Interesting Observation for 12x15 Coil

[hodr]

I have posted my impressions of the 12x15 before, basically it boils down to the thing being crazy sensitive to EMI. But if you take it out into a field or big park you can run it hot enough to go deep. Well, today I went to a school and forgot my backup coil. Try as i might I couldn't calm the 12x15 SEF down, ran the rx down to 3 and the sens to 60 and it would still false like crazy.

I cycled through the frequencies one at a time, and on each offset trying to find the quietest setting. Then when I found the best combination (7.5khz +3 offset; still falsing pretty bad) I decided to change the filter as well. As soon as I changed from 5 high to 5 band, the thing went silent. I was able to crank it to 12 70 92 and it stayed completely quiet.

At this setting I was hitting coins 7-10 inches for the next 4 hours. I air tested the first dime I found (a silver Rosie) and could easily detect it from a foot in the air.

While I prefer to run all three frequencies (helps with those pesky bottle caps), I was pretty amazed at the performance of the coil once I found the combination to knock the EMI out. Not sure why the filter change did the trick, but i'm not complaining.

 

[Rob (IL)]

A BAND pass filter passes a group of signals within a certain range, and attenuates signals on either side of that range.

A HIGH pass filter passes a group of signals within a certain range, and attenuates signals below that range.

A lower filter number is more suitable to slower sweeps and may get a little better depth. Higher filters may help with recovery. High pass is less sweep speed sensitive while band pass cuts EMI better.

Also you could try these two if you want 3 frequency.
EMI
1)Switch to a band pass filter as the band pass filters will filter out more noise than the high pass filters
2) Try different frequency offsets
3) Try the various single frequencies
4) Lower disc sensitivity
5) Try correlate mode rather than best data (if in 3 frequency mode).
6) Switching to Salt Compensate helps with EMI if you want to work in 3F mode.
7) Change coils (concentric)
Try wired headphone

 

[hodr]

Thanks Rob, I appreciate how informative your posts always are.

In my particular case nothing else on that list (and believe me, I tried every one of those options) put much of a dent in the chatter until I adjusted the filter. But it makes sense now that you have explained the function of the filter.

I also noticed that for this coil (and again this is anecdotal and only seems to be for this coil), if I run in mixed mode the falsing tends to stay in the lower vdi's where I would normally discriminate (so I don't really hear it), at least as long as I am not in a heavy EMI area. When I switch out of mixed mode I start getting chirps from 60-95 on the vdi.

-Hodr

 

[cypearl]

Sounds informative this about the filters. And good saying about the coil. Thanks guys,