RF Insulating a box

Only on topic because it involves RF. The temperature controller on my motorbike heated jacket packed in years ago and recently I decided to introduce a new box. I have to say I bought cheap here. It uses pulse width modulation to control the average output power to control the heat of the jacket. It works great, and is beefy enough to handle around 6 amps at 14.2 volts.

Unfortunately it appears to be interfering with the keyless RF system (first world problems I know!). I could put it in a metal case, but is there a way of coating the inside of a plastic case to reduce RF leakage? I can imagine it is the sort of thing someone in this community has had to do from a radio perspective.

Thanks, Mark.

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I couldn’t resist asking ChatGPT:

There are a few low-cost materials that can help insulate a plastic box and suppress radio frequencies:

Copper tape: Copper is an excellent conductor of electricity and can effectively shield against radio frequencies. You can use copper tape to line the inside of the plastic box, ensuring that it is well grounded.

Conductive foam: Conductive foam is a low-cost material that can provide excellent electromagnetic interference (EMI) shielding. You can cut the foam to fit the inside of the box and use it to line the walls.

Aluminum foil: Aluminum foil is another low-cost option for shielding against radio frequencies. You can line the inside of the box with aluminum foil and connect it to ground.

Ferrite beads: Ferrite beads are small magnetic devices that can be placed on cables to suppress radio frequencies. You can use them on any cables that enter or exit the plastic box.

Shielded cables: Using shielded cables to connect any electronics inside the plastic box can also help suppress radio frequencies. Shielded cables have a layer of conductive material that surrounds the wires and helps block any unwanted signals.

Note that it is essential to ensure that any materials used for insulation are well grounded to prevent the buildup of static electricity, which can cause damage to electronics and increase the risk of fire.


Given that Aluminium foil is hard to solder to I’ve decided to go down the self-adhesive copper foil tape route. I’ll give it another go and see if it is air radiation that is causing the problem, if not I’ll have to think about choking the in/out cables.


Hi Mark,
Before you go taping up the inside of the box - take a look at the PCB board and see if there appear to be holes for missing components on it - if so after EC/FCC certification the manufacturer has removed the RF decoupling capacitors to save 3 cents - put them back and you may find the RFI goes away …

73 Ed.


How do you know it’s radiated interference and not conducted? If it’s not radiated you’d be wasting your time screen the plastic box!

Should be fairly trivial to prove if it’s radiated or conducted.

I can’t help thinking that the heating elements in the jacket might be acting as an antenna, with the signals being conducted out of the box on the leads. As Andy says it should be fairly easy to differentiate. Wrap the box in screening material or put the jacket in a screened bag. Some chokes on the input and output might help. If it’s a PWM switcher it clearly has plenty harmonic content. Another power source for the jacket would confirm if it’s conducted emissions back into the bike electrics.
Apologies if it’s a Grandmother and eggs situation.

Yes, that’s what I’d do. Power the blighter from a SOTA LiPO and see if the keyless stuff now works.

The Law of Sod says there’ll be some of both :wink:

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Of course I didn’t think of this and it is entirely plausible, if a little scary, that I’m wearing a wrap around antenna!

Some great answers here, but yes @MM0FMF is right, it is mostly likely to be both radiation types! I can certainly power it off a LifePo4 and see what the result is for sure!


It will be switching a somewhat inductive load on and off with a sharp edge. That will have consequences :wink:

Thinking about the “wraparound jacket antenna” a bit more. I’m guessing the heated jacket is worn under the main motorcycling jacket with the RF keyfob in an outer pocket. You could try lining the inner surface of the motorcycling jacket pocket with foil to provide a screen between the heated jacket and the keyfob, yet leaving the outer face of the pocket clear of shielding to allow the keyfob to interact with the ignition system.

Shielding and filtering is unlikely to be the correct fix.

A jacket heater only needs simmerstat control, i.e. very slow speed pwm e.g. 2-5 second long pwm cycle. It does not need a SMPS type voltage control.
If this unit uses an SMPS chip (i.e. there is a power inductor in there) then it is the wrong type of control.

  1. You want slow control - reduce the pwm frequency at least down into the 10’s of Hz
  2. There should be rise time control of the switching fet gate so the current has a slow rise time e.g. 100us. A series RC, or just a series R (e.g. 100k) to the gate, since power fet gates are very roughly 1nF
  3. Those electros should be across the input, but not the output.

Never had a heated jacket, but handlebar heaters are a gift from the gods.


Just a quick update, and to say thank you for everyone’s suggestions. I normally have the key in my trouser pocket, and I know the system is very sensitive to having it within range anyway - there is a little compartment on the tank that is the suggested location, but of course if you happen to walk away from the bike forgetting to take the key someone has a free ride (well, apart from the GPS tracker).

I added ferrite chokes to the 12v supply and jacket output leads and this definitely made a difference yesterday. I only briefly had a warning that the key was out of range.

The foil tape came yesterday so I’ve lined the box as best I can, avoiding the ventilation holes. Will see if the combination cures the problem.

Copper foil tape is really tricky to apply!

Oh and if you have a motorbike I highly recommend the Roadlock - it may be expensive but it never gets left behind and doesn’t cause damage if you forget it is there and set off…

Regards, Mark.

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An alternative or addition could be this shielding spray that is designed for plastic cases.

Kontakt Chemie 35

KONTAKT 260: Shielding varnish, EMI 35, 200 ml, highly conductive at reichelt elektronik

73 Joe

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Hi Mark,
I don’t see in the thread the use of silver based conductive paint. You could apply it to the inside of the box, but avoiding common mode currents as the conductors pass into the enclosure is always a nightmare.

We went for a ride today and although my wife was wearing the heated jacket we didn’t have any problems. I had lined the inside of the control box, apart from the vent holes, with the copper foil, ensured it was connected to ground and that both halves had conductivity between them.

Hopefully that is the problem solved.
Thanks, Mark

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