Audio Interference - AM - Car Radio

[whissper]

Hello

Has anyone some good ideas on how to eliminate the following problem.

The AM/FM/CD unit in the car is fine except for some unbearable harsh car electrical interference across the entire AM band. (Here in Aus about 500khz to 1400khz)

It it impossible to listen to anything on AM.

With the engine off AM is fine but with the engine on cannot get past all the static, interference, crackling etc.

Can I possible shield the radio from the electrical interference, put something on the antenna cable at the radio end to eliminate.

The radio is only two years old and have always had this problem.

Regards

Whissper

 

[JimB]

First you need to work out how the noise is getting into the radio.

Disconnect the antenna from the radio and listen for the noise:
- if the noise is gone then the noise was coming down the antenna
- if the noise is still there then the noise is coming down the supply cable.

Noise via supply cable:
Ensure the case of the radio is securely connected to the body metalwork of the car.
Connect a choke in series with the supply line.
Connect a capacitor from the radio side of the choke to the radio case / car body.

Noise via the antenna.
Ensure the case of the radio is securely connected to the body metalwork of the car.
Ensure that the screen of the antenna is connected to the radio case and the car body (at the antenna end of the cable).
Where is the antenna located, it may be useful to move it to the rear of the car.
How is the antenna cable routed? Keep tha cable clear of car wiring.
Ensure that the bonnet (hood to our USA readers) is earthed correctly. If necessary connect an earth strap across the bonnet hinges, use the hinge mounting bolts on the hood and body and ensure there is good contact.

General.
Is the ignition system in good condition? Are the plugs, plug leads and distributor cap in good condition?
Is the alternatorin good condition?
Is the battery in good condition? Are the connections good, including the earth strap?

Anything else?

JimB

 

[flat5]

and make sure the spark plug wires are the type made to reduce radio frequency interference

 

[shokjok]

Have you looked inside the radio? Solder lugs to the case may have broken from unfinished radio repairs, the antenna cable may be frayed, or fault exists in the power supply. Also, the mounting bracket may not be electrically grounded to the car frame.

 

[Hero999]

It's AM, it will be noisy, if you don't want noise then listen to FM radio. Sorry not very helpful but true.

 

[audioguru]

Every car I have had plays its AM radio without much interference. None from the car. The AM radios do not have high treble audio frequencies and have pretty high distortion. They are made to hear an announcer read the news, weather forcast or a traffic report.

 

[crusader235]

AM radio interference

I have a Ford Excursion,, the AM is horrible,, 2000 model.. grandson has Toyota. Parked side by side same area, the Toyota sounds like FM,, guess ansewer is to buy a Toyota....Crusader

 

[audioguru]

I think the interference heard on the huge Ford truck is caused by it rusting away.

 

[flat5]

AM car radios used to have narrower IF selectivity than table radios. I don't know if that is the case anymore. It helped to lower the noise level a lot.

 

[Dean Huster]

AM is an afterthought on auto radios these days. Back in 1960, an AM radio was the only thing you had in a car. It was all vacuum tubes, had a vibrator power supply, and the better ones had a tuned RF amplifier and a push-pull audio output using a pair of 6AQ5s. They were really nice radios. Today, you get an AM radio that's nothing more than a low-cost IC with lousy sensitivity and lousy selectivity. The older auto AM radios were great for AM broadcast band DXing and had great audio.

Most AM QRN is caused by hash coming in through the power supply, and is caused by the alternator (if the hash frequency is high-pitched and changes with engine speed), the ignition system (lower frequency and changing with engine speed) or heater blower motor (hash frequency changes with fan speed). There could be other sources: windshield wiper motor, fuel pump, power windows, power seats, etc. -- even static discharge in the wheels. A simple filter available from Radio Shack will often stop the hash entering through the power supply. Also make sure your antenna cable has a valid shield. The ARRLs ham radio handbook has a section on mobile operation and reducing interference.

Dean

 

[transistor495]

That might be a problem with the spark plug transmitting radio frequency interference. My home-made AM radio about 30m away from the roadside catches engine interference from a few vehicles. The sound is quite loud and feel like a two -stroke motor cycle effect. It's especially from petrol engines.

 

[MountainMike]

Hello

Has anyone some good ideas on how to eliminate the following problem.

The AM/FM/CD unit in the car is fine except for some unbearable harsh car electrical interference across the entire AM band. (Here in Aus about 500khz to 1400khz)

It it impossible to listen to anything on AM.

With the engine off AM is fine but with the engine on cannot get past all the static, interference, crackling etc.

[with no engine running, that tells you it is your electrical system - versus an "outside source" - generating the RF Interference]

Can I possible shield the radio from the electrical interference, put something on the antenna cable at the radio end to eliminate.

[the antenna cable is supposed to be entirely shielded - there's your shield.]

The radio is only two years old and have always had this problem.

Regards

Whissper

Sounds like my Model Year 2000 Subaru, where the A/C fan noise is so bad I have to turn off the A/C!

I know part of the problem is an ungrounded ("unearthed" to you Scottish readers) antenna at the antenna mount. I've checked the continuity between the outer shield and the car body - nothing!

The other part of the problem may be an unfiltered d.c. power lead at the fan blower motor! I'm still searching for a replacement antenna or a shop that has the tools to remove and check the current antenna, then I will tackle the motor filtration issue, if it's still a problem.

Switching to the FM band really does not solve the basic problem. Yes, you won't hear the RF noise, but to a lesser degree, it may still compromise your reception on that higher band. The higher frequency (+/- 76-108 MHz.) has less noise propagation than the lower (0.5-1.7 MHz.) band.

Most of the comments before mine are really good suggestions! It's sort of like playing detective with your radio - trying to find the source of interference!

Mike

 

[audioguru]

Why does anybody listen to an old AM radio?? AM is nothing but interference and lousy muffled sounds.

 

[PatM]

I have a Ford Excursion,, the AM is horrible,, 2000 model.. grandson has Toyota. Parked side by side same area, the Toyota sounds like FM,, guess ansewer is to buy a Toyota....Crusader

Fords have always been notorious for ignition noise.
Forty years ago, I could tell when a Ford was within about 500 feet of my receiver.
I would think that by now that would have been fixed.

PatM

 

[MikeMl]

Have you replaced the distributed resistance high voltage harness with solid copper, by chance?

 

[audioguru]

Does anybody listen to lousy-sounding AM radio anymore?
The frequency response is from 200Hz to about only 2kHz.
The important audio from 2kHz to 15kHz is missing.

 

[microtexan]

AM noise

I think the interference heard on the huge Ford truck is caused by it rusting away.

LOL You got it!

 

[Bob Scott]

Does anybody listen to lousy-sounding AM radio anymore?
The frequency response is from 200Hz to about only 2kHz.
The important audio from 2kHz to 15kHz is missing.

The low end of AM audio is not limited to 200 Hz. You get all bass frequencies.
The high end of AM audio is not limited to 2 kHz. The channel is limited to +/-5 kHz, but the chief engineer of CKY AM told me they broadcast up to 15kHz (but I doubt his competence. He's a P. Eng.)
You've been thinking of the limited telephone frequency response.
It is the receivers that limit your high frequency response. Delco car radios are reeeeeally bad for this.

I used to manage a car radio installation facility (16 bays!) My recommendations for the car radio noise:

Check that the antenna ground at the fender has good continuity.
Check that the shield of the antenna cable has continuity from one end to the other.
Check that the car radio chassis is grounded to the frame by way of a metal brace. In some instances, the dash frame might not be grounded to the rest of the chassis, but it should be.
Check that spark plug wire is good and the resistive (carbon) type.
If the noise is a whine, it is the alternator that is generating the noise. If it is a VW, there should be a capacitor mounted to the alternator. It looks just like an old ignition "condenser".


Bob

 

[audioguru]

The low end of AM audio is not limited to 200 Hz. You get all bass frequencies.
The high end of AM audio is not limited to 2 kHz. The channel is limited to +/-5 kHz, but the chief engineer of CKY AM told me they broadcast up to 15kHz (but I doubt his competence. He's a P. Eng.)
You've been thinking of the limited telephone frequency response.
It is the receivers that limit your high frequency response. Delco car radios are reeeeeally bad for this.

I used to manage a car radio installation facility (16 bays!) My recommendations for the car radio noise:

Check that the antenna ground at the fender has good continuity.
Check that the shield of the antenna cable has continuity from one end to the other.
Check that the car radio chassis is grounded to the frame by way of a metal brace. In some instances, the dash frame might not be grounded to the rest of the chassis, but it should be.
Check that spark plug wire is good and the resistive (carbon) type.
If the noise is a whine, it is the alternator that is generating the noise. If it is a VW, there should be a capacitor mounted to the alternator. It looks just like an old ignition "condenser".


Bob

What are you talking about?
AM radio stations in North America are only 10kHz apart. Then interference will be caused if they transmit audio higher than only 5kHz. The transmitter must roll-off the high frequencies at about 3.5kHz so that the response at 5kHz is low. Some AM radio stations are allowed to broadcast audio frequencies that are higher because there are no other stations to interfere with.

AM radios roll off the high audio frequencies above only 2kHz so that they do not produce a heterodyne whistle when they receive two stations that are only 10kHz apart. An audio response to only 2kHz is extremely muffled.