Yet Another RFI Thread!

[austinmgrant]

Greetings everyone! I feel like I've read every thread on every site regarding CB radios and RFI, but I wanted to post here to make sure I'm understanding correctly and not missing anything. I'm getting some RPM related engine noise, and just want to make sure I've correctly identified the source (or eliminated non-issues.) Here's my setup:
2016 F150 5.0L
4' Firestik Firefly
ProComm 2015+ F150 Hood Groove antenna mount
ProComm HS818T-NKT 3/8-24 STUD MOUNT 18Ft RG-58U COAX
Uniden Bearcat 980 SSB and a Uniden Pro510XL

I've tried both radios with the same results. Power wire directly to battery, ground wire to a nearby spot that's grounded to the frame. Here are the 3 relevant scenarios that I've tested, all with the engine running:

1. Radio hooked up to coax and antenna: Lots of RPM related noise. ANL helps a ton, but doesn't completely eliminate it.
2. Antenna removed from the antenna mount, coax still attached to radio: Almost completely eliminates all noise, but some VERY faint RPM noise still present.
3. Antenna AND coax removed from radio: Complete silence.

Based on the above tests, is it safe to assume all my issues are coming from radiated RF from the truck? From my reading, I believe that if it were alternator whine or another power source issue, it would still be noisy with the antenna and coax disconnected. Is that correct?

Finally, what steps would you suggest to help mitigate the issue? I'm guessing the majority of it is unavoidable at this mounting location with this antenna. I've tried several routing options for the coax cable, but none seem to make an impact. Would a double-shielded coax cable make a difference?

Thanks for any suggestions, or for at least confirming or rejecting my suspicions on the cause.

Austin

 

[K4EET]

Hi Austin,

It sounds like, as you said, alternator whine. If it is, it is probably coming over the DC power line in which case you need some type of filter such as these towards the top portion of the following webpage.

CB Noise Filter: Amazon.com

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Read the reviews and see what the folks that bought the various units had to say about each product.

Keep us all informed on what you decide to try and see if anybody has any particular comments on your proposed action plan.

Dave

 

[austinmgrant]

Hi Dave,

Thanks for the reply. In my post, I actually suggested that I think it is NOT alternator whine on the power line because it disappears when the coax and antenna are disconnected. I was under the assumption that if it were on the power line, it would still be present with the coax and antenna unhooked. I believe it could still be the alternator or some other ignition component, but that it's just coming through the antenna and a bit on the coax.

It's also possible I'm misunderstanding all of the evidence I've gathered and reading I've compiled, which is why I came to you guys. Again, thanks for any input! I'll certainly checkout those filters.

Austin

 

[prcguy]

Its probably spark plug noise, alternators can dirty up the 12v system but they don't radiate RFI unless something is really wrong. If it were a bad alternator the noise sometimes sounds louder when the antenna is off because the noise up the power leads getting into the radio audio circuits is not competing with static coming in from the antenna.

Listen to the noise very carefully at idle without the noise blanker or ANL and you should hear a distinct ticking noise, one or two plugs usually make more noise than the rest, then when you run the engine RPM up the ticking turns into a whrrr or buzz. The only time double shielded coax might improve things is if the antenna were mounted far away from the engine but the coax had to run past the engine. Even then, the 40dB + shielding of the coax is not going to let much through compared to the antenna doing its job of picking stuff up.

Most vehicles produce spark plug noise of some level and mounting the antenna to the hood will amplify the problem. Your hood is not grounded very well and is acting like a big antenna picking up RFI below it and channeling it right to your antenna. If you could electrically bond the hood on all sides to all the surrounding metal body it might reduce the noise pickup but then you would not be able to open the hood. Moving the antenna to the rear of the vehicle might get rid of most of the noise but maybe not, some vehicles are just noisy and you need to look into resistor type plugs or wires and/or shield some ignition components.

You can make a RF probe antenna using your antenna mount. Remove it from the vehicle and make about a 2 or 3" dia loop of wire that goes from the ground side of the mount to the hot. Then use that to probe around the engine compartment while the engine is running and the CB is on to locate whatever is making the noise. You might need someone sitting in the truck listening to the radio signaling you when the noise is up or down or just remove the CB and run it off a power supply or battery next to you as you probe around the engine.

I've had to deal with hundreds of radio installations with noisy vehicles. I can always find the source of the noise but most of the time I can't fix it because the vehicle owner is not willing to do what it takes to quite things down. Sometimes grounding the hood will reduce things some, or grounding the exhaust system is a big culprit in HF amateur installs because the entire exhaust is floating in rubber and all the spark plug wires are right next to the headers inducing noise onto the exhaust making it a big antenna. The most successful were shielding the distributor and some of the HV wiring in braid but you risk changing the timing and making the engine run rough. That took made impossibly noisy vehicle that destroyed HF through VHF into a quiet one, but it also had aftermarket high performance ignition parts that made a bigger spark.

My current truck is a 2008 Toyota Tundra and it makes zero RF noise when running. I can operate CB or 160 through 2m and turning the engine on or off makes no difference. I'm going to keep this truck until I die because its so hard to find one that is quiet and operating radios is very high on my list of priorities in life. So probe away, find the noise source and hopefully its not a difficult or expensive fix.

Or dump the Ford and buy a Tundra.

 

[austinmgrant]

Thanks for all the info, prcguy!

My main goal here was to identify that it is in fact coming from some ignition component, and that there wasn't an easy fix that I'd overlooked. Your post certainly supports that theory, and I appreciate all of the info. I've got a spare CB radio and some coax, so I'll definitely assemble that RF probe and see if I can identify the main culprits.

Resistor type spark plugs seems like a simple starting point. Any suggestions on a model / brand?

Thanks again!

Austin

 

[prcguy]

I'm not up on spark plugs or wiring but using factory components for your specific model and year is a good start. The truck left the factory with components that did not interfere with the stock AM/FM radio and a lot of time was spent at the factory figuring this stuff out. If the truck is older and you have changed out plugs and wiring that would be a good reason it makes a lot of noise at CB frequencies now.

Many years ago, probably in the late 70s I did put original factory wires and plugs in my vehicle and it did reduce ignition noise.

Thanks for all the info, prcguy!

My main goal here was to identify that it is in fact coming from some ignition component, and that there wasn't an easy fix that I'd overlooked. Your post certainly supports that theory, and I appreciate all of the info. I've got a spare CB radio and some coax, so I'll definitely assemble that RF probe and see if I can identify the main culprits.

Resistor type spark plugs seems like a simple starting point. Any suggestions on a model / brand?

Thanks again!

Austin

 

[JayMojave]

Hello All: Another great posting from PRC Guy, thanks. I have made Alternator Noise Filters that go into the coax line between the antenna and radio, simply using a 88 Mh choke and a .1 uF in a Parallel wiring in series with the center coax wire in a metal box. This worked pretty good reducing the Alternator Wine Noise in and around the early 1970's. But newer Alternators are quieter as compared to the older cars and newer cars. I believe the Diodes inside the Alternator when turned off generate a RF Noise in some Alternators.

Using a Probe to Probe the Noise Field is a great way of finding the noise source. But this noise finding has some points that need to be said. I have used my coax 1 inch loop to locate engine noise, and even fuel pump noise, and engine computer noise, but the secret to using it is have the input of the probe loop on the coax Attenuated that is reduced its input signal as to allow the operator to pin point the noise source. The Radios RF Gain can be used for this as it can significantly reduce the received signal noise. I use the test probe with a 1000 Ohm variable resistor on the loop end of the coax to attenuate the input signal even more especially when the noise signal is really loud. And a pair of old headsets and a 25 foot extension cord allow the operator to hear the radios noise pickup while right there in the engine compartment or were ever on the vehicle.

My 1977 Ford 4X4 F250 Truck was real noisy with Ignition Noise, even with new high quality Silicone Wires, and new Rotor, Cap, and spark Plugs. As PRC Guy also suggested, I installed a braid wire from RG8 Coax, connecting the Fenders to the Firewall, to the Hood. Bang! Noise significantly reduced to a livable level. Then I added the $200.oo dollar DSP Noise Reduction Unit between the Radio and External Speaker and I am totally satisfied. Drive down the Freeway and listening the Radio just like at home base. Is big Ten-4 G.I..

Now if only they would have a Digital Squelch allowing only voice signals to open the squelch and NOT noise I would be set....

Jay in the Great Mojave Desert

 

[W5lz]

That 'digital squelch' thingy is basically the same as that DSP filter. I would have to say that it wold set you back more than a very good DSP. But then, learning to use a very good DSP filter (level adjustment) is not easy. There's no simple and cheap solution. Good luck.

 

[JayMojave]

The price of the DSP Squelch would be worth it. Installing larger towers, Coax the good stuff, and 7 element beam antennas is all worth it as to make contact with those stations others can't even hear. My status should be classified as a "Super CB'er" Breaker....

I will throw in the digital circuity wanting a accurate S-Meter that has been programed adjusted to show 6dB per S-Unit, and certainly more pretty lights, and chrome knobs.

Jay in the Great Mojave Desert.... "only 150 miles to the next fillin station"