Interference on 10.6 kHz with Tesoro Silver uMax

[TallTom]

I recently bought a Tesoro Silver uMax metal detector.

I think I'm having a problem with radio interference from
a powerful transmitter that uses the same frequency that
my MD uses, which is 10.6 kHz, according to Tesoro's specs.

Of course, the interference might come from a transmitter
on a harmonic frequency, such as 21.2 kHz. (At least, that's
my limited understanding. I'm no radio expert.)

Here's what I encounter when I go MD-ing.

I turn on my machine and (usually) everything works fine.
I can do air tests and surface tests of coins and other
metals, both in Disc and All Metal modes, and I get the
response from the Silver uMax that I expect. I can go
to my back yard, or a nearby park, or the beach, and
I get good results. By the way, I live in the Los Angeles
area. The beach is usually Santa Monica beach.

But sometimes, I suddenly start getting weird interference
on my detector. If I leave it in Disc mode, I start getting
constant false signals. I have to turn Disc up to maximum
and Sensitivity to near minimum in order to stop most of the
false beeps. But I still get so many false beeps that detecting
becomes nearly impossible. Of course, with Disc and Sensitivity
set like that, I'm not going to detect much anyway.

This is not just true at the beach, by the way. It also happens
in my back yard and in a nearby park, both of which are about
15 miles from the beach.

But the big problem becomes more obvious when I switch to
All Metal. That gives me constant chattering that sounds
remarkably like Morse Code. I don't "speak" Morse, but I've
played with it enough over the years (Dad was a ham) that
I recognize the relative lengths of the dits and dahs, and
the spacing between characters and words. There's a little
extra beeping mixed in, but it's amazingly like Morse code,
transmitted pretty fast.

At first, I thought my detector had gone bad, maybe a loose
connection that was causing intermittent shorts when I moved
it around, causing these beeps. I finally sent it to Tesoro
to check it out. I hadn't formed my radio interference theory
at that time, so I just explained that it gave me a lot of
chattering in AM mode, and at that time I thought it might
be worse when I had made contact on the coil with moist sand
when I was at the beach. So I told them that.

Anyway, they said "Final Test Good!" and returned it to me.
It still has the same problem. I used it Friday, April 6,
and from about 5:45 PM to 6:05 PM it worked fine. Then the
"Morse Code attack" began. It continued until about 10:05 PM,
although I wasn't monitoring it the entire time. However,
it was pretty continuous from 6:05 until at least 8 PM;
at 10 PM I turned it on again and it continued, but stopped
within a few minutes after that. (All times are Pacific
Daylight Time.) I used the detector from 10:05 to 10:55 PM
or so and it was fine.

Has anyone else encountered this type of problem?
Is 10.6 kHz the only frequency that receives Morse Code?
I've checked the usage of the radio spectrum and it shows no
usage for 10.6 kHz. (See http://www.vlf.it/trond2/10-15khz.html )

And finally, it there anything I can do about this? Should I
ask Tesoro if they can modify my Silver uMax to use a different
frequency? And if so, what is better?

By the way, there are interesting explanations about VLF and its
uses in Wikipedia at:
http://en.wikipedia.org/wiki/Very_low_frequency
and
http://en.wikipedia.org/wiki/Lualualei_VLF_transmitter

The big Lualualei transmitter uses 21.4 kHz to transmit. That's close
enough to 21.2 (twice my 10.6) that I wonder if it's the cause of
my problem. Radio transmissions to submarines!

Thanks in advance for any info you can provide that might help me
figure out the cause of this problem and what to do about it.

 

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[signal_line]

Never used one, so I don't know. Try a different coil. You might have a leak. I had that problem. You can try to squeeze it to see if any air leaks out, then dry it out and repair. Make sure battery is good and there is no debris inside the coil cover.

 

[woof!]

Tall, it's probably either military VLF as you suspect, or something up in the VHF-UHF range sneaking into the electronics.

If it's VLF, the orientation of the magnetic field will usually be close to horizontal, in which case the pickup will be at a minimum with the searchcoil in the horizontal plane such that the lines of force don't go through the area subtended by the receiver coil in the searchcoil. Also, if you hold the searchcoil in the vertical plane and then slowly rotate 360 degrees, the signal will be strongest when the plane of the searchcoil is pointing at the source. You can probably travel 10 miles away and get a similar result. If you are near a large mass of metal such as a steel building, or near buried infrastructure such as pipes and cables, the shape of the magnetic field will be distorted and because of differences in phase, you may not be able to find a null. However you may be able to trace the route of the underground utility.

Many years ago I used a metal detector to RDF (radio direction find) to approximately locate a VLF source northeast of San Francisco, presumably Navy sub communications, from well over 100 miles away.

With a VHF-UHF source, travelling ten miles is usually a game-changer unless it's a source you brought with you such as a cellphone or a wireless security system. There are a few spots in So Cal that are so hot with military radar that most metal detectors go berserk. Ten miles away, you're fine.

Since you're pretty sure you're hearing something that's like Morse code modulation, I'm betting on a military source in the low 20 kHz band, as you suspect. With the abandonment of the Omega radionavigation system, there's not much high powered transmitter activity in the 11 kHz region these days although the military may be moving with new stuff to fill the void. The modulation you hear is probably not OOK (on-off keying) but FSK (frequency shift keying). And the code itself probably isn't straight Morse.

When you get it figured out, please let us know.

--Dave J.

 

[TallTom]

Thanks for your answer, Dave. I think you're correct, but we'll see.

I just got a response from Robert at Tesoro (to my posting of this
same problem over in the Tesoro forum). He said to send it back
in to Tesoro to his attention and he will change the frequency.

I'm going to do that and I'll post the results after I get it back.
Thanks!

 

[TallTom]

I just posted the message below in the Tesoro sub-forum under the Brands
forum. I'm also posting it here, since I posted the original problem in both
places. Problem solved!

=============================

I received my Tesoro Silver uMax back from Tesoro. They said that I
had an "intermittent problem in the receive windings of my coil", so they
had to replace the coil. Since the problem wasn't constant, it passed
their tests the first time I sent it to them.

I've taken the detector out and used it twice since getting it back, for a
total of about seven hours, and it works fine! No more "Morse code
interference". Tesoro also switched my frequency from the standard
10.6 kHz to 9.8 kHz, but it seems clear that the solution was replacing
the faulty coil, not the frequency change. I can't notice any difference
with the new frequency (depth, sensitivity, etc.) so I'm happy to leave
it this way.

Did the faulty coil actually cause my Silver uMax to receive Morse code
signals, possibly even ones to submarines? Or was I just hearing noise
that sounded to me like Morse? I can't really say. All I can say is that
it seems to be fixed and working fine, and I'm happy that Tesoro was
able to fix it and turn it around to me quickly both times I sent it to them.

So add my voice to the others you've heard (seen) here and elsewhere
saying that Tesoro stands behind its products and goes out of its way
to make things right for its customers. My dealings with Robert and
Rusty have convinced me that they treat customers the way they would
want to be treated.

Compared to numerous other companies I've had to deal with (in other
fields, not metal detecting), that's very refreshing. Almost unique, in fact.

--Tom

 

[signal_line]

You're welcome.

 

[TallTom]

Thanks for your feedback. I wish I'd had a different coil when the problem came up.
I could have tried changing coils and narrowed down the problem sooner.