figure 8 coil????

[Arnie S.]

Little help.
When coils are tuned in full resonance having the tuning caps in the control box instead on the coil assembly will result in the coil cable acting as additional antenna, because it becomes part of the resonance circuit.
Bad idea.

Thank you- I am smiling here and I really came to a full halt. Had to think about that and trying to disagree. You are right. I am concerned with matching a small indiference ;with the system being as good as it can be from the factory.
I do not coil my cable and try to keep it as strait as I can, knowing that coiling it can cause the feedline to act as a choke-though at this frequency, it is debatable. The feedline is not the load, but small ammouts of capacitance can affect the feedline as well as inductance-it is part of the resonant circuit, like it or not. The feedline is shielded ;compensating for the system's mismatch will deliver more power :Xl=Xc. like it or not the feedline is part of the load and I was just wondering if a better match would transfer more power to the from the machine. At the feedpoint, adjustments are ideal. The skinny feedline can not be ideal, IMO. How accurate are the xmt coils matched to frequency as in say White's factory coils? Aftermarket Coils? Oh yea, here is another question for you, what do think about a relflector-tuned and above the coil? Bad idea, or one worth looking into?
Respectfully,
A.S.

[Qiaozhi]

Thank you- I am smiling here and I really came to a full halt. Had to think about that and trying to disagree. You are right. I am concerned with matching a small indiference ;with the system being as good as it can be from the factory.
I do not coil my cable and try to keep it as strait as I can, knowing that coiling it can cause the feedline to act as a choke-though at this frequency, it is debatable. The feedline is not the load, but small ammouts of capacitance can affect the feedline as well as inductance-it is part of the resonant circuit, like it or not. The feedline is shielded ;compensating for the system's mismatch will deliver more power :Xl=Xc. like it or not the feedline is part of the load and I was just wondering if a better match would transfer more power to the from the machine. At the feedpoint, adjustments are ideal. The skinny feedline can not be ideal, IMO. How accurate are the xmt coils matched to frequency as in say White's factory coils? Aftermarket Coils? Oh yea, here is another question for you, what do think about a relflector-tuned and above the coil? Bad idea, or one worth looking into?
Respectfully,
A.S.

Personally, I'm not aware of any metal detector search heads that have a reflector above the coil, and I've no idea how to implement this. The only benefit I can see is that the coil would be less sensitive above the coil and could conceivably reduce unwanted pickup. However, if the idea is to reflect more TX power into the ground, then the benefits would be somewhere between minimal and zero. It takes a huge increase in TX power to make any measurable difference, and this also causes other problems in the RX circuitry.

[Arnie S.]

Thank you,
I am speaking in terms of increasing the effective radiated power. How do you get 1500w of power to act like 25Kw? You use an antenna array, composed of gain antennas. I found two such references to what was called a "focusing" coil placed directly above the xmt coil and spaced,- while searching bucking coils. I posted these sites for forum readers in the General interest area.
http://www.geophex.com/Publications/...%20Nov-Dec.pdf
Broad Band Detector: google bucking+ coil+ M+d
http://www.cflhd.gov/agm/geoApplicat...alDetector.htm
Focusing+ coil+M+D
http://diwww.epfl.ch/lami/detec/INSIGHT2000.pdf
Focusing+ coil+M+D
http://www.hamradioindia.org/circuits/wireant.php
loop+ground +reflector- (non perfect applications at the beach)

One does not need to increase the xmt power to get a greater effective radiation. The focusing coil as it is described, sounds like a classic reflector. Ideal placement for distance is compromised, but the results are in reflecting some of the unused radiation in phase. This is a commercial application for demining. Would any increase in direction of radiation help? What is factual here is, that it has been applied and is in use in a comercial product and that to me means, it works. Can we explore this for our benefit?
I have built such of a design (2el) for an HF band antenna that had a very closely spaced reflector for several reasons. First is the immunity for noise and radiation in the opposite desired direction , xmt & rcv(dead quiet). Second was an increase of erp and rcv sens( a hotie). It was not the 'ideal' design, but according to the design parameters it did what it was supposed to, and with great effect(it was a regular 'secret weapon'). Other operators in the field of radiation would ask "who are you talking to" the ones behind heard nothing, and would ask the first group the same question. Sometimes you have to stand on your head to look at things in a new perspective to see an old rule; work the equasion from right to left. Use what is normally designed out for desired results. <over>
A.S.