Why like this?

[billr]

Surf and Barracuda using the same coil with a resistance of 1.8 ohm pick up a signal at 14" air test. Swap them out for a coil with 3.2 ohm and both achieve identical range at close to 14".

Try the 1.8 coil on the MPP Beach only achieves 9.5" range.

Try the 3.2 coil on the MPP and the range improves to nearly 11.5"

Altering damping resistor values does very little.

Removing and adding coil winding's to the MPP coil helps but not a great deal.

Why the huge difference between both types of detectors and their preference for coil match? Aren't they all PI's?

If anyone has a simple mono coil that does have good range on the MPP can you tell me what it is, size of wire and how many turns etc.
Making the coils for the first two units was simple but the Beach is proving to be more demanding.

[green]

Wire size? Coil diameter? Number of turns? For 1.8 ohm and 3.2 ohm coils.

What was the target that detected at 14 inches?

[surfdetector]

The TX pulse width of the machines should ideally be 5x the TC of your coil. Are your TX pulse widths all the same or do they vary?

Cheers

[6666]

What are the measured inductances of the coils, what type of construction are they, and are they home made , what type of shielding do they have ?

[6666]

Was the damping resistor matched for each coil on all machines

[billr]

green . US Nickle
Surf . Yes stable
6666 As per the coil calc on this forum. Home made. None
I'm just curious as to why two units react similarly and yet the third is so much different to the same coils when it is of the same genre. Was looking for a proven coil build for the MPP with sizes dims etc to help ascertain what the difference between the three units where. This is the best way for me to learn.
I can develop a suitable coil to match the others in performance but the question of "why is the MPP so different in coil choice compared to its relatives " still remains. and why do other proven coils read less when connected up.
Call me curious. I like to learn by asking questions on even the simplest of subjects.

[surfdetector]

Surf . Yes stable

I should have been more exacting in my question. What are the TX pulse widths for the 3 different detectors? In order for the coil to perform optimally, then it would seem that all three would need to be matched to the 5x TC of the coil in question. Thanks.

[Carl-NC]

In order for the coil to perform optimally, then it would seem that all three would need to be matched to the 5x TC of the coil in question.

Practically no one does that in reality. Coils have a typical tau of 100us and running a 5-tau pulse consumes a lot of power and generates a lot of heat. Most PI detectors run about a 1-tau TX pulse which means the current is close to a ramp. Therefore the peak TX current is dependent on supply voltage, inductance, and pulse width.

On the other side is the RX circuit. Bill, do these 3 detector have identical damping, preamps, integrators, and sample timing? If they don't, then any of these can account for differences.

[billr]

Thanks all for the replies.
This all started off with me trying to get the greatest range of detection I could from a given coil. It now feels like I may be able to exploit an unknown error I made in the builds. Hence the question. What caused it?
The coil with 1.8 ohms is different in design to the 3.2 coil. My thinking is I may be able to increase the 1.8 to 3.2 by adding more winding's and detect a nickle at 17" or more with the detector still happy to work as it did before. 17 inches would to me at least be quite some range for a coin.
Of course there would be drawbacks but I learn best when I ask myself. What if?

[surfdetector]

What are the inductance values for the two coils?

[billr]

I have no idea as I don't have anything to test them other than use the coil calc as a starting point. All I can go on is the resistance.
The Surf does well without any damping resistor. It losses a bunch of range with one fitted. The Cuda works well with the 330R. 470 seems to work OK with the MPP. Not much change with a 680R fitted.

[billr]

The TX pulse width of the machines should ideally be 5x the TC of your coil. Are your TX pulse widths all the same or do they vary?

Cheers

On your video step 3. I noticed you adjusted the TX pulse width to 100us and the PP to 1ms instead of 400us and 8ms. What was the reason for that?

[surfdetector]

That is to better match the coil that I am using.

[green]

Thanks all for the replies.
This all started off with me trying to get the greatest range of detection I could from a given coil. It now feels like I may be able to exploit an unknown error I made in the builds. Hence the question. What caused it?
The coil with 1.8 ohms is different in design to the 3.2 coil. My thinking is I may be able to increase the 1.8 to 3.2 by adding more winding's and detect a nickle at 17" or more with the detector still happy to work as it did before. 17 inches would to me at least be quite some range for a coin.
Of course there would be drawbacks but I learn best when I ask myself. What if?

This all started off with me trying to get the greatest range of detection I could from a given coil. I start with https://www.geotech1.com/forums/atta...5&d=1600272466
Example: MPP detects a nickel at 9.5inches. S/N would have to increase 8times to detect the nickel at 14inches with a 200mm coil.

[surfdetector]

I have no idea as I don't have anything to test them other than use the coil calc as a starting point. All I can go on is the resistance.
The Surf does well without any damping resistor. It losses a bunch of range with one fitted. The Cuda works well with the 330R. 470 seems to work OK with the MPP. Not much change with a 680R fitted.

Check out one of these on Bangood or Aliexpress a handy tool to give you an idea of your L etc:
LCR T4 Mega328