simple choke for power supply - how ?

Hi All,

I'm having interference issues on weak stations with a car radio from two-way radio equipment and I figured a choke on the 12V power supply would be a way to address this.. I was figuring on using a Pi or "T" filter (LC/LL), and I know how to assemble such a thing.. _but_ I have no idea how to determine component values.. I don't know the frequency of the interference and have no way to measure it.. (ie. expensive equipment like CRO) so I need a choke with a fairly wide band..

While I'm at it I'll kill two birds with one stone.. I want to boost the signal strength of the weak stations too.. the car has an on glass antenna (flat coil shaped strip on back windsheild) with an existing antenna booster built in (without which the signal strength of even local stations is very poor) , but this is not sufficient.. I would like to clearly pick up stations from interstate (300+ KM) away without interference from power lines, etc..

If someone can help me with both or either of the above it would be greatly appreciated.. (I have minimal electronics theory behind me, half of which I never actually used and since forgot)

Regards, Chris

Have you established that the interference is entering via the power supply? It could also be picked up on the speaker wires, the antenna or the booster. Does it afffect cassette tapes and radio reception? AM and FM? Is it still there if you unplug the antenna?

What sort of two-way radio interference is it? 27 MHz CB? Taxi? Hams? License exempt? Most are VHF or UHF.

You could make an LC low-pass filter with a low cut-off frequency. This would pass DC power and audio frequencies, but attenuate RF frequencies. Alternatively, you could make a "balun" by winding the speaker and/or power cables through a toroid - like the lumps often seen in power leads of consumer products. You will find plenty of info on choke baluns in radio amateur literature. Hams often deal with interference to radio and television caused by their transmitters.

LC filter components can be obtained from standard tables. Choose your filter type e.g. Butterworth, input and output impedance, and cut-off frequency. The impedance choice is a bit tricky for power and speaker leads! Try the lowest impedance that yields practical component values.

The choke baluns might be easier ...

Only occurs when the radio is on (not tape/cd).. and largely weak AM stations.. (don't notice it on FM or stronger AM) Guess that means it comes in via the antenna then and not the power supply.. it's worse with the booster disconnected (power antenna wire disconnected..)

Taxi.. intergrated computer (lcd display shows job information) and 2-way radio.. I'm not sure if it's VHF or UHF..

Wind the power/speaker leads around a ferrite toroid you mean ?

Regards, Chris

So, the booster is not at fault. Do you have another AM radio? Can you hear the interference on that? Do you live near a taxi transmitter? It may be the taxi office is broadcasting spurious signals in the AM band. If that's the case, you can't filter them out! You'll have to raise the issue with them, or take it to your national radio licensing authority.

Yes, as close to the radio as possible. This could still make a difference.

Spurious products can even be generated by rusty metalwork forming a rectifier which acts as a mixer. It happens! It could be a nearby fence, or even your car bodywork.

Thanks for the help, but I'm not sure I was clear enough in my previous posts.. the two-way equipment and the car radio are _both_ inside the taxi.. the owner listens to a mainland AM station most of the time instead of anything local..

We don't live near transmitters, and the interference is pretty constant regardless of where the car is and whether it's moving or stationary.. the car is also brand new so rust, etc is not an issue..

Regards, Chris

OK, I see now.

So, does the taxi equipment cause interference all the time, or only when it is transmitting?

In your original post, you said you also wanted to improve reception. Have you tried an external mag-mount vertical on the roof?

From the minute it's turned on to the minute you switch it off it interferes (doesn't matter if it's transmitting or not)

You mean just a roof mounted antenna ? I have tried just plugigng one in and holding it, the reception is better than the on-glass antenna, however would like to boost the glass one.. drilling holes or glueing is not an option.. and it can't leave a mark.. there is the ski-bar I suppose (roof rack bar that the taxi highlight is attached to).. it would have to have some kind of external plug so the bar could still be removed tho.. I'm not sure what cutting and shutting coax leads does to gain.. (I don't have a crimper)

Regards, Chris

The power lead could be radiating; you might one or more supression techniques (LC filter / screening / toroidal balun / re-routing / shortening) on that.

The interference is almost certainly in the AM band - you can verify this using a small handlheld transistor radio inside the cab. This makes it very hard to seperate wanted from un-wanted signals. Supression may be the only way to go.

So put the LC filter on the 2-way power supply and the radio, or just the 2-way ?

How do I calculate input/output impedance for the filter ? (so I can find a table listing component values)

Regards, Chris

It's trial and error, I'm afraid. Try the easier things first.

Actually, it's not that critical. We're not matching a 50 ohm signal. I might try a PI-network with (say) 100n capacitors to ground and (say) 100uH in series. The capacitor leads need to be short.