Has anyone tried a ferrite choke to reduce EMI

I read that someone tried this on his F75 and it helped to reduce EMI so I tried it on my T2 just behind the display. I tested it in my house where I always get EMI. Once installed I was able to turn the sensitivity up to 75 without any chatter, normally I would have to reduce the default sensitivity down to around 60 without the choke. It doesn't seem to affect depth or GB or VDI. Is there any reason That I should not do this? The link to the ferrite choke on radioshacks site is [www.radioshack.com]
HH
John

Interesting I will wait too see what Tom has to say before I try it.

LowBoy

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I think I remember Tom saying that almost all EMI interference comes in through the coil, so a ferrite choke should have minimal effect. There seems to be a lot of people reporting improvements in the stability of the detector though, so there could be some benefit.

The choke is put onto the coil cable reducing the EMI transmitted from the coil to the detector. My question would be does it also reduce the good part of the signal coming from the coil. It seems to me that it would. It seems to me that if EMI does not fall within the detectors 180 degree phase then the engineers would be able to remove it from the equasion, but I could be wrong... it is just a guess. I'd test the choke on some low gold conductors and see if it affects depth or sensitivity in any way.

J

jbow Wrote:
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> The choke is put onto the coil cable reducing the
> EMI transmitted from the coil to the detector. My
> question would be does it also reduce the good
> part of the signal coming from the coil. It seems
> to me that it would.

I am a ham radio operator in addition to being a metal detector guy. We deal with EMI as radio operators also. The normal signal present on the line from the coil to the control and signal processing head on a metal detector is a "differential mode" signal. That is at all points along the line the signal currents on the conductors inside the line are equal but opposite in direction. Thus the normal signal currents do not generate an EM field around the line. Thus an external "choke" has no effect on the normal signal currents. EMI generated by external sources (radio towers, power lines, etc.) however, finds its way onto the line as a "common mode" current. That is EMI currents in the conductors inside the line are all flowing in the same direction. This in turn results in the generation of an EM field which can be attenuated by a choke. Ham radio operators use external chokes on coax cable all the time to reduce EMI without any affect on the normal RF signals we are receiving. The problem is, at VLF frequencies, it may take quite a few chokes along the line to get the EMI choked off. Also, some of the chatter we hear on our detectors is not EMI. At very high sensitivities the electronic circuits are operating right on the edge of instability and you start hearing this as chatter. Nothing can be done about this other than utilizing some very expensive integrated circuits which would be cost prohibitive to hobbyist consumers.

dgc is correct in every respect. Also..........there may lie a few 'unknowns' into this equation............and the ONLY real method to ascertain a scientific answer.....is to test a choke(s) across the spectrum. This means testing fringe targets of varying conductivity.........and with varying EMI interjection. Yes, nearly ALL EMI enters thru the coil.

When Sens is boosted to max ... and Disc is dropped to '0'.......internal circuit cross-talk/noise occurs.......which has nothing to do with the definition of EMI.

I put a choke on my F75 LTD and I've been able to run my sensitivity at 99 while hunting in Oakland which I wasn't able to do prior to the choke (had to run it around 80). Not a major improvement, but might be the difference between getting a deep silver or not (and most of them are DEEP where I hunt).

Just verify that deep targets don't 'go away' whilst choke is installed.

That comment makes me think there is a potential down-side to using a choke. I would like to try one, what size/rating do I ask for?

Scully Wrote:
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> That comment makes me think there is a potential
> down-side to using a choke. I would like to try
> one, what size/rating do I ask for?

The snap on type pictured in the original post are fairly common (Radio Shack or google snap on choke). The choke needs to fit snuggly for best effect but make sure its not so tight that it cuts the outer cable jacket. Best thing is take detector into a Radio Shack and try them out.

Quote
Tom
When Sens is boosted to max ... and Disc is dropped to '0'.......internal circuit cross-talk/noise occurs.......which has nothing to do with the definition of EMI.

There has to be some outside factor to this though. Because some places you can hunt 0 disc with 99 sens. Other places you can't.

Yes, very true. There have been very few places that I have been able/capable of running the unit wide-open........and achieve max stability. . . . BUT...... it HAS happened. Internal cross-talk between components seems to happen the most when the unit is pulled out of a hot car.....into the cold air..............and................the other way around. Moisture on components induce a problem.

I'm still concerned on how this choke would work on a shielded cable.......and still have some doubts.

That's why this is such a great website. Where else are you going to have such an indepth discussion like this? I'm going to try a few tests with and without the choke installed on fringe low conductive targets I find with the choke off. I'll report any useful info I find. Thanks again Tom and everyone else. This website's the best.
HH
John