Coaxial PI Coil

[Anonymous]

Someone told me that a coaxial PI Coil has the same coverage or scan width as a PI mono coil? Is this true or false? Also, how do these coils compare for target size and depth?

 

[Anonymous]

Dave,
The one that I used had TX and RX coils the same diameter, so logically the scan width would be the same as a mono of equal diameter. My arrangement also used the middle coil as TX/RX for the all metal channel. The two outer RX coils were just for the discriminate channel. In air, the discriminate range was not far short of the range from the all metal channel but the signal varied a lot depending on the conductivity, or if it was ferrous. Different types of ground modified the discriminate range considerably.
Eric.

 

[Anonymous]

Just thought; did you mean coaxial stacked. or coaxial coplanar?
Eric.

 

[Anonymous]

Eric,
I meant coaxial stacked. Although the seperation between the coils is less than a Dual D and the ground signal will therefore be stronger, is it fair to say that a stacked coaxial will still have less ground pickup than a mono? Also, what would be the effect of spacing the three coils further apart?

 

[Anonymous]

Hi Dave,
Yes, a stacked coax coil will have less ground pickup than a mono because the TX coil is separated by a vertical distance from the bottom RX, which is close to the ground. In the case of a mono, the TX and RX being the same coil, give the tightest coupling to the ground. It is the surface mineralisation which gives the greatest signal variations with coil type. Mineralisation at depth (6in or more down) gives less change in signal between coil types. Increasing the separation in a coax coil will reduce ground effect, and also signal, as the TX gets further away. The signal from a small object will decrease faster than the ground so there is no advantage here. Another feature of the stacked arrangement is good noise cancelling in the discriminating channel.
Eric.

 

[Anonymous]

Hi All
With the co-axial,is the RX coils out of phase to each other.
Regards Frank Wallis

 

[Anonymous]

Hi Frank,
That is correct. The transmitter coil induces equal signals in each RX and because they are connected in opposition, the signals cancel. The electronics in the front end preserves the accurate balance needed. The opposed coils also cancel outside electronic noise.
Eric.

 

[Anonymous]

Hi Eric
Many thanks again for your answers to the techinal workings of the coils
Regards Frank Wallis