short while as the chap in an adjacent car was getting very upset that his key fob wasn’t working and he couldn’t unlock his car and I had a feeling that I might have been the cause. I thus went slilent and very soon after his key fob worked fine and, much relieved, he got into the car and drove off. I have a feeling this may have happened somewhere else which gave me the idea but this time I was pretty confident that I was the culprit.
73 Viki MBWA


I don’t think you have caused a malfunction.

The power consumption of the key is noticeably higher than if you only have a remote control to open and close the car.

When the voltage is lower, there are sometimes drop-outs. A low voltage is shown on my car’s display.

The position of the key outside must be more precise. In my car, the range is quite short anyway.

73 Armin

If the key uses 433MHz (common in Europe) then UK 70cms FM operation, simplex 433.5 +/- and repeater duplex 433.0 +1.6MHz +/- will normally totally disrupt car keys. A lot of comfort keys have now moved to 868MHz in Europe now as the band is less congested meaning 70cms is no longer a killer.

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It’s an interesting problem. An approach used for keeping RF inside / outside screened rooms / buildings is to make any penetrations into waveguides in cut-off at the frequency range of concern. So instead of a full Faraday cage it could be sufficient to drop your keys into the bottom of a cylinder, open at one end. For example, a 100-mm diameter tube has a cut-off at 879 MHz. It needs consideration of just how long it needs to be for the decay of the evanescent wave to attenuate the transmission sufficiently that even with a state-of-the-art LNA it’s not practical to recover the signal. Nevertheless I’m on the lookout for a nice decorative brass shell casing. :smiley:

But anyway the other side of the attack is the need to prompt the UHF transmission with the LF transmission of the car ID (150 kHz). So I wonder just how effectively a bad actor can do this if you just put a good number of metres between the keys and publicly-accessible spaces. Are those Faraday pouches effective against the LF or actually only the UHF? Presuming they’re not (especially in the near field), what would be effective?


A lot of the repeater attacks (criminal-1 stands by your car repeating the probing transmissions the car sends looking for its key, criminal-2 is by your house retransmitting the probe signal at high strength and listening for fob response and sending that back to criminal-1) are effectively disrupted by the key fob going to sleep. A lot of modern fobs have motion detectors in the fob and when the fob is stationary for several mins it switches itself off and doesn’t respond to probes. Only when you are moving about with the fob in your pocket etc. will it send its unlock signal. This makes the pouches effectively unnecessary as the key wont respond when hanging on a hook by the door, or in a dish in the hall.

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Wave a stick through your letter box.

I don’t have it. Mine was an ex-demo car but it’s not fully loaded (no auto unlock, no reversing camera, no auto-park, no rear heated seats). If it had come with it I’d have got the dealer to disable it if I couldn’t turn it off in the 200 billion pages on online menu options.

I’ve often been concerned that parking my car by various TX sites could cause problems. i.e. when at Anthorn last week, was there enough LF RF that it could overload the car RX and I’d not be able to start it again. Not an issue. Likewise when photographing Skelton (multiple 250kW HF transmitters). Not on the same frequencies but just so many V/m that the car gives up. But you see workers cars on the site so it must be OK.

Gerald is the only person I’ve heard off who has had issues with the key fob. All other issues I’ve heard/read about have always been RF blocking the car RX.