In search of answers to RelicDigger's original post.....MattR.UK.

[MattR]

[quote RelicDigger1]Just got back from what started out as a lackluster day of relic hunting, a flat button and some scrap brass. At the end of the day, on a whim, I swapped frequencies and BAM, BAM, BAM...targets everywhere, and in a TINY spot no bigger than a car hood a lot of detectors have been tried (It's a bullet hole).

What are we doing when we increase/decrease the "frequency"? Are we actually going from 13kHz to 13.2, say?

It was simply phenomenal to watch. Changing the frequency seemed to light the ground up despite a heavy nail concentration.

Thanks[/quote]

_________________________

RelicDigger

 

[Brainwasher]

I guess my question to this is would a true multi-frequency detector, such as a Minelab Explorer have hit the targets without having to stumble upon a frequency change like that? If you truly didn't know you had the interference and the detector seemed stable, what would the chances be that you would change the frequency? I finish my tour in Iraq in January and fully intend to purchase the F75 when I get home. I've already made up my mind on the F75...I think. From what I have read, it is pretty close in depth, but has a much faster recovery speed than the Explorer, which in my mind means covering more ground in a set amount of time, which would logically mean more opportunity to run the coil over good targets. However, posts like this one scares me into wondering if I'm making the right choice. Covering more ground in a set amount of time isn't going to mean much if I'm leaving the goodies in the ground behind me. Maybe someone can let me know if I'm about to make a mistake here. Thanks. Jeff

 

[Streak!]

sometimes its possible to "overanalyze" a point or observaton. Also..............one of the tenets of detecting..and setting up your machine....is that what might work for one person in any given situation......may not (and probably wont) work you YOU....and the situation you find yourself in at any given time. RF interference comes in many many forms......and likely affects our machines in ways we cant possibly know...be it an Explorer OR F75. (and thats not even taking into account all the other variables..IE: ground conditions, mineralization, amount of moisture in the soil..etc..etc..)
My point is....quite simply, anything you may read on a forum..no matter how valid it may be for the poster, may have no relavance at all for YOU and any given situation you find your self in.
I've tried changin freq's in may area after detecting in the default freq..and found that it made no difference in the "detectability" of any targets..............AT ALL! Now, thats not to say it didnt help things for someone else..but as Matt so appropriately put it............we simple dont have enough relavant data about the conditions the origonal poster found himself in to make ANY kind of assumptions at all!
To answer your origonal question "quote" (I guess my question to this is would a true multi-frequency detector, such as a Minelab Explorer have hit the targets without having to stumble upon a frequency change like that?) "unquote".
Probably not. While the Ex Is a multi freq detector..that doesnt mean its immune to interference any more than any other machine on the planet. In fact.....multi freq machines dont have the market on depth and performance, AND...single freq machines actually have a few strenghts that multi freq machines DONT have!
Take my advice....buy the F75 and dont look back. While there "may" be that odd time where the Ex might out perform the 75.........there will be MANY times the 75 will outperform the Ex. No matter how much we love our primary machines there is no one magical machine that will do it all. Best we can hope for is one that will work good in the MAJORITY of situations.....

 

[Brainwasher]

Thanks Streak. You're right, I tend to over-analyze everything. Tediously analyzing which detector to buy really isn't working out so well for me. Too many factors involved and it's driving me insane actually. Your post has given me back that warm fuzzy feeling that I'm getting the right detector with the F75. Hopefully, this was just an isolated event for just one person. I don't know much about how interference works, but I was thinking that if it attacks only a certain frequency range that a multi-frequency detector would get around it by pushing signals above and below the interference, but if interference is going to wipe out all the frequencies across the board, then I could see where it wouldn't matter at all. Does this make sense, or am I way off on my thinking?
The other thing that has me wondering a bit is about two years ago I took my MXT, with the stock coil, over a field in Louisiana and found several three ring bullets. I then switched to a larger excellerator coil and went over the same field and found three Enfield bullets. They weren't any deeper than the three ringers, but for some reason the stock coil didn't find any of the Enfields. Maybe I just missed them the first time through, but since all three Enfields were a different type of bullet than I found with the stock coil, that got me thinking that maybe the frequency was slightly altered by the coil change and the Enfields, being made at a different time or place, with a slightly different consistency of lead now showed up. Of course that makes me wonder if a multi-frequency detector wouldn't have hit all the bullets in the first sweep. I know there is much more to it, soil conditions, conductivity of objects, etc...but I was wondering if others are seeing anything like that. I'm still pretty much decided on the F75 though. When I was poking around different forums a couple of months ago, I had some Minelab users suggest the F75, but not too many F75 users suggesting the Minelab. I would still like to hear if others think they are missing targets because of using a single frequency. Thanks again. jeff

 

[Streak!]

depending on the wavelength and amplitude of the interference. I've hunted under power lines that "pulsed". Interfered one minute......but not the next. Dont have a clue as to what causes THAT, but it happen's.
I'm sure there are certain freq's that are less suceptible to outside RF interference, but as I said before having a multi freq machine doesnt make one immune to interference. Some mistakenly believe for instance...that minelabs transmit on 28 different freq all the time.but thats just not true. they test the matrix..and settle on the freqs they deem best for that particular situation. While I guess thats a good thing...it doesnt mean they will settle on one that handles interference very well, unless possibly the imterference is there when it does its thing on startup. Lets ask the minelab guys about this. I know there are a few here that use SE's and Ex II's. how often do you guys encounter interference?
I can honestly say that using both the Shadows.and f75, I only encountered one time where I experienced experience from an external Rf source. I HAVE though......on every machine I ever had...experienced interference from other machines....MUCH more often than I have from external Rf sources. In fact the interference from other machines is much more insidious!
We do a lot of comp hunting.......and during a break one day we decided to test how machines affected each other, as we hunt in close proximity...and also had a DEEP hunt coming up.
We tested a number of machines.and the results were pretty much the same whatever we used.
We would bury a target at the edge of detection depth (the maximum depth where we could still pick up the target reliably in all directions) and then swing over it with one machine as another machine approached. The results were suprising! WAY before we could audibly pick up the other machine,,,,,, the target in the ground would just dissappear. I'm talking 30 40 feet away. It had a tremendous effect on depth, long before the other machine got close enough to actually hear it affect the machine we were testing. I personally think thats a large part of the reason why targets are missed a lot out in the field whe hunting with buddies or in groups. I havent tested the 75 for this yet...........but I have no reason the think it will be and different in this respect than any other machine I've ever used.
Like I said in my previous post, interference affects us in ways we cant even begin to understand......and all we can do is use the machine we like and make the best of it. just have fun and enjoy the hobby!

 

[Mike Hillis]

Think about the threshold setting for a moment. If you set the threshold below audible hearing, small signals that barely influence the threshold settings are not heard. The further negative you set the threshold, the larger the signal that can be silenced. On the positive side, you can also raise the threshold setting to a point that the smaller signals are saturated by the threshold tone. The loud tone actually hides the smaller signals. The louder the tone, the louder the signal that can hide within it.

Frequency shifting is acting just like that threshold control. One of those channels is like the perfect threshold setting. Too low a freq to noise setting and the small signals are gone. You don't even hear them. You don't hear the interference and you don't hear the small signals. To high a freq to noise setting and the noise itself is hiding the signals. You hear lots of noise and you can't tell the difference between the interference and the good signals.

Part of our job as operator at the initial setup is to determine what the proper freq to noise setting is for the site. Too often we just want the noisy interference to go away. We don't actually look for the optimum, correct setting for the site. In RelicDigger's example, he accidentally stumbled onto the optimum freq setting for his site. What we want to do is take the accidental part out and make it intentional.

I think the easiest way to set the freq to noise setting is in the all metal mode by using the threshold setting. Set the threshold to that barely audible range and then set the sensitivity setting to where you just start to hear it breakup. Then shift frequency, searching for the one that allows a stable threshold. You should be able to tell a difference. If you can't, then either raise or lower the threshold setting to where you can tell a difference when you shift. You may even have to lower the sensitivity setting a little more. You may not hear a difference between every setting but you will be able to tell the difference between some. Unless you are real lucky and in a dead zone somewhere you can find the settings that give you the most sensitive machine.

The hard part to this is that this optimum freq to noise setting changes as conditions change. Introduce another metal detector 150 feet away and chances are everything is ok. Move that other detector within 50 feet and things change. Neighbor turns on his CB base station to chat with a friend in Nome AK, Somebody standing behind you starts chatting on his cellphone, The outdoor lights come on, etc.... Your clue to the change is either the small signals have stopped or now increased to where you can't tell good from bad. With some experience you should be able to find the right freq to noise setting and establish a baseline signal response to listen for to alert you to changes.

Of course you can work up to this and not every site warrants this much attention but its good practice for the sites where you do want top performance

HH

Mike

 

[Scully]

The weight difference is substantial. I agree with Streak, buy the F-75. If made a list of good qualities and compared the two, the F75 will far exceed the Explorer.

 

[Scully]

I too have experienced such an effect, but not in a planned test. When hunting with parters and one calls the other "over" to test a target, the original person would sometimes have to shut off his machine before I could pick it up. I am curious which machines were involved in your test.

 

[vlad]

There are a variety of things: poorly designed or shielded front ends [oscillators]; also if a oscillator is not properly shielded or designed it can pull in spurious and ambient signals and transmit them along with the detector's signal [George Paine]; poorly made ground filters [George Paine, Ty Brooke, Troy, Frank Ball, Charles Garrett]. Search coil design is a major factor as anyone that has used a co-ax can tell you. It also seems that 15kHz is the frequency that is closest to 60 cycle noise [George Paine, Troy, Ty Brooke]. When you frequency shift you are not going up or down to a new frequency, as this would cause off resonance to the point that the coils "Q" would be so bad that you would have a severe loss in depth, or it might not work at all [Ty Brooke, George Paine, Keith Wills, MR.BILL]. When you frequency shift what you are doing is looking for one of the even harmonics which are much less affected by 60 cycle interference than the odd harmonics which are affected to an extreme [George Paine]. So doing a frequency shift only quiets down the detector, it of itself causes no target to be more findable. (Also there is no one frequency that is best for all targets, so the best you can do is have an operating frequency that is a compromise [George Paine]). [There is no signal sampling period with VLF DETECTORS which transmit on a constant wave-that term is relevant to pulse induction of which the F-75 IS NOT.]
So besides frequency shifting you have three options with your current detector when encountering interference: changing search coils looking for a different design or one better shielded, turn down your sensitivity which on most machines means "receive sens.", or turning up the TX sensitivity if your detector has one like the Nautilus DMC-2B, or the TREASURE BARON with DEEP-HUNTER MODULE, and simply overpower the interference [A 4TH option would be threshold if your detector has one].
The only other option is a different detector.

 

[jim tn]

Good post and thanks! HH jim tn

 

[Streak!]

mostly Exploders.....Shadows.....a couple of fishers...and tesoros. We did try it with a DFX too.and the resouts were pretty much the same

 

[vlad]

Get a CZ-5 [they are dirt cheap] with a 10.5" loop. You will NOT be disappointed. Contact me for who has best prices on a preowned with a warranty.