One turn coil never crosses 0 while balancing

[samu]

https://www.geotech1.com/forums/show...er-coil-inside

Thank you for your answer. I see how it works, however, I'm still not sure about if the tuning loop should be connected in serie or in parallel and if it is better to connect it to the TX or RX coils. It is not clear in the picture as the wires run into epoxy.

[kt315]

two steps
1/ you have to tune maximally possible to null by the balance loop. it is connected to TX coil, mainly.
2/ do 1 turn of wire. must be closed end to end. not connected nowhere. balance again to null moving it to center of coil and back.
you have to get FULL ZERO on o-scope.

[Skippy]

Whoops. Got my X and R mixed up. I knew what I was trying to say, it was late ( western Europe ) and I was tired... and I can't delete/edit it now, either... plus it's been quoted anyway. Sigh.

Regarding the extra loop: It's NOT connected to either of your search-coils. It's just a 'fake target' , that you fix in place when you've found the 'correct location'. In principle, any bit of metal could have some useful effect on adjusting a coil's null. But most common items will affect both X and R, making them inconvenient. What is needed is something that exclusively ( or primarily ) affects just one component. I have used real targets before to achieve this. The loop of thin wire is another way.

[samu]

two steps
1/ you have to tune maximally possible to null by the balance loop. it is connected to TX coil, mainly.
2/ do 1 turn of wire. must be closed end to end. not connected nowhere. balance again to null moving it to center of coil and back.
you have to get FULL ZERO on o-scope.

egarding the extra loop: It's NOT connected to either of your search-coils. It's just a 'fake target' , that you fix in place when you've found the 'correct location'. In principle, any bit of metal could have some useful effect on adjusting a coil's null. But most common items will affect both X and R, making them inconvenient. What is needed is something that exclusively ( or primarily ) affects just one component. I have used real targets before to achieve this. The loop of thin wire is another way.

My head is going to explode now... So The balancing loop is not connected to anything is just a ring of wire... That explains a lot of things. Let my try this and will post the results. Thank you, thank you, thank you...

[Skippy]

Some maths for you, related to single-turn wire loops:

The time-constant / corner-freq of a loop is very dependant on the wire diameter, and varies very little with loop diameter.

For a 20mm diameter loop, the time-constant can be calculated as:

TC = 31 x D^1.67 ... where D is diameter in mm, and TC is in microsecs

So, for example a 0.25mm diameter wire produces a target with TC = 3.1 microsecs; or f_c = 51 kHz

A loop made from the suggested 40 AWG = 0.08mm has TC approx 0.46 us; f_c = 345 kHz

I suggest making your loop have f_c = 200 kHz or higher, so it is well over 10 times the detector operating frequency.
This works out as 0.11 mm diameter wire, maximum.

f_c = 1 / ( 2 * pi * Time-constant )

Making the loop larger will obviously cause a stronger nulling response.

[kt315]

My head is going to explode now... So The balancing loop is not connected to anything is just a ring of wire... That explains a lot of things. Let my try this and will post the results. Thank you, thank you, thank you...

you have not mislead between a balanced loop connected to TX and and isolated closed loop i wrote about above.
its different matter. russians use small piece of copper/foil instead of the loop.

[Skippy]

"Russians use a small piece of copper/foil instead of the loop"

I chose the lowest conductivity metal that's easily manipulated, tin, (specifically lead-free solder). I made small blobs 3mm diameter, for example, then crushed them flat in a bench vice. They then resembled small coins, 0.5mm thick, with short time-constants.

[green]

Hi samu

Wondering if you have found a way to balance the R signal with your coil.

[Carl-NC]

"Russians use a small piece of copper/foil instead of the loop"

I chose the lowest conductivity metal that's easily manipulated, tin, (specifically lead-free solder). I made small blobs 3mm diameter, for example, then crushed them flat in a bench vice. They then resembled small coins, 0.5mm thick, with short time-constants.

You can do the R-null with any eddy target. However, a target with a lot of reactive content (high conductors) will also move the X-null so then you have to iteratively work back & forth between positioning the dummy target and moving the coil(s). Moving the coil(s) almost purely affect X-null, and so the best dummy target is one that almost purely affects R-null. Then you have independent knobs for X-null & R-null.

Also, make the R-null target as small as possible to get the job done. A larger R-null target will work but then becomes a larger target to variable ground (ground reflection effect) and can cause falsing.

[Skippy]

"You can do the R-null with any eddy target. However, a target with a lot of reactive content (high conductors) will also move the X-null"
".... and so the best dummy target is one that almost purely affects R-null."
Yes, pretty much what my posts #18,#20 were saying.

"Also, make the R-null target as small as possible to get the job done."
That was the idea of the small tin discs, they were typically 5mm - 8mm diameter, with target frequencies 150kHz+. Lead is obviously lower-conductivity, but isn't easily melted with a soldering iron. If you can get thin lead sheet, then you can cut it etc to shape it. Thin lead sheet with a sticky tape backing is used by golfers to tinker with their club head weight/balance.

I also think it's best to try and maintain some symmetry when placing these tuning items ( ferrite lumps or short TC non-ferrous ). For example if you found a 8mm square bit of ferrite placed in a particular spot near the 'toe' of a DD coil gave a good null, replace it with two 6mm square bits - one at the original location, and one at the 'equivalent' spot near the 'heel' of the coil. There is then symmetry from heel to toe.
I've never made/tuned a figure-8 coil, so I'm unsure how this would apply, but keeping the left-half of the 8 the same as the right-half seems like a reasonable aim.

[green]

You can do the R-null with any eddy target. However, a target with a lot of reactive content (high conductors) will also move the X-null so then you have to iteratively work back & forth between positioning the dummy target and moving the coil(s). Moving the coil(s) almost purely affect X-null, and so the best dummy target is one that almost purely affects R-null. Then you have independent knobs for X-null & R-null.

Also, make the R-null target as small as possible to get the job done. A larger R-null target will work but then becomes a larger target to variable ground (ground reflection effect) and can cause falsing.

Don't have a VLF but I can look at Rx when Tx is on with my PI. My coil isn't the same shape as samu's but is similar, figure8 Rx surrounded by Tx (Tx and Rx, AWG28 wire). My coil isn't quite I.B. but can balance it by squeezing it or using a ferrite core, looks the same either way when watching the scope. Ferrite core was used to balance the coil. Don't have 5mm wire (used to make samu's coil) so used some AWG6 (4.11mm dia.). Using another piece of AWG6 to balance the signal caused by the imbalance of a piece of AWG6 seems to be the only way I can rebalance the coil. Wondering where I'm looking at it wrong or is VLF different? Last scope picture, tried squeezing the coil and moving the ferrite core to get a zero signal (I.B.) but couldn't.

[Carl-NC]

If you imbalance one lobe with a particular tau then you will need the exact same tau for the other lobe. Your plots look as I would expect them to.

[kt315]

just repeat this pic.

[green]

If you imbalance one lobe with a particular tau then you will need the exact same tau for the other lobe. Your plots look as I would expect them to.

You can do the R-null with any eddy target. However, a target with a lot of reactive content (high conductors) will also move the X-null so then you have to iteratively work back & forth between positioning the dummy target and moving the coil(s). Moving the coil(s) almost purely affect X-null, and so the best dummy target is one that almost purely affects R-null. Then you have independent knobs for X-null & R-null.(reply 24)

I'm confused. What should I see on the scope when working back and forth between positioning the dummy target and moving the coil(s) when trying to balance a high conductor target with a low conductor target?

[Carl-NC]

My point was, you cannot balance a a high conductor target with a low conductor target. They won't balance.