I have an Icom IC-707 in my truck, a 2003 F150, with a stainless 102 inch whip mag mount on the roof. It has a a coil pack 6 cylinder.

Without it running I have no problem; as soon as I start the truck up I have horrible interference all the way up to S9.

I have tried pretty much everything now that I can think of.

I have researched about spark plugs with resistors: special sorts of cables with high resistance, I've ohmed out my shortest cable and it had 8,000 ohms my spark plugs had about 3200 to 3,600 ohms.

It's not coming from the feed line because I can run it off a separate battery not connected to the truck and it still happens; it's not the fuel pump because I turn the ignition on and it primes it up. You don't hear it it's picking it up through the antenna.

Does anyone have any advice about what I need to do to stop the ignition system, which I believe is a culprit just stopped this interference? Do I need to get spark plug wires with even higher resistance? Also, these are brand new wires, I tried that, and the plugs are not that old, maybe about 6 months. I've also tried grounding the radio to the frame and the antenna to the frame: no difference. I'm 90% sure it's my ignition.

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    $\begingroup$ Hi! Welcome here :-) I tried to structure your text a bit; hope that's welcome! $\endgroup$ Aug 31, 2023 at 17:13
  • $\begingroup$ Whoever told you that you need to measure the resistance of your spark plugs certainly was kidding. That makes no sense at all; so I'll also ignore the very unphysical measurements that you've got (these are spark plugs; they literally are just a spark gap, electrically – their resistance to anything that your measurement device could generate should be "infinity"). The resistance of your wires is beyond doubt; there's very little a manufacturer could "do wrong" there, without your motor not working well anymore. So, these measurements would've made sense on a 1965 VW Beetle, not on your car. $\endgroup$ Aug 31, 2023 at 17:16
  • $\begingroup$ What could indeed be a reason for heightened EMI would be if sparking occurred outside of the motor block, for example if a cable broke, or was not actually making good contact (but even then you'd notice by significantly reduced drag in your car – one cylinder would fire unreliably). So, maybe check whether all cables are snug on their connectors, don't use much violence $\endgroup$ Aug 31, 2023 at 17:25
  • $\begingroup$ How old are the spark plug wires? I have seen deteriorating insulation cause RFI. Arcs from wires to the engine could be seen in the dark. $\endgroup$ Aug 31, 2023 at 17:43
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    $\begingroup$ I had terrible interference on an HF radio from my 2012 GMC SUV, like you AI7OW, it was only when the engine was actually running, and it was not ignition noise, which is usually a pulse, like rapid ticking; it was more like wide-band white noise, but interestingly, its intensity went from not being able to be heard to S9, every 20kHz, so because of the sound of the noise, I concluded it was likely CANBUS communications noise that was interfering with the mix-down stage of the radio, which is why it came and went every 20kHz. $\endgroup$ Sep 1, 2023 at 12:55

2 Answers 2


There are many things you can still do. Here are a few quick suggestions to start with:

Proper antenna ground. A mag mount is marginally OK at VHF / UHF, but there isn't enough capacitance at HF. The antenna needs a solid ground connection to the vehicle body.

Assuming basically no grounding from the mag mount, you have a sort-of dipole antenna, with the whip at one side and the coax+radio on the other side. This unwanted part of the antenna extends to include the microphone cord and the power cables which run all the way to the battery, the battery negative-to-engine block strap and finally the engine ground strap. This maximises the pickup of RF noise from the engine compartment.

If you must use a magnet mount, you should find a way to ground the outside of the coax properly, just as it enters the vehicle. Cut off some of the jacket, solder some braid to the coax, and bolt this to the vehicle. Or install a (SMA) connector, and mount a F-F barrel to the vehicle. Ferrites on the indoor section of the coax will also help.

Vehicle Panel Bonding: The next step is to bond the body panels together with earth straps - first the hood, the box (might not be connected to the chassis) and then perhaps the doors. This blog post shows some photos and gives some more hints. I think the straps don't need to be as thick as they show (this is not for lightning) so rather use a few small straps with self-tapping screws. Clean off the paint first, and re-coat after assembly for corrosion protection.

Panel bonding will help with both the performance of the antenna, and the pickup of interference from inside the vehicle.

Ferrite beads on the power cables: to reduce the transfer of noise from the engine you can try clipping some ferrites over the power cables. Important to clip over both + and - leads, the ferrites won't work if they see a large net DC current flowing through them. You could try beads both ends of the wire. If you've tried a separate battery then perhaps this isn't the most important step.

Noise Blanker: The spark-plug noise will be hard to eliminate completely. Most radios have a NB feature which mutes very short impulses, without spoiling the sound overall. If you have spark plugs, you will need the noise blanker.

You don't describe the noise in detail - I'm assuming it is ignition noise, which is most likely, but there can also be alternator noise and alternator whine, and more general interference from the vehicle electronics.


Just a few random ideas which might help.

With the truck running, tune the 707 all the way down to 160 meters and measure the noise. Then turn on a portable AM radio, or the AM radio in the dash, tune it to 1600kHz, and measure the noise. If the noise is radiated EMI then the AM radio will pick it up too. If it is conducted then the AM radio will not pick it up.

Next turn on the 707 and tune it for a good strong noise signal. Then disconnect the antenna coax and measure the noise. If it disappears with the coax disconnected, it is radiated EMI.

Get you an oscilloscope and make a little air-core loop antenna for it. With the engine running, pop the hood and sweep the antenna around within the engine compartment and look for radiated noise at the frequency of interest. You might be able to focus in on the noise source by moving and rotating the loop around.

Another note: does the noise scale up in frequency when you rev the engine? If so, it's got something to do with the fuel injection, ignition, or charging system. If not, it might be coming from the main "black box" computer that controls the fuel and ignition systems.

  • $\begingroup$ Thank you very much. Very useful information. This week and I was doing a little bit of experimenting and I'm almost positive it's coming from the fuel pump. Apparently this is a very common issue with Fords and they used to sell a type of filter you install right at the fuel pump which is discontinued because of the age of my truck. It's odd five megahertz and there's pretty much no interference. I forgot to check on my truck's AM radio I will do that but I would imagine if there's none on the icom there will be none on my truck's AM radio. If I get home at a decent hour today I'll try more $\endgroup$
    – Plumber90
    Sep 5, 2023 at 16:09
  • $\begingroup$ If your Ford is fuel-injected, would not the fuel pump be inside the fuel tank and hence incapable of radiated EMI (because the tank would serve as a faraday cage)? This suggests the interference would be conducted i.e., broadcasted through the wiring harness- and capable of being easily filtered out of the DC power line connection to the ICOM using a series inductor and a shunt capacitor. $\endgroup$ Sep 6, 2023 at 6:52
  • $\begingroup$ Plastic my brother, plastic! But good thinking. It's not coming from power line. With antenna unplugged its gone. $\endgroup$
    – Plumber90
    Sep 7, 2023 at 9:31

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