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   Home   Help Search  
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Author Topic: Ignition noise  (Read 8294 times)

Posts: 65

« on: March 12, 2014, 11:44:54 AM »

I'm getting what seems to be ignition noise in the receive of my 2m APRS radio in my SUV.  The setup is a 1/2 wave larsen on a fender mount.  The rg58 runs into the engine compartment for about 18 inches before passing the firewall.  Then about 3-4 feet in the cab before connecting to the radio.

The truck has a coil/distributor/ht wires, not coil-over-plug design.  The only suggestion I was able to glean from was new plugs and wires (truck is 17 years old, and the wires look original).

I was wondering though, there isn't a good ground connection to the hood.  Would there be some benefit to tying the hood to the antenna ground with a could good ground straps, and using that to have the hood hide the engine compartment from the antenna?  Are there other things I could try?  Right now when the engine is running, I can't decode any APRS packets.

Posts: 682

« Reply #1 on: March 12, 2014, 12:07:50 PM »

New sparklers and leads is a very good start, the old leads could be breaking down.
Also while you are at it (cheap enough) change the distributor cap and rotor arm.
 Yes bond the bonnet (hood) to the rest of the body.
If this doesn't sort it then change the coil. Then thats about the lot for the ignition
As they say, every little helps

73 de Tony

73 de Tony
Windows 10:  Making me profane since March 2017

Posts: 14491

« Reply #2 on: March 12, 2014, 12:22:16 PM »

Are you sure that it is ignition noise, which is a "pop pop pop" that increases in rate with the engine speed? Alternator noise is a "whine" that increases pitch with engine speed. If it's a constant "hash" then it might be the fuel pump or computer noise. The fuel pump noise usually appears for a few seconds and then stops when you turn the ignition on but don't start the vehicle. That's because the pump runs for a few seconds to pressurize the lines before you start the engine.

Bob  AA4PB
Garrisonville, VA

Posts: 65

« Reply #3 on: March 12, 2014, 01:17:11 PM »

thanks.  Yeah the noise is a tick-tick-tick that increases speed as the engine rpm increases.  Listening to sounds online, that seems most likely.  I suppose it could also be fuel injector noise, but it seems the likelihood favors ignition.

Posts: 1050

« Reply #4 on: March 13, 2014, 05:54:33 AM »

I was wondering though, there isn't a good ground connection to the hood.  Would there be some benefit to tying the hood to the antenna ground with a could good ground straps, and using that to have the hood hide the engine compartment from the antenna? 

Even if it doesn't, bonding the hood will improve the efficiency of the antenna system. You may find you need to retune your antennas as usually doing the bonding and improving the ground plane reduces the resonant frequency of the antennas so they need a little shortening.

Posts: 1

« Reply #5 on: March 15, 2014, 01:48:09 PM »

One simple way to check out if your ignition wires or any wires are arcing is to wait until dark and then open your hood with your engine running you would be surprised at some of the light shows,(arcing) you could see! If you have a friend you could have him turn on different accessories while you stare under hood or dashboard to see if you notice anything.It's a free,  simple, and quick way of ruling out a Lot of possibilities! Good Luck!!!

Posts: 3289

« Reply #6 on: March 17, 2014, 09:46:08 AM »

Please, follow Alan's advice first.  Anything we say here is just repetition....

-Replace the wires and plugs. 17 year old wires are leaky, I guarantee.
-Drill a hole and mount a 'real' antenna.  Magmounts are much more susceptible to noise.
-Bond the hood, doors, trunk
-Bond ALL the sections of the exhaust, separately.

Tip:  Remove the antenna lead at the radio. If the noise is still there, it is an alternator problem coming thru the wiring.  If the noise goes away, it is an ignition/fuel system problem and is being radiated thru the air to the antenna.


Posts: 65

« Reply #7 on: March 17, 2014, 05:42:51 PM »

Thanks.  As I said, it sounds nothing like the sounds I've seen associated alternator noise (pitched whine) and sounds exactly like the sounds associated with ignition or maybe fuel injection (tick-tick-tick).  The antenna is hopefully "real" its a Larsen 2/70 on a fender mount.  There is nowhere on the vehicle to drill a hole due to other considerations (sunroof and roof rack) so the fender mount seems to be the least worst option.

In reading the section of his site on ignition noise I didn't see anything about bonding grounds which was why I asked about grounding the hood which I picked up from some other unknown source.  Right now I'm a little too broke to just throw money at the problem, which is why I was asking for more specific advice.  I've already grounded the hood with minimal effect, so next I'll be replacing the plugs and wires next (~$200).

Posts: 9749


« Reply #8 on: March 18, 2014, 06:26:05 PM »

I do a lot of shielding work for RF stuff, and I am also a gear head. I also designed some ignition stuff years ago.

I doubt grounding the hood would have any effect on VHF. To shield for VHF, you could not have any insulated gaps longer than an inch or two between the hood and body. That is impossible without butchering the vehicle.

Plugs sometimes have booster gaps. These are small gaps in the insulator's rod that allow voltage to build before firing the gap. Also, the distributor always has clearance between the rotor and the cap terminals. There is always an arc at that point. With all those arcs and an HEI ignition, I'd expect an issue with a front fender mount antenna no matter what shape the plug wires are in.

Resistance wires are reduce emissions at low frequencies because they slow the rise and fall and limit current, so the loop resistance of the path from coil through wire to plug and back through the engine to the coil is dominated by that resistance. Unfortunately, VHF suppression is significantly less impacted because even a few inches of lead will radiate quite well.

At VHF and higher, you can't really ground the hood or fenders together. You can't even bond the fenders to the inner fenders or other body panels very well without ruining the vehicle. You just cannot get bonding every few inches. You can't reduce wiring radiation as much as at lower frequencies; just a few inches of lead will radiate quite well.

You really should just move the antenna away from all the arcs and sparks.

By the way, injector backpulse (at least in my Ford vehicles) is a HF issue that rolls away pretty fast on VHF. My Power Stroke is pretty quiet at VHF, but is horrible at AM BC frequencies. 

Posts: 10248


« Reply #9 on: March 19, 2014, 05:07:21 PM »

I have something to add to this issue.

FM only transceivers shouldn't be effected by AM pulsed noise, but they are for several reasons. Probably the most pervasive one is poor design.

Commercial transceivers of 20+ some year ago, cost many times what a similar amateur transceiver cost at the time. They were almost impervious to AM pulsed noise. The MoCom 70 I once owned almost never opened its squelch unless there was a severe lightning storm.

Nowadays, with all of the wide-coverage receive features, including AM aircraft bands, I'm really not surprised at all of the interception of AM noise, including ignition RFI. It is the price we pay for "cheap" gear!


Posts: 817

« Reply #10 on: March 31, 2014, 10:28:24 PM »

On a distributer ignition motor, you have the rotor to cap gap as well as the plug gap.
This exposes more RF to radiation. Check the inside of the cap for carbon tracking and cracks.
Be sure the wires are fully seated in the cap towers.
I would get the antenna feed line out of the engine bay no matter how you have to do it..
18" is near perfect for pickup at the frequency in question and you don't know how good the outer shield is.
I run Ford COP fuel injection and do have minor ticking some times on 2m .
On HF 40m, the radio picks up a single ignition noise but the Noise blanker takes it out and makes  the signal useful and the radio does not shut down it's AGC in response to the noise as it does with the blanker off..
I have ferrites on all the injector leads but could do more to get rid of the remaining noise when I get 'around-to-it'.
If you tend to hear individual ticking it's generally one cylinder your hear with a problem of wire leakage or open carbon plug wire and sometimes even a routing issue..
You may even hear it on AM radio at some spots on the dial.
You might also be getting something from the 12 volt line. A scope would confirm how much.
In my case I can look at the individual cylinder misfire monitors with  Scanner and see any cylinder with a high misfire count long before an check engine light comes on or a code is set and clean it up with a boot and or coil change.
A home made filter might do you very well made out of a large 7/16 link with threaded thimble that closes the link, if there is noise on the 12 volt feed..
Wind as much #12 stranded as will fit and still close the link. On the out end of the winding connect a large value electrolytic capacitor of 3 -5000 mfd at 25 volt rating and ground the other end.
Tape the mess up and put in some kind of plastic box.
This filter will be good for nearly 40 amps and prevent most everything from getting to the radio riding on the DC.. The large cap does two things, it shunts AC signals to ground and as a DC power sink to the radio.
Be sure you fuse the supply lead at the power source to just above the max current the radio draws for the most safety from fire should a ground out occur..
All this works out for me or I would not have it in place.
Good luck.

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