On the technical advantage of the X-terras

[Anonymous]

As to receive winding being exactly matched to transmit frequency, you can have the receive windings slightly out of synch. The target, depending on its composition will change the returned signal. For those who remember A.H. Electronics "Super Pro" detector, the loop was a printed spiral wound design transmit loop, with no receive winding. The detector was tuned to a low hum, and when a target entered the field it changed the coil frequency and alerts the operator.
There have also been some questions about Sovereign/Excalibur loops. Early models had a searchcoil which contained a circuit board that was part of the transmitter and preamp; later coils only have the preamp.

 

[Ralph Bryant]

Was the Super Pro a pulse design ? If not, how did it manage to operate with no RX winding ? Curious.

Ralph

 

[Varmint]

There are several ways, but one would be to monitor the drive to the TX coil closely, and look for variations in the amount of current required to drive the coil. This is based on coupling the signal to the target (which in this case also includes the ground, everything is a "target" so to speak. The more coupling that is going on, the higher the current required to drive the coil to a given voltage.

The transformation then is current to voltage, and the output signal is a direct relation to the amount of "target" within the "field of view" of the coil. If you are familiar with the Theremin "musical instrument", the concept is pretty similar. As the musician's hand approaches the antenna, it detunes the circuit causing a variation in output. I said similar, not identical, so try not to dwell on this concept as the be-all, end-all comparison.

If you are familiar with op-amps, the negative input would be coupled to the "drive" side of the TX signal, while the positive input would be coupled to the coil side of a sense resistor in the actual drive line to the coil. In this way, "loads" to the coil (targets and the ground) create a variation in the op-amp output that corresponds to the difference between the master (buffered) drive signal, and what the coil "sees".

In actual practice there would be more op-amps to condition the signals and shift the output into a useful range for the rest of the electronics, but the generic concept is as stated.

DAS

 

[Varmint]

coil design is everything, and it requires a "Farady Shield" around the coil in order to negate the effects of capacitive or electrostatic loading from non-conductive, non-mangeto-responsive things like hands and other things we would rather not have a detector respond to.

Attached is a pic of a coil I built.[attachment 12955 DSC01571.JPG] I was planning on building my own detector just like I did way back when, but once I saw the advances the new detectors had made I considered it a fools mission to put in the tons of effort required only to end up with higher expense (my time is worth $$$ too!) and untold hours of weaking, program modifications, and all the sort.

In any event, notice it is composed of a nearly continuous copper tube. At the left [attachment 12956 DSC01572.JPG]is a gap in the tube where you can see the windings themselves, at the right [attachment 12957 DSC01573.JPG]is where the windings enter and exit the coil assembly. Yes, it's quite a bit of "fun" to wind something like this, but in the end you have a well shielded coil that has near zero response to things you don't want it to respond to!

DAS

 

[Varmint]

n/t

 

[Anonymous]

It was a OFF RESONANCE DISCRIMINATOR. When something entered the field it changed the frequency and alerted the user. Info courtesy of Ty Brook.

 

[Anonymous]

Wish you would build some pigtail adaptors allowing us to use other brands of small loops on C$ and Excel, and for that matter X-terra.

 

[Joe in austin]

Looks like a good loop antenna for DF. Next time you're lost at sea, make sure you take that coil with you.

73 Joe

 

[Varmint]

I did build a 10M loop (octagon) from copper water pipe and 22.5 degree elbows that was REALLY quiet (noise) but hot as heck on signals TX and RX. Just like a loop is meant to be!

Pic is of the feedpoint, with fiberglass rod providing the stability to keep the ground and hot sides stable. The copper "strap" holding the SO-239 is the same copper pipe pounded flat to form a box.

DAS

 

[Joe in austin]

You should try that plumbers special on your detector...maybe you could work China, or at least find it!

I'll be listening for ya on my 1270!


Joe