NOISE: my battles with EMI

[cockpitbob]

I've had 2 serious encounters with EMI making my radios almost useless.  One battle is won and the other is ongoing.  I thought others may benefit from my ongoing adventure.


Mobile (car) Noise: 
I did an installation of my FT-857 in my SUV that I was rather pleased with.  The remote head is on a microphone gooseneck up front and the main body is in the baggage area.  I installed jacks in the center console for the mic, Morse key, headphones (or plug into car stereo) and for the remote head.  It looked great.  However, with the engine running I had S8 noise on just about all the bands.  The noise did not change with engine speed. 


First, with straps I grounded everything:  doors to body, body to frame (body is on rubber isolators).  That didn't help.  Then I was told to ground the exhaust system to the body.  That worked!  On a quiet day it was below S2 with the engine running.


Then, after not using it for a year I put the radio back in for a long drive and the noise was S9!  I assumed a ground broke so I re-did some grounds and added some.  Didn't help.  After lots of reading I concluded it might be the fuel pump.  On fuel injected cars the fuel pump is in the tank and runs at the same speed regardless of what the engine is doing.  A clever test to perform is to note the noise with the key off, then turn the key on, but not start the engine.  The fuel pump will turn on for 1-2 seconds to pressurize the fuel system.  I saw the noise jump to S9 then drop back down.  To confirm the theory I put a 6' piece of wire at the end of the antenna cable and went sniffing around the car.  Everything was quiet except the fuel pump wires at the fuel tank, which were real loud.  I made a filter from a 0.1u, 100V cap and a couple large ferrite beads and spliced it into the pump wires close to where then enter the fuel tank.  That got the noise down to S3.  I can make a better filter, but S3 is plenty good for me.  I hate doing wiring up under the car.


I don't know why the pump got noisy, but my instincts tell me it may be thinking about dying.


Home EMI:
I don't know when it got bad or why I didn't notice it earlier, but my home station has had S8 of background noise for some time now, even on the 15M band.  With the rig running from a battery I threw the house's main breaker.  Noise went to S0 :D .  I hadn't seen that in a long time.  After so much breaker flipping that one of my Verizon set top boxes lost its programming and won't work, I found 4 breakers that cause noise of S5 - S8.  One is from a wall wart power supply.  One is the supply for the Verizon cable box >:( .  Those I can filter or replace with linear versions.  I haven't tracked down the noise sources on the other 2 breakers yet.  My family hates when I shut down parts of the house when they're home.  I'm going to have to track them all down though since even the weakest will keep my S-meter at S5.  I'll post a follow-up as I make progress.

[Luigi]

Plasma and LCD TVs are also a serious emitter of noise when running. I wonder if anyone has figured out how to block the noise from them.
Luigi

[RadioRay]

Mobile:  I HATE commercial AM/FM, so the first thing I did with the stock radio in the Bronco was to tear it out and toss it into my parts bin. My old Bronco was a mobile radio station since the 1990's (may it rest in peace.).  Major sources of noise were [like you] the fuel pump, which I cured with a .01 capacitor across the pump motor and wrapping the feed harness through many ferrite cores. Dirty DC power supplied to the HF transceiver (FM VHF/UHF didn't care) straight from the battery had RF  HASH on it. Under the hood, my added power lines were LARGE and shielded. In the cab, just as the cables entered the firewall, I added a 12 volt 25 Amp DC line filter designed for those 'thump!-itty-bump!'  car sound systems and that portion of the broadband hash went away.

With a 16' military whip tied forward, it worked VARY well, even NVIS.  With the whip UP (stationary) it was a daily occurrence to talk with Europe on 17m with VERY GOOD signal reports.  I used copper strapping from the base of the auto-coupler and it ran panel to panel beneath the vehicle.

Now - I have a tiny Toyota, selected for gas mileage, and have since given a friend that 16' whip and mounting base.

[APX7000]

Electrical noise can enter the radio through the power line or antenna.

If your getting noise in your mobile, disconnect the antenna. Did the noise go away?  If so, the noise is coming in through the antenna/coax.  Many times re routing the coax or upgrading to better shielded coax will fix the noise problem. If that doesn't fix the noise problem, the antenna is picking up interference from something in the car. If this is the case, use the AM radio trick in the next paragraph to locate the offending noise.

Now if you disconnect the antenna and the noise is still present, then the noise is coming into the radio via the power lines. Something in the car is generating noise and it's feeding it into the radio. It could be computer noise, alternator, water pump noise or a loose/dirty spark plug wire or ??. It could be just about anything in the vehicle causing noise. What I do to locate the noise is to take a portable AM broadcast radio with headphones and tune it to a blank frequency. Then wrap the AM radio completely in aluminum foil to reduce the sensitivity of the radio. Now use the wrapped AM radio as a probe around the engine. When the AM radio gets close to the interference generating device, you will hear it on the AM radio. Works like a champ!  Usually if something is causing lots of interference, it's defective in some way or fashion and must be replace or repair.

Another trick for getting rid of electrical noise is to twist the power line going to the radio. Twisting the power cord sounds crazy, but it WORKS!  Twisting prevents cross-talk of noise. That's why Ethernet cables are twisted and each pair is twisted at a different rate.  If you locate a noisy DC motor, and the motor is good, try twisting the power wires feeding that fan. Give it a try next time, you'll be shocked how well this simple trick works. If you have added DC fans to keep your equipment cool, make sure the DC power wire up to the fan is twisted.

Also, almost all cell phones have Li-ion cells and the charging process causes lots of hash and interference noise due to the constant switching on / off. Charging cell phones in the car can generate lots of interference. Anything with a Li-ion battery can cause interference when charging.