Uncle Willy
New member
The bottom where the red disc is located is the size of the area your signal is covering at peak depth.
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What your detector coil signal really looks like - a hair net on steroids.
- Thread starter Uncle Willy
- Start date
n/t
That version looks a tad fuzzy. Probably from being overly enlarged.
Here is the original I posted that might look a tad better..
Even mine is a bit fuzzy because I compressed the image so much.
The original was sharp as a tack. I should still have it somewhere, but
it's probably on a different older HD by now..
I used actual antenna modeling software to generate that. "EZNEC"
I modeled that vaguely after the Ace 250, and used the same 6.5 kc frequency
in the model. It shows the bi-directional pattern as it should.
I ran the coil through eznec, and generated the pattern, then added the
graph, color bars, etc last. I guess I was kinda bored that day..
Here is the original I posted that might look a tad better..
Even mine is a bit fuzzy because I compressed the image so much.
The original was sharp as a tack. I should still have it somewhere, but
it's probably on a different older HD by now..
I used actual antenna modeling software to generate that. "EZNEC"
I modeled that vaguely after the Ace 250, and used the same 6.5 kc frequency
in the model. It shows the bi-directional pattern as it should.
I ran the coil through eznec, and generated the pattern, then added the
graph, color bars, etc last. I guess I was kinda bored that day..
bearkat4160
New member
n/t
Uncle Willy
New member
Yeah I forgot I had that. You put that up quite some time ago. Do you remember when, I can't ? Did you post it here or send it to me. Can't remember that either. HAA.
Bill
Bill
Uncle Willy
New member
Quite a bit. As you can see at peak depth you ain't covering much real; estate.
Bill
Bill
It's been a while. I think in the first year that I had the 250, so probably 3+ years ago. ??
I'd have to find one of the original bmp's to check the exact date. I checked the one on
the server, but it says 09, so is not the original that I first posted. I think I had to dig it up
and put it back on this server when I switched ISP's. I posted it here, and I think
two or three times since I made it, when the topic comes up about coverage at max
depth.
Some stuff I have is on different drives, and I have to actually change the drives in this
puter to get to them. I rebuilt this box 3-4 months ago, and it only lets me run two of
the older EIDE drives at a time. "My old box let me run four at a time". But this one
lets me run a lot more SATA drives.. I need to get another big SATA drive so I can
place everything from the old drives back on the puter.. I'm already running a 1 TB
SATA+ a 500 GB EIDE now, but I record lots of movies, and fill the drives up fast..
I'm pretty much full to the brim on this 1500 GB... :/ I just checked my folders, and
I currently have 551 recorded movies on just these two drives..
Actually, I have two 1 TB SATA drives, but I killed one of them by accident..
I dropped a TV remote control off the top of my puter case, and it fell down along
the side of the case, and the corner hit the edge of the drive.. The shock killed it..
Lost a lot of good movies with that dumb mistake.. It's got a bad head now, and
just clicks when it tries to run..
I'd have to find one of the original bmp's to check the exact date. I checked the one on
the server, but it says 09, so is not the original that I first posted. I think I had to dig it up
and put it back on this server when I switched ISP's. I posted it here, and I think
two or three times since I made it, when the topic comes up about coverage at max
depth.
Some stuff I have is on different drives, and I have to actually change the drives in this
puter to get to them. I rebuilt this box 3-4 months ago, and it only lets me run two of
the older EIDE drives at a time. "My old box let me run four at a time". But this one
lets me run a lot more SATA drives.. I need to get another big SATA drive so I can
place everything from the old drives back on the puter.. I'm already running a 1 TB
SATA+ a 500 GB EIDE now, but I record lots of movies, and fill the drives up fast..
I'm pretty much full to the brim on this 1500 GB... :/ I just checked my folders, and
I currently have 551 recorded movies on just these two drives..
Actually, I have two 1 TB SATA drives, but I killed one of them by accident..
I dropped a TV remote control off the top of my puter case, and it fell down along
the side of the case, and the corner hit the edge of the drive.. The shock killed it..
Lost a lot of good movies with that dumb mistake..
It's got a bad head now, and just clicks when it tries to run..
Mike Hillis
Well-known member
That is what it looks like in the air. It doesn't look anywhere like that in the ground. 
If it did look like that in the ground, why would you bother overlapping your sweeps by 50% or more?
Looks pretty but it is very misleading.
HH
Mike
Uncle Willy
New member
If you look closely, at peak depth it is only covering an area the size of a half dollar or less plus the signal weakens and shrinks in size upon entering the ground. That's why the signal is often described as cone-shaped.
Bill
Bill
If that is what it looks like in the air, the pattern should be basically the same
in the ground. It would just be attenuated the way I see it. I don't think the pattern
would change much if any. If you believe that the pattern is accurate in the air,
why do you believe it would change in the ground? Or maybe to put it another way,
what do you believe the "altered" ground effected pattern would look like?
If the pattern does look like that in the ground, the need for overlapping passes
seems fairly obvious to me. But it mainly would effect the deeper objects, which
has always been the usual claim.
Now, I'm not stating it's a perfect example, as I've haven't done careful tests to
confirm the modeled pattern exactly matches the real world pattern. But it shouldn't be
far off. Attenuation of a signal does not effect the pattern. It only lowers the level,
which in some cases could make that pattern seem to shrink in depth a bit more vs
the above coil plot if the attenuation was severe. The signal might have less usable
depth due to the attenuation, but the pattern shape itself should be pretty much unaltered.
Which would make overlapping swings even more critical vs object depth if that were
the case. Course, I'm always open to other theories on this.
in the ground. It would just be attenuated the way I see it. I don't think the pattern
would change much if any. If you believe that the pattern is accurate in the air,
why do you believe it would change in the ground? Or maybe to put it another way,
what do you believe the "altered" ground effected pattern would look like?
If the pattern does look like that in the ground, the need for overlapping passes
seems fairly obvious to me. But it mainly would effect the deeper objects, which
has always been the usual claim.
Now, I'm not stating it's a perfect example, as I've haven't done careful tests to
confirm the modeled pattern exactly matches the real world pattern. But it shouldn't be
far off. Attenuation of a signal does not effect the pattern. It only lowers the level,
which in some cases could make that pattern seem to shrink in depth a bit more vs
the above coil plot if the attenuation was severe. The signal might have less usable
depth due to the attenuation, but the pattern shape itself should be pretty much unaltered.
Which would make overlapping swings even more critical vs object depth if that were
the case. Course, I'm always open to other theories on this.
Mike Hillis
Well-known member
Hi NM5K,
The ground does stuff to the signal and it just doesn't come out like you would expect it to. I've done some in ground tests just to see what the detection pattern looks like so I'd know what was going on under my coil. It is some pretty weird looking stuff. Changes the way you detect. Changes what coils styles you perfer to run for various depth ranges, too.
Try it yourself. Use a wooden yard stick and mark a center point on about the 16" mark. Then lay a coin on the ground at the center point. Sweep your coil over it, recording how far away the center of your coil is from the coin when you first hear a response. You may have to use some clear packing tape or something to draw crosshairs in the center of your coil so you can get a acurate measurement. Then progressively buring the coin deeper an inch at a time and repeat, recording the results. Once you get to the depth where you can no longer hear the coin, take your result and plot them. I'll wait for you to post the WOW!. Then you too, will respond to these type posts, with a "thats how it looks in the air".
What effect does the sensitivity setting have on the detection pattern? Repeat the same experiment with low, middle and high sensitivity settings. Yep...the pattern changes.
Do this with and without headphones and you'll learn how important headphones really are.
HH
Mike
The ground does stuff to the signal and it just doesn't come out like you would expect it to. I've done some in ground tests just to see what the detection pattern looks like so I'd know what was going on under my coil. It is some pretty weird looking stuff. Changes the way you detect. Changes what coils styles you perfer to run for various depth ranges, too.
Try it yourself. Use a wooden yard stick and mark a center point on about the 16" mark. Then lay a coin on the ground at the center point. Sweep your coil over it, recording how far away the center of your coil is from the coin when you first hear a response. You may have to use some clear packing tape or something to draw crosshairs in the center of your coil so you can get a acurate measurement. Then progressively buring the coin deeper an inch at a time and repeat, recording the results. Once you get to the depth where you can no longer hear the coin, take your result and plot them. I'll wait for you to post the WOW!. Then you too, will respond to these type posts, with a "thats how it looks in the air".
What effect does the sensitivity setting have on the detection pattern? Repeat the same experiment with low, middle and high sensitivity settings. Yep...the pattern changes.
Do this with and without headphones and you'll learn how important headphones really are.
HH
Mike
The ground would mainly attenuate the signal if anything. But if the attenuation were equal
throughout the area of ground being detected, the pattern would stay the same, as all
angles will be attenuated equally. Of course, pattern and ultimate depth performance are two totally
different things. You can keep the same pattern , but still lose depth performance.
But in a case like that, all the angles will be attenuated equally. It's not going to effect one angle,
and ignore the others. I think the attenuation is causing you to think the pattern is changing
shape, when actually it most likely isn't. But I'll try and plot them out one of these days to see how
it's jiving with the puter plot.
But another thing that makes me think it's the attenuation rather than change of pattern
shape, is your claim that changing the sensitivity is changing the pattern as well.
That is totally false. You will never change the radiation pattern of an antenna or coil
by merely changing power or receiver sensitivity. Ever! Just doesn't work that way.
For one thing, with many or most machines, adjusting the sensitivity only adjusts the
receiver. Not the signal going into the ground. The signal going into the ground stays
the same.
But even if we were to actually adjust the power level of the signal going into the ground,
that would never change the pattern. Why? Because all angles will be changed
at an equal amount. You cannot change the gain levels at some angles, but not others.
The only way to make that happen is to use a different coil with a different pattern.
That's why we all use different coils for different jobs.
The only thing that can ever change the pattern is the antenna "coil" itself in such
a case.
The amount of power applied to an antenna, or the amount of sensitivity used on a
receiver never changes the antenna pattern shape. It will always change at equal amounts
for every angle, and keep the same shape.
So I totally disagree with your statement about sensitivity changing the pattern shape.
It's basically impossible.. The shape is staying the same, but being as you are
using different sensitivities, you are changing how much of that pattern you are
able to detect, assuming that the signal into the ground is always the same.
If you lose two inches at the center of the plot, you will also lose the same two inches
at the edges of the plot. In the radio antenna world, you would measure that plot in db.
If the signal level into the ground is being changed, the pattern is still the same, but
the change in power will cause the signal to vary as to how deep it goes, compared to
the original power setting. But it will always vary exactly the same for every angle.
And thus the antenna pattern is unchanged. This is just proven basic radio antenna theory.
But saying all that, I'll still try a plot test when I get a chance just to see how they
jive with the modeling program. But from what I've seen in the course of normal
detecting, I haven't noticed any great departure from the pattern shown in the
graph. But like I say, I wasn't too careful about doing exact measurements.
throughout the area of ground being detected, the pattern would stay the same, as all
angles will be attenuated equally. Of course, pattern and ultimate depth performance are two totally
different things. You can keep the same pattern , but still lose depth performance.
But in a case like that, all the angles will be attenuated equally. It's not going to effect one angle,
and ignore the others. I think the attenuation is causing you to think the pattern is changing
shape, when actually it most likely isn't. But I'll try and plot them out one of these days to see how
it's jiving with the puter plot.
But another thing that makes me think it's the attenuation rather than change of pattern
shape, is your claim that changing the sensitivity is changing the pattern as well.
That is totally false. You will never change the radiation pattern of an antenna or coil
by merely changing power or receiver sensitivity. Ever! Just doesn't work that way.
For one thing, with many or most machines, adjusting the sensitivity only adjusts the
receiver. Not the signal going into the ground. The signal going into the ground stays
the same.
But even if we were to actually adjust the power level of the signal going into the ground,
that would never change the pattern. Why? Because all angles will be changed
at an equal amount. You cannot change the gain levels at some angles, but not others.
The only way to make that happen is to use a different coil with a different pattern.
That's why we all use different coils for different jobs.
The only thing that can ever change the pattern is the antenna "coil" itself in such
a case.
The amount of power applied to an antenna, or the amount of sensitivity used on a
receiver never changes the antenna pattern shape. It will always change at equal amounts
for every angle, and keep the same shape.
So I totally disagree with your statement about sensitivity changing the pattern shape.
It's basically impossible.. The shape is staying the same, but being as you are
using different sensitivities, you are changing how much of that pattern you are
able to detect, assuming that the signal into the ground is always the same.
If you lose two inches at the center of the plot, you will also lose the same two inches
at the edges of the plot. In the radio antenna world, you would measure that plot in db.
If the signal level into the ground is being changed, the pattern is still the same, but
the change in power will cause the signal to vary as to how deep it goes, compared to
the original power setting. But it will always vary exactly the same for every angle.
And thus the antenna pattern is unchanged. This is just proven basic radio antenna theory.
But saying all that, I'll still try a plot test when I get a chance just to see how they
jive with the modeling program. But from what I've seen in the course of normal
detecting, I haven't noticed any great departure from the pattern shown in the
graph. But like I say, I wasn't too careful about doing exact measurements.
So, I got a question Uncle Willy. If that red spot at the bottom is the coverage area, do you get better coverage when use use all the different sweep techniques?
I just wander if you had a faint target and you instead of swinging the coil, rotate it left and right close to the target then back and forth, still rotating and at diagonals?
Or is that the purpose of the S-sweep, figure 8, narrow oval, flat and cross oval sweep. I'm still thinking of just rotating the coil versus sweeping for an elusive target. I imagine that's crazy, but the only way I know to find out these things is to ask the experts.
However, just thinking about this has gave me a headache and after reading NM5K and Mike Hillis's posts, my head is spinning like a top. All 3 of y'all seem to know your stuff, but it's so far over my head, it's hilarious. Thanks for posting.
I just wander if you had a faint target and you instead of swinging the coil, rotate it left and right close to the target then back and forth, still rotating and at diagonals?
Or is that the purpose of the S-sweep, figure 8, narrow oval, flat and cross oval sweep. I'm still thinking of just rotating the coil versus sweeping for an elusive target. I imagine that's crazy, but the only way I know to find out these things is to ask the experts.
However, just thinking about this has gave me a headache and after reading NM5K and Mike Hillis's posts, my head is spinning like a top. All 3 of y'all seem to know your stuff, but it's so far over my head, it's hilarious. Thanks for posting.
Wow! what a discussion. Sounds like we've got some folks who know what they are talking about. Sadly, that doesn't include me. But I'll repeat someone else's question. Is the sniper coil the same pattern and wil it just be a bit shallower? It's a lot easier to pinpoint with the sniper so I'm guessing that the target area will be smaller and not as deep Yes? No? Maybe?
MrGee
MrGee
Uncle Willy
New member
Yeah it's a concentric with what is referred to as a"'cone-shaped" signal pattern because of the narrow detection area at the bottom. The Sniper signal is smaller and narrower, and sharper, but it will get nearly seven inches or so in depth which is surprising for a 4.5 inch coil.
Bill
Bill
Uncle Willy
New member
Interesting test but you left out what the effect " Metallurgical Phenomenon" has on freshly buried coins in many areas - like no signal at all. You can find many posts on this forum where folks have buried a coin and could not get any signal no matter how hard they tried. I asked the Garrett engineers about it and they have heard of it but can't explain it. It just happens in many cases. Sometimes you can dig a coin up, throw it back in the hole and pass the coil over it and get absolute silence, or you can cover it back up with the same dirt and still get absolute silence. Just one of the mysteries of electronic physics.
Bill
Bill
Yep, that would be the end result with the sniper coil. The main advantage is being
smaller and detects less of an area. That way you can go into trashy or busy areas
and not be overwhelmed with signals. The sniper coil is one of my favorites..
I'm not claiming to be an expert on detector coils. I haven't built any yet, and there
may be effects from the ground that I could be overlooking. But generally I would
think the #1 effect would be attenuation of the signal compared to air.
This is the main reason detectors use such a low frequency. The very low frequency
suffers from less attenuation from the ground compared to the higher frequencies
used for most radio, etc. But RF is RF, and all antennas, including loop types follow
the same rules. Most detector antennas are called coils, but actually they are very
small loop antennas. The reason I say "very small" is because they are very small
compared to the wavelength they are being used for. Normally, any loop antenna that
is physically 1/10 of a wavelength or smaller is considered a "small loop". This is the
same in the radio world. For the very low frequency of a usual detector, IE: 6.5 kc
for an Ace 250, the detector coils are downright tiny and quite inefficient for the
wavelength being used. But we live with it, because we need to use that low frequency
to partially avoid the attenuation issue. That's the reason submarines also use very
low frequencies. They tend to use around 20-30 kc I think, but I'm sure can vary.
But that's still a very low frequency when you consider that the upper end of the longwave
spectrum is 300 kc. You will hear aircraft NDB's around that area.
Anyway, like I say, I'm no real expert on detector coils. There may be small details
about certain things that I'm missing.
But on the other hand, as far as radio and loop antennas in general, I have a pretty good
background. I've studied radio and antenna theory for years, and like I say, for the
most part, what applies in the LW radio spectrum also will apply to the even lower LW
spectrum most detectors are using. None of the usual rules will change.
I've been a licensed ham radio operator for 33 years, and have passed the extra class exams,
so I didn't just fall off the turnip truck about such things.. I have built several small loops for
radio reception. Usually LW, and MW..
So I can not state for certain that Mike Hillis is not seeing a narrower perceived pattern in real
life performance. There may be some quirk with the ground that I'm missing. So I'm not disputing
that without doing some testing.
But with the claim of receiver sensitivity or even power output to the coil effecting the coil pattern,
I can say with 100% confidence that is hogwash. It may seem like it's changing, but it's really not.
That would break every rule in the book if that were to occur. And most of these rules are 100
years old and pretty much considered etched in stone due to being verified over and over through
the years.
If you increase or decrease power to any antenna, it's pattern never changes because all
angles of the plot are effected equally.
smaller and detects less of an area. That way you can go into trashy or busy areas
and not be overwhelmed with signals. The sniper coil is one of my favorites..
I'm not claiming to be an expert on detector coils. I haven't built any yet, and there
may be effects from the ground that I could be overlooking. But generally I would
think the #1 effect would be attenuation of the signal compared to air.
This is the main reason detectors use such a low frequency. The very low frequency
suffers from less attenuation from the ground compared to the higher frequencies
used for most radio, etc. But RF is RF, and all antennas, including loop types follow
the same rules. Most detector antennas are called coils, but actually they are very
small loop antennas. The reason I say "very small" is because they are very small
compared to the wavelength they are being used for. Normally, any loop antenna that
is physically 1/10 of a wavelength or smaller is considered a "small loop". This is the
same in the radio world. For the very low frequency of a usual detector, IE: 6.5 kc
for an Ace 250, the detector coils are downright tiny and quite inefficient for the
wavelength being used. But we live with it, because we need to use that low frequency
to partially avoid the attenuation issue. That's the reason submarines also use very
low frequencies. They tend to use around 20-30 kc I think, but I'm sure can vary.
But that's still a very low frequency when you consider that the upper end of the longwave
spectrum is 300 kc. You will hear aircraft NDB's around that area.
Anyway, like I say, I'm no real expert on detector coils. There may be small details
about certain things that I'm missing.
But on the other hand, as far as radio and loop antennas in general, I have a pretty good
background. I've studied radio and antenna theory for years, and like I say, for the
most part, what applies in the LW radio spectrum also will apply to the even lower LW
spectrum most detectors are using. None of the usual rules will change.
I've been a licensed ham radio operator for 33 years, and have passed the extra class exams,
so I didn't just fall off the turnip truck about such things.. I have built several small loops for
radio reception. Usually LW, and MW..
So I can not state for certain that Mike Hillis is not seeing a narrower perceived pattern in real
life performance. There may be some quirk with the ground that I'm missing. So I'm not disputing
that without doing some testing.
But with the claim of receiver sensitivity or even power output to the coil effecting the coil pattern,
I can say with 100% confidence that is hogwash. It may seem like it's changing, but it's really not.
That would break every rule in the book if that were to occur. And most of these rules are 100
years old and pretty much considered etched in stone due to being verified over and over through
the years.
If you increase or decrease power to any antenna, it's pattern never changes because all
angles of the plot are effected equally.
Uncle Willy
New member
Doesn't it take a certain amount of juice to drive the signal into the ground and battle and penetrate the varied matrix and streaks of minerlization or large accumulation of same. Doesn't this weaken the signal the deeper it goes and draw it in ( compress it's size and shape )?
Bill
Bill
TRIPLE-SSS
Well-known member
WOWWW