Is the output power to the search coil the same as....

[thedeserttortoise]

I really like my Xtera 70 and have been using it for prospecting. What I am wondering is since the Whites, Fisher, and Tesoro all use 8 size AA batteries are they producing a deeper penetrating detector? Does this higher operating voltage equate to a higher power coil and transmitter? Any of you out there with the technical side of things? PS.. Been out of touch for a bit, had to have some new plumbing done on the old heart. Doc says one more tune up should get it... Hope someone can answer this. Tortoise

 

[Digger]

Glad to hear your "plumbing work" is going well. Get that all fixed up so you can do some detecting!

Having more batteries in the control housing doesn't equate to necessarily having a higher power coil and transmitter. By using the headphone amp of the codec as a transmitter, Minelab is able to generate more than 20V peak across the coil, from a source of 3.3V.

I happen to believe that George Payne is one of the best (if not THE best) at understanding and explaining how metal detectors work. Here are exerpts from a couple short articles where Mr. Payne examines transmit voltage and sensitivity:

For metal detectors there are a ton of items that effect overall target sensitivity. But I would like to discuss the one I consider the most basic and least understood. Also, it was overlooked for years as a way to increase detector detection depth. In the following explanation the coil is in parallel with a capacitor to form a loop tank circuit. Also, the loop and capacitor combination is driven by an oscillator at the tank resonant frequency. The loop magnetic field strength at a particular point is related to what is called (amp) times (turns) or AT. This is the loop rms current times the number of turns in the transmit coil. You can have a Xmit coil with one amp and ten turns for 10AT. In another coil you can have 10 amps flowing with only one turn. The result again is 10AT. Both of these coils would produce the same sensitivity if used as a transmit coil in a loop because both are rated at 10AT. The air sensitivity of both coils would be the same. However, the coil impedances are entirely different. To establish the required current, 1 amp for one coil and 10 amps for the other, each coil would require different drive voltages. In this case the coil with 10 turns would require a much greater voltage across it to develop the required current. In other words, these coils would require two different voltages for operation but the resultant detector sensitivity would be the same in both cases.

The AT produce by a coil is a function of the coil turns and loop voltage. It has nothing to do with the operating frequency. It

 

[BarnacleBill]

Per se the output voltage is not determined by the battery voltage. You can generate tens of thousands of volts with a battery. i.e A stun gun/taser. It's a combination of voltage & current that determines how quickly you chew through batteries along with the battery mah ratings. With an Induction Balance detector, which is the most commonly used, boosting transmit power soon arrives at a point of diminishing returns. i.e. You may boost transmit power to gain a 10 to 15% increase in depth in mild ground, but pay a battery life penalty of 75% for the resultant depth. On the Pulse Induction side there is an advantage to boosting transmit power because that technology works on a different principle.

The most efficient way to gain depth is on the receive side by improving filtering of ground noise and coil design. Remember the signal makes a two way trip, with attenuation losses caused not only by distance, but by the media(soil) it travels through. I guess a somewhat close analogy might be the difference of shouting to someone on a beach 100ft away, as opposed to walking into the water and trying to shout underwater the same distance. The distance hasn't changed, but the media(water vs air) sure has.

And actually there is some similarities between FishFinders/Sonar and metal detectors. Here's a quote from a three frequency FishFinder manual:

"Very high frequencies (455kHz) are used for greatest definition but the operating depth is
limited. High frequencies (200kHz) are commonly used on consumer sonar and provide a good balance between depth performance and resolution. Low frequencies (83kHz) are typically used to achieve greater depth capability."

HH
BarnacleBill

 

[David]

Tortoise, get well soon! They say vegetarians have less much heart problems due to less artery pug ups caused from animal products. So maybe go green(er) Tortoise!

The X-Terra's(with 9" Concentric) use 4 AA's and the Musketeer Advantage(with 7.5" DD--TM 800) uses 8 AA's. Randy said they both have about the same depth with those coils so the extra batteries do not make any difference in depth but just maybe last longer.

The MXT has 8 AA's so does it have more depth than the X-Terra 70 with 4 AA's? No I do not think so.

 

[thedeserttortoise]

I really appreciate the lesson here. This is all starting to make sense. My basic theory with what you guys are saying tells me that in addition to the simple power equation we are also adding inductance from the coil design and target characteristics, as well as power loss through the earth medium. Hence the need for so much more current to maintain a higher wattage as an end result. To get it either on the transmit side or the receiving end would become impractical. And earth, being the medium we have to deal with is so dence that that increase will just see more earth. I like the water analogy a lot. Radar is much more effective in air than sonar is in water too. I'm sure I'm off here somewhere but you have definitely made me aware of some dynamics that I will have to look into a bit more to fully understand. What would be good reading to better understand the cube laws? Are these values that are also associated with radio transmitting and reception? I can see now that its all a matter of power consumption and each models way of dealing with that issue. Randy, Dave, Barnacle Bill. Thank you. Tortoise PS>> also explains why the actual depths of both my Etrac and Xterra are so close at times.