Radar rocket science

[What?]

Ok kids, a little poser...
I have fitted the Stebel Magnum horn to the Bavarian Money Waster, and all is cool. Except that, when I use it, it causes my Escort 8500 to give a laser alert. If I switch laser detection off, the Escort is not upset by the horn at all.
I checked the voltage drop across the detector power supply whilst activating the horn, and this was only 0.03 volts (engine not running) or 0 volts (engine running). Exactly the same thing happens with a different Escort as well, so it does not appear to be the detector at fault.
Flashing the headlight, or hitting the starter does not cause a laser alert - only the horn does it (didn't happen with the stock horn).
I am thinking that perhaps the wire to the horn may be physically too close to the detector power wire at some point, but why would that give a laser alert?

[Lou Girardin]

I had the same issue on my RF. I never did get to the bottom of it. It hasn't happened on either Bandit.
Just don't use the horn, use a finger.

[Blackbird]

Same for me with the original Uniden I had, used to drive me nuts. Then I bought the Escort and the problem went away - same power take-off and everything. Sorry I couldn't be of more help but at least others have had it too.

[What?]

Just don't use the horn, use a finger.

I thought about that, but using a shotgun is frowned upon in some circles...

[skelstar]

It could be noise generated in the electrical circuit that has a frequency too high for a voltmeter to detect. Not laser freq high, just something that would unsettle the laser det circuit.

Its unlikely but thats all I can think of.

[Ixion]

Well, you can test this easily enough. Disconnect the horn wires (or the detector power supply), and power the disconnected item up from a separate battery. But leave them in their normal positions. Now there is no electrical connection between them. try it. If the horn still makes the radar go off, it's not interferance throught the electrical system , it's a harmonic of the horn itself. If isolating the two systems removes the problem, you can probably solve it by running a separate feed to the horn direct from the battery, through a relay. So only the coil of the relay, not the horn itself will be on the same circuit as the detector. The battery is a big enough sink that it's unlikely any high frequency stuff will get through it.

[skelstar]

The battery is a big enough sink that it's unlikely any high frequency stuff will get through it.
All depends if they are sharing the same battery feed or earth.

[Sparkz]

The laser detector is a bunch of (or just one) infrared photodiodes with a high gain amplifier behind them to amplifiy the very weak signal (several micro Amps). This circuit will be sensitive to electromagnetic interference, such as that radiated from the horn. The noise from the horn is radiated as radio waves, and conducted along the supply wires, but I reckon it is the radiated noise that is being picked up.

The photodiode amplifier will pick up broadband noise where the radar receiver is a very narrow band receiver and is not affected.

You can try moving the horn further from the radar detector, but otherwise you may be stuck with it.

[skelstar]

I like that explaination much more than my guess :).

[XP@]

... the Escort is not upset by the horn at all...
This is something you should be concerned about?
I thought it was their job!

[What?]

You can try moving the horn further from the radar detector, but otherwise you may be stuck with it.
Hmmm. Any further apart, the horn would be monted on the number plate. I was guessing RF interference, just couldn't quite tie it together.
Might try powering the horn via a relay and shielded cable. Can't see it happening tonight, though.


This is something you should be concerned about?
I thought it was their job!
:first:

[bungbung]

Try a suppression capacitor on your horn terminals to reduce the rf interference generated by the horn.

[iwilde]

How oftern do you use your horn? I tend to use my right wrist rather than my left thumb.

[Fatjim]

I get this problem sometimes when overtaking cars. I don't hear their horns though.

I've had to turn the laser detection off because I'd piss off the cages by cutting them off and then braking hard because the detector went off.

[Ixion]

Hmmm. Any further apart, the horn would be monted on the number plate. I was guessing RF interference, just couldn't quite tie it together.
Might try powering the horn via a relay and shielded cable. Can't see it happening tonight, though.


:first:

You could try a Faraday cage round it? Wrap the horn in fine wire mesh, and earth the mesh. I don't know what frequency we are talking though so I have no idea what size mesh would work.

In theory a Faraday cage will shield out RF signals, but allow noise to go forth (by acting itslef as a diaphragm if nothing else)

[What?]

Try a suppression capacitor on your horn terminals to reduce the rf interference generated by the horn.

OK - ya wanna get a bit more technical? All other options have proved fruitless (outside of running an external power source)

[Biff]

Pop into Dickie Smith and buy yourself a cheap ferride filter (or a capacitor). You feed your cables through it (just like the ones you'll fins on some digi camera USB leads etc). Eazy squezzy.

If you follow Ixion's advice ensure you earth the mesh for best results.

What you're experiencing is more than likely an RF harmonic. Filter it out, or buy a better radar detector - like a Bel, as they have some pretty wizzy RF filtering on them in order to avoid false alarms and such like.

[What?]

Cheers Biff. I will go see Dickie in the next few days.
The cage idea went out the window early on - the noise is definitely travelling through the wires, not the atmosphere (although plenty of other noise is).
Interestingly, from reading some Merkin fora, it seems that this issue is not limited to Escorts, and does happen with Bel's as well. 2 different detectors have the same problem on my BM, but neither are effected in my ute (same horn).
Some of the aforementioned Merkins report laser falsing happening (in cages) just under hard acceleration! Also falses caused by window wipers etc.