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The DFX, other detectors, and EMI (Electromagnetic Interference).

February 20, 2011 11:14AM avatar
Quote
SquareNail
Speaking of the DFX. I just finished reading the White's DFX Engineering Guide and noticed under the External Interference heading it reads:: that power line interference is stronger at lower frequencies. Is this true.

Well, like most of us, I am just an enthused detectorist who likes to search a variety of sites, and do so without a lot of interference. When hunting at different locations I might experience some noise or chatter and can often deal with this problem by reducing the Sensitivity level. This might have nothing to do with the operating frequency.

In some locations I might note that while I seem to be annoyed by noisy interference when using a larger-size coil, I can switch to a smaller coil and operate much quieter. Operating frequency doesn't seem to have a bearing on this.

There are also times when it might be a chttery environment and I can simply opt for a different metal detector which might not be so noisy, even with a higher Sensitivity setting. These detector changes might reflect a model with a different operating frequency, or they might not.

Several years ago I hunted a torn up sidewalk in the heart of downtown Portland, Oregon. There are a lot of businesses with some high-power transmitter devices, like a nearby police station or fire station, plus other businesses with their high-rise power sources, not to mention the overhead power lines.

That day I had two metal detectors with me. I first hunted a wide torn out sidewalk and into the dug up street using my White's XLT. I opted for my favorite search coil, the 6½" concentric (5.3 BullsEye) and my AC Sensitivity was higher with my custom program, but due to noisy operation I had to reduce my Pre-Amp Gain from '10' down to somewhere from '4' to '6' to stabilize the detector.

I hunted a block of renovation mess and found a nappy Indian Head, a few very early wheat-backs, a 'V' nickel, and about 80% of the way down I popped an 1887 Seated Liberty dime. There were a lot of nails strewn about and I thought I might have better performance with a slower-sweep model with a quicker response. Besides, my blood sugars were a bit low and I needed to get to my rig and guzzle down some orange juice.

I had been able to comfortably work this area with a high-tech XLT and smaller coil, but after I felt better I decided to switch to my White's IDX Pro, which also had a 5.3 BullsEye coil mounted. When I crossed the street to where I first started, I turned the detector on and it was the noisiest operation I had heard in quite some time.

My model was modified with external Threshold and Ground Balance controls and I tried to ensure I had everything properly set, but even reducing the Sensitivity control to the minimum level the detector was just way too chattery to even try and detect. I moved down from the corner about a third of the block to see if there might have been more interference from the power lines directly overhead, but no success. Noise! EMI caused noise.

These two detectors were from the same manufacturer, were each using the same 6½" coil, and worked in the same environment the same day. The more technical model worked reasonably quiet with only a little sputter now and then, while the generally efficient, quicker response model was totally inoperable. Same brand, same coil, and both worked at the same 6.59 kHz frequency. Thus, frequency was not the main cause for the instability. I went back to my XLT to finish hunting that day, and the following two days, in that downtown area. On the next two I only had a little success with the IDX Pro on a weekend when, I can only guess, it meant that there was some EMI source that wasn't in use.

Once, when working with the 2nd or 3rd DFX I had, I was trying to deal with some noisy operation in a different town, 30 miles away where I live now, and working under power lines doing more sidewalk repair. My friend with her 6.59 kHz XLT wasn't having a problem. I worked some of this area with my 6.59 kHz IDX Pro without a problem, even at maximum Sensitivity (where I usually operated with it). I also hunted there with an MXT which is right close to 14 kHz and not a problem. So, with the DFX in hand, I had the opportunity to try and narrow down the noisy operation problem and I opted to select a frequency to search in. 5 kHz or 15 kHz were my options and the lower frequency was the quietest with similar higher Sensitivity and Pre-Amp Gain settings, while the higher of the two frequencies was noisier. So much for lower frequencies having more power line EMI, at least with one detector at in a noisy environment cause by overhead power lines.

In the past two weeks I have demonstrated several models from four different manufacturers, and with different results. In four open playgrounds that were at a park and school in one town, and at two schools in another town, and this is working close to the school building where I can often have some interference, or in the case of two of these sites I have very nearby overhead power lines, I had the most instability with a Teknetics Omega when closer to the school and power lines. The next most chattery was the White's M6. One might think the 7.8 kHz Omega was noisier because it works at a lower operating frequency than the 14ish kHz M6, but wait!

The quietest operating models were the Teknetics Delta, also operating at 7.8 kHz and the 19 kHz Teknetics G2. Only slightly noisy but not as bad at these sites were the also 7.8 kHz Teknetics Gamma and 6.59 kHz XLT, and all models were operated at a Sensitivity as high as possible. The Teknetics T2, operating at 13 kHz, isn't a lower frequency in this comparison, but was able to be calmed down with a reduced Sensitivity setting. So, the low or high operating frequency didn't really seem to mean much in this case. Naturally, all of these models are a "lower frequency" if we allow for the fact that they are all in the VLF (Very Low Frequency) range between 3 and 30 kHz, but it's obvious that other things entered into the noise condition other than simply operating frequency.

Maybe two weeks ago I was closer in to downtown Portland in an urban neighborhood where the streets are so narrow that only one car can get around a parked vehicle at a time. A lot of overhead power lines, too. I was demonstrating a few models to a fellow in the dirt parking strip and I used the 7.8 kHz Omega. It was a bit noisy at full Sensitivity but I could reduce it to its setting of '65' and it worked fairly quiet and I used an Indian head cent and nickel and Quarter as sample targets on the dirt.

I then went to a Teknetics Delta and Gamma, at the same frequency, and the Delta at the default setting of '8' could be increased to '10' (top Sensitivity is '12'), and the Gamma could be raised to '85' or '90' and was stable. The higher frequency T2 at 13 kHz was just a bit noisy at the turn-on default Sensitivity of '60' but was calm enough to work, and smoothed when reduced to its setting of '55.' Then, since he was interested in a nugget hunting model, I grabbed the new 19 kHz Teknetics G2. This unit can be increased from a low Sensitivity of '1' to a maximum setting for it that reads '100' and, most of the time, I can run the G2 at its Sensitivity level of '100,' or maybe reduce it to the low '90s', but in that parking strip location with overhead power lines, the 19 kHz G2 was totally inoperable. Totally! Even at its minimum Sensitivity setting of '1' it was a constant loud and pulsing racket!

So, in this scenario, the highest frequency was the noisiest, and one of three lower frequency models was second noisiest, but controllable. Also, the 19 kHz G2 wasn't the most powerful and deepest-seeking model compared. Actually, the G2 is the weakest performer on depth of detection of all of them, while the T2 is the deepest, by far. Even at the default Sensitivity setting of '60' or reduced to '50', it still matched or bettered the depth potential of any of the others.

So, whenever I read anything that is a technically scientific statement like "power line interference is stronger at lower frequencies", I seem to insert a little word change and say
"power line interference is might be stronger at lower frequencies" and then make a final determination of what is and isn't most effected in a real-life hunting environment.

As for the DFX, I usually found it to work a little quieter at the lowest of the two frequencies it offers. So much for EMI science. With any detector I use, regardless of the operating frequency, a lot depends upon the detector's electronic design, the search coil used, and then I simply decide if it does or doesn't work quietly in a particular location.

Monte
Subject Author Views Posted

Frequency in Iron Sites

SquareNail 889 February 15, 2011 01:49AM

I presume you're asking about handling iron trash, not finding it, and frequency doesn't really factor in.

Monte 2282 February 16, 2011 12:17AM

Re: I presume you're asking about handling iron trash, not finding it, and frequency doesn't really factor in.

SquareNail 692 February 17, 2011 03:06AM

Re: I presume you're asking about handling iron trash, not finding it, and frequency doesn't really factor in.

Monte 664 February 18, 2011 12:51AM

Re: I presume you're asking about handling iron trash, not finding it, and frequency doesn't really factor in.

SquareNail 626 February 18, 2011 11:57PM

The DFX, other detectors, and EMI (Electromagnetic Interference).

Monte 1851 February 20, 2011 11:14AM

Re: The DFX, other detectors, and EMI (Electromagnetic Interference).

SquareNail 682 February 21, 2011 03:29PM

Re: The DFX, other detectors, and EMI (Electromagnetic Interference).

Monte 1230 February 22, 2011 02:46AM

Monte...I sent you email

SquareNail 624 February 25, 2011 01:23AM

Thanks, 'SquareNail, I got the e-mail.

Monte 569 February 25, 2011 03:54AM

Re: The DFX, other detectors, and EMI (Electromagnetic Interference).

ron_c 606 February 22, 2011 04:32AM

Using a big coil to "see" through (below) trash? Usually doesn't happen.

Monte 643 February 22, 2011 05:32PM



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