PI Frequency & Coils relation

[1843]

Hi all,

Would you please explain why we use low frequency for large coils and high frequency for small coils???
Someone claim that for achieving to deep penetration it is necessary to use 100Hz or a bit higher frequency when using 1m x 1m coils...But I see although Lorenz factory uses 700-1300Hz for his PIs, their PIs have an excellent depth penetration..... !?!?!?

Best regards

[Max]

Hi all,

Would you please explain why we use low frequency for large coils and high frequency for small coils???
Someone claim that for achieving to deep penetration it is necessary to use 100Hz or a bit higher frequency when using 1m x 1m coils...But I see although Lorenz factory uses 700-1300Hz for his PIs, their PIs have an excellent depth penetration..... !?!?!?

Best regards

Hi 1843,
you are right... first answer is "FALSE SIGNAL".

Using low frequencies gives you the ability of making a coil without shield at all thus eliminating one big problems of big coils... a big shield!
This is that way cause you could use that unshielded coil without worring too much of false signals, cause at lower frequencies there are much less skin effects and disturbance from e.g. wet surfaces (soil, grass etc).

So when you read you need lower frequencies translate as " less troubles ".

Of course, there are PI detectors with extremely good ground penetration (some examples include deepstarII (3000+ Hz range, tunable), lorentz's as you stated, but also e.g. GS4 (450-550 Hz tunable), XR-71 ...) and with relatively hi-freq (hundreds of hertz or thousands of) that are really good for depth search. Right but not only... generally speaking those are even better cause they get very good signals on smaller objects and e.g. on gold items!
DeepstarII is a good example, goes extremely depth and have really good gold detection too and not only in beaches (but be aware of pottery stuff ) and runs at 3000+!
Why this ? Ok, they are good detectors, well balanced, with good coils... everything fine and they work fine in right conditions!

Now second response: "COMSUMPTION"

Making big coils usually means stay low with number of turns (to keep low inductace and suit detector front-end needs)... less turns and bigger wire usually too... you have a drop in resistance... cause you also need to send big bursts of mag collapse by coil and all this take you to overall increase of consumption.

Ok. Now assume you have to switch-on for say 100uS the magnetic field...
you use lot of energy from battery with that coil... but if you slow down stuff and repetition rate (frequency) you have less bursts in the time unit, so less battery drain, so smoother operations... (cost is you lose some pinpointing and can not see a small target between two tx signals...). Is what e.g. DP does, less freq. (around 100Hz).

Only one reason to "blame" deepstarII is consumption: 350mA ! It's a monster! But if you want to find gold rings on beaches (at bigger depth than others) you would pay that cost!

Now third response: "FREQ. ATTENUATION IN GROUND"
This could be controversial...but someone reported that lower frequencies have better "penetration" on soil.
My point of view is (always the same) you see S/N. If you can increase S/N ratio by decreasing freq. you would find much stuff that way... and often this is the case. But some people think in different terms and design machines for the thousand hertz scenario... and (not a big surprise) they have success cause they make good designs (optimal S/N, optimal shields...) and then higher freq is not a problem anymore !

Lorenz's are that way. DP is on the other. And end of responses

Kind regards,
Max

[1843]

Many thanks for responding.

As I & Geo know shielding the coils become important when the sample delay is less than 30us and is independent of frequency...
What is your idea?

Best regards

[Max]

Many thanks for responding.

As I & Geo know shielding the coils become important when the sample delay is less than 30us and is independent of frequency...
What is your idea?

Best regards

Hi 1843,
yes it is. I know the same thing. Older Pi used this kind of delay and often deep seeking devices use longer delay than e.g. 10-15uS. That's because longer delay does affect only small items detection, where big ones are detected well... and in deep seeking you look for big things.
In normal conditions, with 30uS no shielding is required.

In some soil you could also run lower delay without shielding on big coil... and still having no or few false signals. But is very gnd composition related... e.g. absence of iron minrals, dry soil etc

Best regards,
Max