IDX problems

[Koala]

good sens on the vic penny,

the double on close pass is the target signal phase plus the Tx or Rx coil detuning minutely introducing enough of a phase shift to give another bang.

If u put a det over say a radiator, the Tx shifts by tens of Hz!

Many thanks for the explanation.

Overall very pleased with the coil. Need to get out over some more productive land to learn its little ways. It really likes a swing past the target unlike the factory concentric coil which you can get away with wiggle over the target.



Really thought it was a scrap coil the first time out when it kept falsing until I realized it was the ground balance.



First coil I have made that the shield has worked as good as the factory coil. Mixture of graphite and varnish get a thumbs up from me.

[golfnut]

did u do the shell and over the winding bundle too?

what varnish, graphite and ratio?

[Koala]

Did not use a shell at all


used layers of the foam that normally goes under laminate flooring and glued them together. photo in post 26


Shield was made from hardboard added two square of copper slug tape at opposite ends and soldered two tap wires.


Graphite I bought from eBay and vanish was the cheapest I could find at B and Q



Put a tablespoon of each and mixed felt a bit dry so added just enough varnish to make it runny again. One was runnier than the other which showed in the resistance. one ended up at 100 ohms the other 500



Once dry test with the two tap wire. Then sealed with a top coat of varnish and cut one of the tap wires off. Resistance did go up when sealing in but can't remember by how much




Placed one of these shield disks top and bottom nothing else. Nothing over the wires and nothing around the edge


Did space the shield away from the windings with a layer of foam as in a factory coil but don't know if that necessary


then fiber glassed over the whole thing

[golfnut]

very industriuos, well done for keeping trying.

Sounds like your onto something - good luck.

[Koala]

Spent a day detecting. Not many finds but got more than most people. Starting to get the hang of the coil. Super stable if ground balanced positive. Only negative with the coil is it picks up surface iron a long way away. Plough shear can be 6" or more away from a target and it still interferes causing a second beep. Found a new technique. Test the area in all metal mode to identify all the targets then in disc mode wiggle over the dead center of each. Still got it a bit wrong on a double beep. Identified two targets about 8" apart dug the first and it was iron but the second was part of the nose cone of a anti aircraft shell.

Came away with a lead solder and a silver ring.


Going to have to dig a target out of my lawn. Its about 12" away from a scaffolding pole. With the factory coil its a nice clean dig all day beep. With the new 10" DD is a iron grunt even with the disc very low. Which will be right ??

[mhak1]

Hi guys
Why in VLF metal detector , the transmitter frequency is 1 to 2.5 kHz higher than receivers frequency ?
While the frequency of both should be the same.
Thank you

[Qiaozhi]

Hi guys
Why in VLF metal detector , the transmitter frequency is 1 to 2.5 kHz higher than receivers frequency ?
While the frequency of both should be the same.
Thank you

Theoretically it would appear that tuning the TX and RX coils to the same frequency (resonance) would be the obvious solution. But in the real world it's not so simple. Due to manufacturing difficulties, component value drift with time, cost, etc., the coil tuning is always offset to improve stability and maintain consistency of performance between detectors. When tuned off-resonance, any variation in component values, due either to time or temperature will have much less effect on the detector's operation. Only Nexus Detectors make resonant coils for their machines, and each coil can take up to a week to produce. The coil manufacturing process is complex and time consuming, and only suited to low volume production.

Sometimes people get confused when thinking about the RX coil being tuned to a different frequency than the TX. For example, if the detector is transmitting at 10kHz, and the receiver is tuned at 9kHz, it doesn't mean that the RX signal is 9kHz. If you examine the output of the preamp on an oscilloscope, you will see that the received frequency is actually 10kHz. It's just at a lower amplitude than it would be if the coils were tuned to resonance. Maybe that's what you're referring to?

[kt315]

Mhak call the source at first where you found this foolishness.

[WM6]

For example, if the detector is transmitting at 10kHz, and the receiver is tuned at 9kHz, it doesn't mean that the RX signal is 9kHz. If you examine the output of the preamp on an oscilloscope, you will see that the received frequency is actually 10kHz. It's just at a lower amplitude than it would be if the coils were tuned to resonance. Maybe that's what you're referring to?

Or with graph (be careful: it included naked curve - something very like titties !!!!):

Grab quickly, cause Qiaozhi is crazy at my naked pics and I expect this graph with naked curve will be deleted soon too:

[Qiaozhi]

Grab quickly, cause Qiaozhi is crazy at my naked pics and I expect this graph with naked curve will be deleted soon too.

Very funny.
But at least it's technical, and not designed to stir up tension between Geotech members.

[WM6]

What tension? You warn Funfinder and me, but there was nothing like wrangling or hot dispute.
Funfinder is my best friend and I only try to stimulate him to contribute here with more of his excellent and lucent debate.

[Qiaozhi]

What tension? You warn Funfinder and me, but there was nothing like wrangling or hot dispute.
Funfinder is my best friend and I only try to stimulate him to contribute here with more of his excellent and lucent debate.

I have replied to you in the Off Topic forum.

[mhak1]

Theoretically it would appear that tuning the TX and RX coils to the same frequency (resonance) would be the obvious solution. But in the real world it's not so simple. Due to manufacturing difficulties, component value drift with time, cost, etc., the coil tuning is always offset to improve stability and maintain consistency of performance between detectors. When tuned off-resonance, any variation in component values, due either to time or temperature will have much less effect on the detector's operation. Only Nexus Detectors make resonant coils for their machines, and each coil can take up to a week to produce. The coil manufacturing process is complex and time consuming, and only suited to low volume production.

Sometimes people get confused when thinking about the RX coil being tuned to a different frequency than the TX. For example, if the detector is transmitting at 10kHz, and the receiver is tuned at 9kHz, it doesn't mean that the RX signal is 9kHz. If you examine the output of the preamp on an oscilloscope, you will see that the received frequency is actually 10kHz. It's just at a lower amplitude than it would be if the coils were tuned to resonance. Maybe that's what you're referring to?

Thanks for the answer
Yes, that was the order

[kt315]

open WHITES PATENT ON THE COIL and give me A PLACE WHERE YOU SEE ABOUT RESONANCE IN RX -!!!

US4293816

[Koala]

You can alter the RX capacitor to increase the match which gives more depth. I would advise against it. Discrimination alters and the detector is far too twitchy to use in the real word. All very impressive on a YouTube clip when testing on the bench but swing the detector through some long grass or over uneven ground and it will be chirping away.