[OT] RF Interference

[Skeleton Man]

Hi all,

I'm looking for a way to stop RF interfence in a taxi, from the
2way/computer, etc on weak radio stations.
The whole setup includes a 2way, taxi meter, computer (used for dispatch),
and security camera all integrated together.

Whenever the equipment is turned on, weak stations (such as ones from
interstate) are subject to whining type noises that are sometimes constant,
and other times vary with acceleration/deceleration.

I might add it's an on-glass antenna with booster (oem installed) for the
radio, I know that makes for weaker reception.

Any help on where to start would be great..


Merry Xmas All,

Chris

 

[Guest]

Speaking as software engineer who knows some electronics (ie not a car
electrics expert)...

Interference can be radiated as rf energy from other equipment, or present
on the power leads.

If it is radiated rf energy then this can be fixed by reducing the rf energy
radiated by the interfering devices - the action required may be device
dependant. A better placed antenna would probably help against this kind of
interference -ie not close to interference sources - above the roof would be
aa good place shielded from the interfering equipment by the metal roof.

If it is present on the power leads then this can be reduced by filtering
the power leads (blocking high frequencies) -an inductors in series with
each power line, and a capacitor across the power lines will help this.
These could be placed close to the radio power input, and it may also help
if they were placed close to the power inputs to interfering equipment. I
imagine there must be standard components available for car environments for
this job from somewhere like Halfords (a UK car parts shop).

Do you mean that the whining varies pitch as the engine speed varies? If so
then my guess is that this component of the interference could be due to
inadequate filtering on the alternator or the ignition system - possibly a
capacitor has broken down and needs replacing. If the interference is only
there when the extra equipment is turned on then that suggests that it is
more likely alternator than ignition (as the ignition effect would stay
constant, the alternator current would have increased).

Hope this helps

 

[Skeleton Man]

Interference can be radiated as rf energy from other equipment, or present

on the power leads.

If it is radiated rf energy then this can be fixed by reducing the rf energy
radiated by the interfering devices - the action required may be device
dependant. A better placed antenna would probably help against this kind of
interference -ie not close to interference sources - above the roof would be
aa good place shielded from the interfering equipment by the metal roof.

Firstly thanks for the reply.. much appreciated..

The on-glass antenna is simply a flat coil on the rear windscreen (looks
much like the window demister, except a loop not flat).
An external antenna isn't really an option as the car is brand new and I
would prefer not to drill holes, etc. (as would the owner)

Is there some kind of filter or shielding or something I can place near the
equipment, etc ?

If it is present on the power leads then this can be reduced by filtering
the power leads (blocking high frequencies) -an inductors in series with
each power line, and a capacitor across the power lines will help this.

You mean like a Pi filter ? How do I determine the required values of caps
and coils ? (I was thinking 2 coils in series with +12V, and 2 caps between
power and ground)

Would I be best using variable capacitors and adjusting untill I get the
best result, or just a fixed value that covers a wide frequency range ?

Do you mean that the whining varies pitch as the engine speed varies? If so
then my guess is that this component of the interference could be due to
inadequate filtering on the alternator or the ignition system - possibly a
capacitor has broken down and needs replacing.

I double checked and the interference from the equipment doesn't vary, the
one where the pitch varies with engine speed occurs when the equipment is
off.. (probably occurs when it's on too, but the first problem drowns it
out).

I would very much doubt a broken down cap as the car is brand new (forgot to
mention in my original post). Will a Pi filter on the power leads to the
radio help with this interference too ?

Regards,
Chris