coplanar coil

[leaud]

Re: Dave, oops left out the decimal point

How can I do a test without capacitor on the receive coil?

[Russ_NY]

Dave, sent you an e-mail as requested

Dave,
Thanks for the help. Sent you an e-mail as requested.
HH

Russ_NY

[Dave Emery]

Re: Dave, oops left out the decimal point

There should never be a capacitor to tune the receive coil, NEVER - EVER - NO - BAD - GET RID OF IT!!! I admit that I have seen some poor designs which have attempted to use a tuned receive coil. They were all very poor performers. The receive coil should connect to an amplifier such as an operational amplifier or op-amp. Measure the signal at the output of the op-amp. Good luck, Dave. * * *

[Robby_H]

Re: Dave, oops left out the decimal point

Dave is right and some consideration should also be given to the tx. I live in Australia and the ground is generally reasonable near the coast but very bad on the goldfields and some beaches.
It is actually an advantage here to make sure the tx coil isn't tuned to the operating frequency as ground minerals will easily pull both the frequency and amplitude unless you use a more complex tx circuit that securely locks both of these in place. You can see this effect quite easily using a scope.
It's been a while since I played with vlf but I think a ferrite bar from a radio arial presented to the coil will show how much the tx coil properties change in your case. Ideally it shouldn't have any effect and if it is substantial then your ground balance control will be overridden making the unit noisy on mineralised ground/beach.

[Dave Emery]

Re: Dave, oops left out the decimal point

Robby, Australian ground is of course the ultimate test for any detector.

The main reason that a tuned receive coil does not work in a VLF is that it is almost impossible to keep the phase of the receive signal correct.

The phase will vary due to temperature variations of both the coil and the tuning capacitor (and as you mentioned, the ground minerals)

Even a crystal derived transmit frequency does not solve the problem. The result is that your ground cancel and discriminator settings will vary.

A lot of people have fallen for tuning the receive coil on their VLF designs. The reason is that a tuned coil (as in an LC) gets rid of almost all the local interference and provides a much stronger receive signal than an un-tuned coil does. There are a few ways to keep everything stable using a tuned receive coil but the circuitry required is not at all trivial.

Well designed VLF's rely on their synchronous demodulators to cancel interference and allow the high gain required to detect weak targets.

[leaud]

Re: Dave, oops left out the decimal point

thank you for your help

[Robby_H]

Re: Dave, oops left out the decimal point

Hi Dave, I think we said more or less the same thing??
All I was trying to do was point out that the tx circuit needs stabilising also. It is very likely that the tx circuits used in some hobby designs will pull fequency (and amplitude) if the coils are tuned exactly to the operating fequency and as the tx is the reference for the synchronous demodulators you mentioned then this can also have a very bad effect. This effect is shown by an inability to find a height to ground balance. The coil appears to go through different layers of ground balance as it approaches the ground. A crystal or pll tx circuit is useless unless the amplitude is also locked as the tx amplitude can be seen to vary wildly on mineralised ground.

The push-pull front end tx circuit from a GT 16000 isn't a bad design.
It's almost a direct copy from one of the old op-amp cook books. It locks both the fequency and amplitude of the tx, can't vary and gives a very clean sine wave that can be set to almost double the supply rail if needed. The untuned receive coil is fed straight to a non-inverting op-amp from memory.

A lot of noise can be overcome by inverting and processing the often thrown away half cycle.
The ETI 1500 throws away one half cycle of the received signal and this makes it prone to noise and a general lack of sensitivity. The Magnum and some other designs, inverts and processes this thrown away half cycle and the drop in noise level and increase in sensitivity is instantly obvious allowing the hobbyist a bit more to play with.

[zum]

COPLANAR COIL

IS IT ENOUGH REVERSE WIRE TO GATE IT OUT OF PHASE AND NULL PLEASE HELP ME.