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simple choke for power supply - how ?

S

Skeleton Man

Jan 1, 1970
0
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
 
A

Andrew Holme

Jan 1, 1970
0
Skeleton said:
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 ...
 
A

Art

Jan 1, 1970
0
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!!
 
S

Skeleton Man

Jan 1, 1970
0
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
 
A

Andrew Holme

Jan 1, 1970
0
Skeleton said:
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.
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 ?

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

Andrew Holme

Jan 1, 1970
0
Andrew said:
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.
 
S

Skeleton Man

Jan 1, 1970
0
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
 
A

Andrew Holme

Jan 1, 1970
0
Skeleton said:
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?
 
S

Skeleton Man

Jan 1, 1970
0
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
 
A

Andrew Holme

Jan 1, 1970
0
Skeleton said:
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)

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.
 
S

Skeleton Man

Jan 1, 1970
0
From the minute it's turned on to the minute you switch it off it
The power lead could be radiating; you might one or more supression
techniques (LC filter / screening / toroidal balun / re-routing /
shortening) on that.

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
 
A

Andrew Holme

Jan 1, 1970
0
Skeleton said:
So put the LC filter on the 2-way power supply and the radio, or just
the 2-way ?

It's trial and error, I'm afraid. Try the easier things first.
How do I calculate input/output impedance for the filter ? (so I can
find a table listing component values)

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.
 
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