COIL TUNING?

[nomad 11]

Can somebody please explain this to me?i've heard of this.how is it done and can it be done to any brand coil?thanks all-

[Sky Pilot]

I believe what you are referring to is matching the coil to the unit or making certain that the coil "runs" on the same frequency as the detector ie., if the unit runs at 7.5khz
for instance, then the coil has to be "tuned" or very basically, the coil must have the correct size wire and the proper amount of turns to operate at this same frequency.

Some detectors (certain Minelab models) "sense" what frequency the coil that is connected to them is set up for and automatically tunes the unit to run at the same frequency.
This is not something that the average user can change or adjust after the unit is manufactured.

Also, a long time ago it seems there was a way to "tune" the coil to perform better by taping certain metal at specific areas of the coil, but I don't think that would help/matter with the new units, but I may be wrong.

Hope this helps!

Regards

SkyPilot

[Kentucky Kache]

Or if you're referring to ground balancing, some detectors don't have ground balance.

[Sky Pilot]

Good catch, KK!

I may have been wrong to assume that he was referring to the technical aspect of it.

Thanks for pointing that out!

SkyPilot

[Kentucky Kache]

Good catch, KK!

I may have been wrong to assume that he was referring to the technical aspect of it.

Thanks for pointing that out!

SkyPilot

I don't know either. I've never heard of actually tuning a coil, so I just wondered if he meant ground balancing. Can't prove anything by me.

[AUDuke]

Or if you're referring to ground balancing, some detectors don't have ground balance.

All modern VLF detectors have ground balance, some have it fixed, some manual and on some its automatic, and some have tracking.

[Frankn]

Lets get into what a coil is. It is merely an antenna. I won't go into electronic formulas but all frequencies have a corosponding wavelength "The length of the antenna or coil wire" The thickness of the wire makes no difference. SO the length of the coil wire must match the output frequency of your detector. There are ways around this, but not for the average person without electronic training. There are half wave and quarter wave antennas. There are loading coils etc. But most are applied in during the design phase of detector developement. Hoped this helped. Frank

[Carl-NC]

A detector coil is not an antenna in the classical radio sense; it is part of a transformer circuit. Radio antenna considerations (quarter wavelength, etc) do not apply.

I think what Nomad is asking about is frequency tuning. In narrowband detectors (single frequency), the transmit and receive coils are often tuned (resonated) to the frequency of the detector. This makes coil operation more power-efficient, and it cuts down on noise. In wideband detectors (including multi-frequency) this can't be done.

Since the TX and RX coils are inductors, tuning is done by placing a capacitor in parallel with the coils. Some manufacturers put the cap inside the coil housing, some put it inside the control box. The transmit coil is usually tuned exactly to the detector frequency; if it's off a little, then power efficiency won't be optimal, no big deal. The receive coil is usually tuned somewhat off of the detector frequency to make the phase response linear. It can also vary a bit with no ill effects. This means that there is some leeway in tuning, which is good because coil inductances vary from coil-to-coil.

You could take a coil and more finely tune it to the detector, but I don't think you will see any practical benefit unless the tuning is way off. This could happen if the tuning cap went bad.

- Carl

[Frankn]

Carl-NC : With all due respect I must disagree with your antenna assumption. The coil of a metal detector serves the exact same function as the antenna of a radio. What you are talking about takes place in the circuits in the detector in the way of filtering and fine tuning the signal. But I think we both reached the same conclusion, that being that the poster has no way of mo ding the antenna.

[rcasi44]

Since Carl is a Whites engineer, I'd have to go with him. I don't think I could call it an assumption on his part.

[nomad 11]

hello fellers-basically what i was asking was i've heard of other members on this forum referring to sending their detectors and coils in to the manufacturer and having their coils tuned to make them hotter.i perceived this to make them hotter as (deeper)being fairly new to detecting i'm dissapointed in the depth i'm getting out of the detectors i've used so far so i thought and that i must have bought used detectors that have gone out of tune per say.but now since i've posted this question i have found out i very pertinent bit of information that has enlightened me and that is i have always run my discrimination all the way up because there is so much trash around here.but i had no idea that i was giving up depth in doing this.so when the weather gets warm enough and i'm able to get out i will test this theory.and simply running in all metal is out of the question for me.i cant stand the threshold sound its like hearing a flock of mosquitos buzzing in my ear and i would be digging more junk than treasure.thanks to everyone that responded especially carl your explanation was way to deep with the little knowledge of electronics that i have thanks all-nomad 11

[Sky Pilot]

No problem for my part!

Sky Pilot

[N2CU]

A search coil consists of a transmit coil and a receive coil. Maximum signal transmitted and received occurs when the coils are resonant. A capacitor connected in parallel with the coil makes it resonant at some frequency. By adjusting the value of the capacitor you can make the coil resonant at your exact detector frequency. Most capacitors might have a value tolerance of +/- 5%, so if a manufacturer uses off-the-shelf parts the resonant frequency could vary by 5% or more. Custom tuning the coil would require adding additional capacitors in parallel until that sweet spot is reached - undoubtedly too much time for manufacturers. By careful parts selection and quality procedures, they can get it pretty close without the extra effort.

The length of wire in a coil is not a resonant antenna at the detector frequency. Half wave resonance at 7.5 kHz would require a wire almost 12 miles long! I'm sure they don't squeeze that much wire into a 8" coil. Just think of the weight too!

Hope this helps.

[JohninNC]

Nomad 11 you bring up a good point. If you run with lower discrimination you will get deeper, but another tip is to turn your threshold down to a point where it is barely audible or right on the line of not being audible. This is what a lot of folks call running in silent mode. There is no sound until you get over a target. It makes for very pleasant detecting. Hope this helps you out.