Why does the ctx ignore gold chains

[sube]

I know it only sees one link at a time .But the detector is suppose to run from 1.5 to 100 kHz with 28 frequency's . Does it choose a frequency according to the ground conditions .My point here is I have a mxt which runs at a set frequency of 18 kHz I think and has no problem of picking up a gold chain , that's in coin and jewelry mode .Why then does the ctx ignore the gold chain link. sube

 

[Southwind]

Actually, I believe the CTX, like the other FBS's, only uses 3 frequencies made up of a combination of 28 difference frequencies.

Gold chains, and odd shaped pieces, are a problem for FBS detectors. They are actually a problem for most all detectors but especially the FBS's. It's one of those things if one detector did it all better than the rest everyone would own one. You might ask why your MXT doesn't correctly ID a silver dime at 10" like the CTX can.

 

[sube]

South wind What I really would like to know is how the detector picks it frequency is it by analyzing the ground or some other method . sube

 

[Southwind]

My understanding, and I could be wrong, is that when you use Noise Cancel it chooses 3 out of the 28 available frequencies based on the least interference. When you manually choose the Noise Cancel frequency you're also choosing the 3 frequencies it will use. My experience has been the lower Noise Cancel, say 1, ends up with the best frequencies for deep silver. I.E. lower frequencies. The high Noise Cancel, say 10, works best for low conductors such as gold because it uses the higher 3 frequencies.

 

[GKMan]

Not so long ago there was a good discussion on this...

http://www.findmall.com/read.php?86,2005119,2005131#msg-2005131

 

[Larry (IL)]

I think Minelab got away from the misleading 28 frequency thing with the CTX 3030. The new info I have read just claims FBS 2 uses multiple frequency transmission which is believed to be three received and processed primary and offset frequencies, all in the lower metal detecting spectrum of 2-5 KHz. That range of frequencies favors higher conductors like Silver and Copper.

Very small gold like fine chains and nuggets, requires a detector operating in the 15 KHz range or higher. The higher the operating frequency, the smaller is the gold targets detected but the detector becomes less sensitive to the higher conductor targets. What operating frequencies to use for any detector is a trade off.

 

[sube]

Thanks Larry . That makes sense 2-5 kHz is not very good for gold . What made me wonder was the 1.5 to 100 kHz . sube

 

[Larry (IL)]

ETrac and before, Minelab said they transmitted or utilized those frequencies as most detectors do anyway counting the harmonics of the primaries. They never said which of those frequencies were received and processed. Very misleading.

 

[DaytonaGold]

Would it be possible to develope a coil that could detect chains or does all this have to do with frequencies?

 

[Overvoltage]

Southwind, so you are saying thay if someone does a manual noise canx and chooses a high chanel then that makes the CTX Xmit at a higher freq, and thus should be better at detecting small gold. This would be subjecting to higher probability of interference from EMF. Correct? Has anyone tested this?

 

[sube]

Overvoltage Yes I tried several different channels from 1 to 11 still no hit .Has to do with it running on 2-5 krz I guess . sube

 

["Dr.Tones"]

Where's Digger, He can explain it best.

 

[Johnnyanglo]

This has been discussed before... but quickly,

The FBS design sends a long pulse and then short pulse trains at two frequencies. The low freq is 3.125KHz and the High is 25KHz, which is a harmonic of the first and not as strong. There are many harmonics in the transmit signal, but they are not used (the detector only sense the low/high return signals). The idea that there are 28 freqs broadcast is true, but they are unused harmonics. Might as well say there are 100 or 1000 frequencies - again, true, but not used.

So, what does this mean? It means the detector is designed to find high conductors (which are usually valuable) and ignore low conductors (which you don't want, like iron trash). Low freqs drive deeper into the ground and do not excite low conductors well (low conductors need many pulses to drive eddies to return a strong signal, but you don't want them to do that, which is the problem with high freq detectors - they find tiny iron).

The Noise Cancel simply shifts the base freq (and all harmonics) about 10% to avoid interference with other detectors. It isn't going to give you significant target sensitivity changes, though some think it does.

A small gold chain, thin ring, small clasp, thin earring, etc. are all going to generate weak eddies when hit with low frequency transmit signal. The high freq helps but is weaker ... so, you probably aren't going to find these thin jewelry items unless attached to a pendant or some other conductive surface. Your best chance to find thin jewelry will be circular shaped targets that generate a composite eddy that circles the target (like a ring). A thin straight pin, or broken ring (broken circuit) is going to be very weak. The tiny individual links in a gold chain individually produce a very tiny eddy (it is not summed) and therefore are below the sensitivity of FBS machines. However, a high freq gold detector would light them up to a considerable depth.

The advantage of the CTX is it's ability to go deep in iron/mineralized soils and still retain reliable target identification. Other detectors can find smaller jewelry at greater depths but the higher the soil minerals become the more the advantage swings to the CTX (and E-Trac). There is no perfect detector.

 

[GKMan]

Johnny, you always post messages that seem very well thought out and technical. But you don't post here much. It's that because when you're not posting your busy designing detectors for Minelab?

 

[Larry (IL)]

Thanks for the explanation Johnny. I have been told by a competitive engineer all along that the CTX and other Minelabs were two frequency detectors while others "in the know" insisted on three. It is good information to know what is really going on.

 

[rsarge1]

very interesting thanks for the explanation Johnny

 

[Southwind]

I think the idea that it uses 3 frequencies came from Minelab saying FBS analyzes/processes 3 channels. "FBS detectors listen in on what Minelab calls three Channels"

It could just be the soil but I know for a fact my E-Trac/CTX hits better on deep silver when Noise Cancel is 1 as apposed to 10. All you got to do is try it. Not much, but it does hit better.

 

[Larry (IL)]

Terry, theoretically, channel one is a slightly lower frequency and should go slightly deeper with all other factors being equal but for some people, channel one may have more interference than say channel 6 in any given location. In that case, channel 6 will detect deeper than channel one. EMI will vary from location to location so I trust the CTX to find the best channel for me.

 

[Southwind]

That's kind of what I figured. The reason it seems to hit harder is more than likely just because channel 1 is the channel with the least interference in my area.

Another thing I've noticed, and the reason I manually set mine to 1 most of the time, is if I'm alone and do a Noise Cancel the E-Trac/CTX will pick a lower channel but when I'm hunting with a buddy and do a Noise Cancel it picks an upper channel every time. Usually 8-9

 

[GKMan]

This has been discussed before... but quickly,

The FBS design sends a long pulse and then short pulse trains at two frequencies. The low freq is 3.125KHz and the High is 25KHz, which is a harmonic of the first and not as strong. There are many harmonics in the transmit signal, but they are not used (the detector only sense the low/high return signals). The idea that there are 28 freqs broadcast is true, but they are unused harmonics. Might as well say there are 100 or 1000 frequencies - again, true, but not used.

So, what does this mean? It means the detector is designed to find high conductors (which are usually valuable) and ignore low conductors (which you don't want, like iron trash). Low freqs drive deeper into the ground and do not excite low conductors well (low conductors need many pulses to drive eddies to return a strong signal, but you don't want them to do that, which is the problem with high freq detectors - they find tiny iron).

The Noise Cancel simply shifts the base freq (and all harmonics) about 10% to avoid interference with other detectors. It isn't going to give you significant target sensitivity changes, though some think it does.

A small gold chain, thin ring, small clasp, thin earring, etc. are all going to generate weak eddies when hit with low frequency transmit signal. The high freq helps but is weaker ... so, you probably aren't going to find these thin jewelry items unless attached to a pendant or some other conductive surface. Your best chance to find thin jewelry will be circular shaped targets that generate a composite eddy that circles the target (like a ring). A thin straight pin, or broken ring (broken circuit) is going to be very weak. The tiny individual links in a gold chain individually produce a very tiny eddy (it is not summed) and therefore are below the sensitivity of FBS machines. However, a high freq gold detector would light them up to a considerable depth.

The advantage of the CTX is it's ability to go deep in iron/mineralized soils and still retain reliable target identification. Other detectors can find smaller jewelry at greater depths but the higher the soil minerals become the more the advantage swings to the CTX (and E-Trac). There is no perfect detector.

And just like that Johnny vanished again...who is this Johnny? who always appears whenever there's a thoughtful insight required.