SRF

[ionut_mtb]

Can someone clear this things up for me, please...

We need to know SRF of the coil only to find out the right dumping resistor OR we just use dumping tool with potentiometer and oscilloscope? With oscilloscope you can see also how fast it is.
Knowing SRF of a coil - is it good for something else?
-does it have anything to do with frequency of the pulse rate? any relation between this two.

-i know higher srf is better for small gold, as it can sample faster. But, for beach hunt and water hunt i'm not interested in small targets, as it can be so much small junk and targets falling from scoop holes...

Now, increasing the coil diameter will not be sensible to small targets. Question is: For gold rings it will go more deeper than same diameter coil with lower srf?

We need fast coils for gold rings? Or we need fast coils for deep gold rings? Shallow small gold target response equal with deep big gold response?

Thanks!

[Wahyu]

Can someone clear this things up for me, please...

We need to know SRF of the coil only to find out the right dumping resistor OR we just use dumping tool with potentiometer and oscilloscope? With oscilloscope you can see also how fast it is.
Knowing SRF of a coil - is it good for something else?
-does it have anything to do with frequency of the pulse rate? any relation between this two.

-i know higher srf is better for small gold, as it can sample faster. But, for beach hunt and water hunt i'm not interested in small targets, as it can be so much small junk and targets falling from scoop holes...

Now, increasing the coil diameter will not be sensible to small targets. Question is: For gold rings it will go more deeper than same diameter coil with lower srf?

We need fast coils for gold rings? Or we need fast coils for deep gold rings? Shallow small gold target response equal with deep big gold response?

Thanks!

Me too
Waiting for answer

[Idefix]

We need to know SRF of the coil only to find out the right dumping resistor OR we just use dumping tool with potentiometer and oscilloscope? With oscilloscope you can see also how fast it is.
Thanks!

Hi,
It is not absolutely usefull to know the value of your SRF coil. Knowing it allows to compare between Coils. So, it is necessary to trim have the good value of damping resistor, and a scope is usefull.

Knowing SRF of a coil - is it good for something else?
-does it have anything to do with frequency of the pulse rate? any relation between this two.
Thanks!

The SRF makes the first µS after the TX to be inusable. For example, 5µs. The readable answers of the target takes place after that, during some 5 to perhaps 30µs. So the next TX could be for example 300µs to 1000µs . It is necessary to have enought energy for the TX sample, but not too much to allow the answer to be read.

It is necessary to have time to sample the answer of the coil, and to make some ground cancellation, and others , so it gives the duration , for example 500µs.
So, this makes the cycling period.
If you shorten the cycle, the electrical consuption will be highter.

-i know higher srf is better for small gold, as it can sample faster. But, for beach hunt and water hunt i'm not interested in small targets, as it can be so much small junk and targets falling from scoop holes...
Now, increasing the coil diameter will not be sensible to small targets. Question is: For gold rings it will go more deeper than same diameter coil with lower srf?
We need fast coils for gold rings? Or we need fast coils for deep gold rings? Shallow small gold target response equal with deep big gold response?
Thanks!

Deep gold rings, means quick target answer due to ring, and small amplitude answer due to the deep.
Quick answers need quick coil, quick preamp inside the detector, and short sampling TX.
Small amplitude needs very low noise detector, ie very good preamp , very good osillator, good EMI cancelling inside detector

[green]

A chart I made awhile back, targets swinging from a pendulum. GEB off, probably 10usec sample. Don't know why the nuggets lost distance when the 25x25mm foil didn't with similar time constants. Probably need to retest.

[Ferric Toes]

A chart I made awhile back, targets swinging from a pendulum. GEB off, probably 10usec sample. Don't know why the nuggets lost distance when the 25x25mm foil didn't with similar time constants. Probably need to retest.

The time constants may measure the same, but the foil has a considerably larger radiating area. Take a lead musket ball and measure the detection range, then hammer it flat and measure the range again. It's the same effect.

Eric.

[green]

The time constants may measure the same, but the foil has a considerably larger radiating area. Take a lead musket ball and measure the detection range, then hammer it flat and measure the range again. It's the same effect.

Eric.

The 10 grain nugget lost a greater percentage in distance when changing delay time than the 4 and 5 layer foil even though it had a higher time constant. Maybe I did something wrong or I don't see why yet.

[green]

A chart I made awhile back, targets swinging from a pendulum. GEB off, probably 10usec sample. Don't know why the nuggets lost distance when the 25x25mm foil didn't with similar time constants. Probably need to retest.

Been thinking about the problem. I see from Eric's reply #5 I didn't explain the problem correctly. Integrator output is the average of the input over the sample time times a multiplier. Targets with the same TC should loose the same percent signal, about the same lose in distance(not % lose) when increasing delay time assuming no noise. Maybe noise is causing the nuggets to loose more distance when increasing the delay time, some of the sample is in the noise level.

Or maybe the measurement needs to be done better.

[bbsailor]

Green,

What is the diameter of the coil used on your target distance test?
What is the value of the damping resistor?
What type of coil?
How many samples are being integrated in your test?

The reason why I ask these questions is because Eric Foster published a coil size chart to optimize a coil size for maximum range on particular targets.

The coil SRF will dictate the damping resistor value and be critical in ensuring that a target is fully enegised by a coil discharge TC being one fifth of the target TC.

Normally, lower delays allow detecting smaller or lower TC targets as the detectable energy in a target normally decays to near the noise level in five TCs. When you integrate many samples you can raise a very low signal out of the noise that may not be detected if it was not integrated many hundred times.

DD coils are normally a little faster than mono coils.

Many of these variables interact and need to be considered when doing a target TC, delay and detecting distance analysis.

Thanks

Joseph J. Rogowski

[green]

Green,

What is the diameter of the coil used on your target distance test?
What is the value of the damping resistor?
What type of coil?
How many samples are being integrated in your test?

The reason why I ask these questions is because Eric Foster published a coil size chart to optimize a coil size for maximum range on particular targets.

The coil SRF will dictate the damping resistor value and be critical in ensuring that a target is fully enegised by a coil discharge TC being one fifth of the target TC.

Normally, lower delays allow detecting smaller or lower TC targets as the detectable energy in a target normally decays to near the noise level in five TCs. When you integrate many samples you can raise a very low signal out of the noise that may not be detected if it was not integrated many hundred times.

DD coils are normally a little faster than mono coils.

Many of these variables interact and need to be considered when doing a target TC, delay and detecting distance analysis.

Thanks

Joseph J. Rogowski

Rx two 200mm round, series connected one inverted 2k damping resistor. Tx oval over Rx about 1k damping resistor. 1000pps

[bbsailor]

Rx two 200mm round, series connected one inverted 2k damping resistor. Tx oval over Rx about 1k damping resistor. 1000pps

Green,

It is good to see someone trying to unravel all the interrelated variables of target TC, physical size and orientation to the coil, coil size, coil type and full stimulation of the target.

Eric's comment about radiating area seems to apply in resolving your test results. Take a coin that is in the range of 1/10th to 1/20th the diameter of the coil and detect it at maximum distance with the coin parallel to the coil. Now, rotate the coin 30 degrees, 60 degrees and 90 degrees and see the max detection distance get less with each rotation.

In reference to Eric's coil chart, if the detection distance of a particular target is just equal to the coil radius, increasing the coil size will not improve the detection distance for that target at the same target orientation to the coil.

As target TCs get smaller you reach the point of assuming that you are fully stimulating the target to attempt to isolate and measure other variables. Assuming your TX coil is 300uH, with your 1000 ohm damping resistor being the primary load (no input resistor and clamping diodes as in a mono coil), your coil discharge TC is 300uH/1000 ohms or 0.3uS. This number indicates that the smallest target that this coil and discharge TC will fully stimulate is 1.5uS. Try to keep your target size 1.5uS but change its radiating area and orientation to the coil and try to work within the coil radius distance and chart these results. I think you will start to see how these other variables come into play. Other variables include how your integration circuit is tuned for the range of target TCs you are detecting and how long the target is within the coil area as the coil is moved over the target (or target under the coil).

Many early meter detector tests were done with metal balls to eliminate the radiating area and orientation to the coil variable issue.

I came to appreciate these type of subtle things when I designed a guitar pickup using a current transformer (Triad CSE-187L or CSE-186L) and attached a very thick hairpin loop about 2.25" long with a magnet in the center to fit under 6 guitar strings. By using thicker wire as the string loop going from AWG 12 to AWG 6, I noticed an increase output because thicker wire produced more induced current but the skin effect keeps the higher frequencies from fully penetrating to the wire center thus allowing me to voice the tone of the pickup by using various wire sizes and/or numbers of smaller wire parallel turns. This taught me a lesson...understand all the variables and how they interact.

I hope this helps.

Joseph J. Rogowski