Tuned coils

[Carl-NC]

Dave E. mentioned below that tuning the coil on a VLF is generally a bad idea. However, isn't this fairly standard practice amongst manufacturers? I know that Tesoro puts a coil cap on the main board.

- Carl

[Beachcomber]

Re: Tuned coils

I was wondering the same thing but not because of anything that Dave had said but just an impression that I have based on things I have read about the latest detector designs. Years ago and I am talking about the 70's and 80's I know the coils were tuned and some even had capacitors in the coils themselves to help achieve the tuning that they wanted. But recently I have gotten the impression that they are not so specifically tuned and are much more broad banded than they used to be. Hopefully someone who knows for sure one way or the other will post and help clear this up.

HH

Beachcomber

[Dave Emery]

Re: Tuned coils

Here is the explanation: A tuned circuit made from a coil and a capacitor is an excellent way to get rid of noise. The capacitors you see on Whites, Tesoro and other detectors do indeed tune the coil but NOT to resonance. The coils are commonly tuned lower in frequency than the transmitters frequency although they could also be tuned higher. The effect is that the receive signal is much lower in amplitude than if the coil were tuned to the transmit frequency. The reason that this is done is to help get rid of the noise that the coil is picking up. As the coil is not resonant to the transmitters frequency there is no threat of phase drift and other instability problems. If you try to use a receive coil which is tuned to resonance you will have drifting discrimination and ground balance settings to the extent that the detector will be unusable. Hope this clears things up, Dave. * * *

[Beachcomber]

Re: Tuned coils

Makes sense to a degree. I keep thinking back to my Amateur Radio days when we wanted everything to be tuned from the transmitter tank circuit to the antenna as well as the front end of the receiver. Matching the tuning of the antenna to the transmitter allowed for the most efficient transfer of power insuring the as much of the tranmitter power was actually being radiated and not being fed back into the tranmitter which caused additional heat. We always strove for an SWR of as close to 1 to 1 as possible.

Since the coil on the metal detector is in essence it's radiating antenna I would have thought tuning the coil to the same frequency as the transmitter would be important if you wanted to make the most of the limited transmit power. I can see your point about not tuning the receive winding or tuning it off frequency.

Thanks for your response Dave!!!!

HH

Beachcomber

PS It's funny how the more things change the more they stay the same. Years ago I used to use Morse Code and SSB to communicate with people around the world. Today I use a computer and the Internet to talk with people around the world Have a Great Day!!!!

[Dave Emery]

Re: Tuned coils

.... .- ...- . .- -. .. -.-. . -.. .- -.--

N2SN also G4FEB

[Beachcomber]

Re: Tuned coils

Thanks I did Guess you really never forget Morse Code either Glad I retained something from back then

WA2EVR if you have any old CallSign Books from the '70's you'll find me there

--... ...--

HH

Beachcomber

[Carl-NC]

Re: Tuned coils

"The coils are commonly tuned lower in frequency than the transmitters frequency although they could also be tuned higher."

This just jogged my memory... when I visited a manufacturer a few years ago, I specifically asked if the RX coils were tuned to the TX frequency. Their response was that they are tuned just off of the TX frequency... I don't recall their explanation, other than I didn't quite understand it. Now I understand. Thanks!

- Carl

[Dave Emery]

Re: Tuned coils

Just off means that a detector transmitting at 6.5KHz will have its receive coil resonated to about 4KHz. This is quite a long way from resonating at the transmit frequency. Being tuned does however mean that a lot of noise is rejected.

[Sun_rise_pt]

I agree with corretamante tuned LC circuits, thinking that the S / N ratio will come out much stronger.
The noise exists in the entire frequency spectrum whatever the tuned LC the set of Rx "antenna".
It seems natural that this circuit is prepared as much as possible to be aware of all the disturbance amplitude and / or phase detection ie: (in essence) The unbalance that occurs in exactly the same frequency of TX. A circuit perfectly tuned to "hear" specially this frequency (from TX) and not any other, will not make sense to be the ideal?

A very well written article by a manufacturer of MD's with great reputation, is accessible to all hobbyists are on the page below:

I leave the suggestion of a special reading of the end of the article referring the matter of resonance ...

Then please read:

http://www.nexusdetectors.com/scienc...detectors.html

[satdaveuk]

Talking as a personal point of view, have been building tuned coils for quite sometime now, far as performance on the field goes theres no noticeable difference between either tuned or non tuned coils.
The advantage of the tuned coil is you can make multiple coils for the same project, where as if you use untuned then your knackered because the chances of getting the same specification every time with diy manufacturing is almost zero.

Will add though can only speak on projects ive built todate, spent alot of time on this very subject with experiments based the TGSL and that was my conclusion.

I do wonder when these manufactures make tuned search coils use the same value caps on each search head or do they vary depending what the final inductance was.
With the pro tools they have proberly the latter but does anyone no for sure?

Regards

[Dave J.]

The Nexus "science" essay is full of errors and misunderstandings.

--Dave J.

[Jerry]

I agree with corretamante tuned LC circuits, thinking that the S / N ratio will come out much stronger.
The noise exists in the entire frequency spectrum whatever the tuned LC the set of Rx "antenna".
It seems natural that this circuit is prepared as much as possible to be aware of all the disturbance amplitude and / or phase detection ie: (in essence) The unbalance that occurs in exactly the same frequency of TX. A circuit perfectly tuned to "hear" specially this frequency (from TX) and not any other, will not make sense to be the ideal?

A very well written article by a manufacturer of MD's with great reputation, is accessible to all hobbyists are on the page below:

I leave the suggestion of a special reading of the end of the article referring the matter of resonance ...

Then please read:

http://www.nexusdetectors.com/scienc...detectors.html

Well I thought everything was making good sense until I read to the end of that article.
I have worked with resonant circuits for many years but do not have a clue as to what
total electro-magnetic resonance means.

"The best metal detectors regarding
depth penetration and discrimination
accuracy are the IB detectors tuned in
total or as close as possible to total
electro-magnetic resonance."

Is he talking about tuning the RX as close as possible to the TX? What is meant by Total Resonance?

I am confused on this one.

Jerry

[Dave J.]

Pretty sure that's what he means, Jerry.

However I recommend that you simply ignore the Nexus essay, rather than attempt to understand it (since so much of it is nonsense).

--Dave J.

[golfnut]

My interpretation of what is said..


They speak of the target phase pulling the Tx tank off and then the Rx tank off. Rather than listening for eddy currents in the target.


If you have a tx tuned cct with a wide bandwidth - resistive coil, dielectric loss, poor quailiy cap etc a small pull on the tank would not be seen.

If the tank is q'ey and narrow band (low loss coil and cap pls low resistance coil etc) the same target pull would be easier to see - greater shift.

I work in an antenna design company and we daily adjust antenna BW for reasons discussed..
s

[Sun_rise_pt]

I have found experimentally that, in terms of a slight mismatch at the optimal frequency of the Rx tank circuit (that receive same TX freq.) have a better discrimination with some disadjust. Obviously, this causes a phase shift, and you can easy discriminate the angular deviations caused by the different responses of the metallic targets constituent.
However if we fail to consider the part of the discrimination, and that the target response has exactly the same characteristics in the signal returned (Rx) (in frequency compared to Tx signal ) the best "antenna" that will be that is perfectly tuned and with a better Q factor. Even for a beginner this is a base rule, that makes sense, coming from far away, since the time of Heinrich Rudolph Hertz, Oliver Lodge, James Clerck Maxwell, many others... ....and could not be denied.

I believe in the Nexus... Is it reasonable for anybody to understand that a manufacturer has available a good research lab that develop your product to compete with those of other manufacturers, which are already numerous. For this, we assume that for a manufacturer seriously not "shoot" to the discredit for making public wrong statements...


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