simple choke for power supply - how ?

[Skeleton Man]

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

 

[Andrew Holme]

Skeleton Man wrote:

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 ...

 

[Art]

FYI: The RF interference from two-way equipment is coming in via the
antenna, normally, being received with the other information you are trying
to listen to.
Increasing the antenna ability to receive information may only acerbate the
problem.
What you are trying to do with a Car Radio may be better attempted with a
commercially available communications receiver and a good antennae array.
Then you will probably still have the interference via the two=way radio
equipment depending on your proximity to the offending transmitter, etc.
Not a simple quandary to rectify/ Happy 2005, Have a Safe One!!
"Skeleton Man" <invalid@guestwho.com> wrote in message
news:ziaBd.97242$K7.75735@news-server.bigpond.net.au...

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

 

[Skeleton Man]

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?

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..)

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

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

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

Regards,
Chris

 

[Andrew Holme]

Skeleton Man wrote:

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?

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..)

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.

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

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

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

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

 

[Andrew Holme]

Andrew Holme wrote:

Skeleton Man wrote:
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?

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..)

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.

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.

 

[Skeleton Man]

OK, I see now.

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

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)

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

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