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Spraiski 12th July 2008 01:01 AM

Antenna noise filters
Does anyone know what kind of circuit is in an antenna noise filter ( eg - )?


1moreamp 12th July 2008 02:33 AM

I built my own by separating the shield of the antenna coax and inserting a ceramic disk capacitor in between the two side after I cut the shield in two. A piece of black plastic tape or shrink sleeve later and VIOLA! a Antenna noise filter.

The big deal is to isolate the ground so to prevent ground loops.

Another way would be to add a RF balanced transformer to do the same thing, but if this is done a signal amplifier might be a good idea to compensate for the loss the transformer will cause in the RF signal strength. And these amplifiers themselves can cause noise issues.

The only possible purpose for doing any of the above is if you have found that the antenna is the source of you engine noise issue because of a ground loop condition it may be causing...Hope this helps :)

Spraiski 12th July 2008 08:18 AM

Well maybe a bit more background to my particular situation is in order. I have a 2001 Honda Civic LX Coupe. It uses the grid on the rear window as an antenna for the radio.

FM reception is acceptable. AM reception is the problem - with the engine off the reception is marginally acceptable, but with the engine running, or when anywhere near another vehicle that is running AM stations are almost completely wiped out. I know that adding an external antenna would cure the problem, but I would like to see if I can get better performance with the window grid.

I'm trying to get together several different solutions so that I can try different things in the same afternoon (avoiding possible placebo effects).

1. First off will be a thorough check for breaks in the grid.

2. Redo the grounds from the battery to the engine and to the body ensuring that I have good metal to metal contact (with a dab of copper antiseize to prevent any future corrosion of the mating surfaces).

3. Add a low pass RC filter to the power leads going into the radio/cd player.

4. Add an antenna filter of some sort...but I'm not sure what sort of ciruit is in there - is it just a high pass RC filter (and if so, what sort of cut-off frequency do I want to aim for)?

5. There is an 'antenna module' at the rear of the vehicle right next to the window - I think this is an amplifier, but I'll have to investigate further before doing anything with it.

Any thoughts?

1moreamp 12th July 2008 06:51 PM

Well your planned approach sounds valid in this case. Most unwanted noise is created under the hood of the radios host vehicle.
Everything from bad spark plug wires to bad grounds etc.. can be the main reason for most of the issues you mentioned.

But please bear in mind that the radio itself can also be a weak link in the scheme of things. Some stock radios are just so poorly designed that the issues your dealing with may be unavoidable.

wakibaki 12th July 2008 07:52 PM

I wouldn't expect a radio in any modern car (especially a Honda) to exibit these problems unless there is a fault. Car radios are, after all, designed to work in cars.

It's a bit complicated in this case 'cause of the screen antenna and the amplifier.

First I'd be looking for obvious faults. Is the antenna cable connected to the radio? In some cases you can still get reception without one. Is there any possibility of ingress of water (condensation) into the antenna cable or amplifier? Is there power to the amplifier?

Then check the radio for obviously faulty capacitors - blown cases or cracks. Also check for dry joints. These can manifest as almost invisible cracks in the solder. If you can, operate the radio in the dark, you can sometimes see tiny sparking in a dry joint. Otherwise you can remake every joint in sight... Most car radios operate in a pretty hostile environment, hot and with high vibration, the failure rate reflects this.

You should be looking for a fault to fix, rather than seeking to bolt on something that will correct it.


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