What did I do wrong?

[porkluvr]

Since I live in Brazil I don't know the D-C of my brand of resin.

So, what does living in Brazil matter (unless you're living in the rain forest and have no electricity).

You would probably need to look up the manufacturer's website, contact their technical support group and ask about dielectric constant of their resin. Maybe they can help. The closer the dc number is to 1, the better. Of course dc=1 is impractical, but 4 or 5 or more would be very bad. Unfortunately, I do not have any good suggestions for a low dc potting compound but I will be on the lookout.

So is the copper wire necessary? Or can I just paint the coil with graphite. Is the break in the graphite also necessary? Also I was wondering if laying the coax across the coil (as shown in the pic of the completed coil) could mess it up.

Yes, no, yes, and yes. You probably need some wire to make a connection, but just like Comp and Maikl hinted, you have over-did it. Your shield (and fastening material) makes too good of a target. One other thing that has not been mentioned is that you may also need to paint the top half of a coil for adequate shielding.

Honestly, you should look at all the criticisms offered because they all had merit, imho. As has already been pointed out (I think), constructing a fast acting spiral PI coil is no easy task.

[Vortxrex]

Did you try just making a simple coil of the correct Inductance/Resistance? That way you know the rest of your MD is working fine.

[Al1g]

What would be defined as "correct Inductance/Resistance? During testing I did wrap some copper wire around a lid as a test coil.

[Vortxrex]

The wire is a single strand from some braided wire. And yes it is in contactt with the graphite. I am using the Hammerhead. Its connected to the cable via standard 75 coax. What about the dreaded "eddy currents" ? Also the damping resistor is about 680 ohms though that isn't perfect.

Even if there is nothing wrong w/ the coil you made, I think the inductance of your coil is incorrect for the HH. Coil inductance, coil resistance, Dampening resistor must all be matched if you want to sample the RX quickly (which of course you do)

Do you have an oscilloscope? I hope so, because you will need it to adjust the dampening resistor value to the coil you made.

Do you understand how the dampening resistor value is directly related to how quickly you can sample?

Do you understand how the resistance/capacitance of the coil is related to how quickly you can sample?

[Al1g]

I believe so, but correct me if I am wrong.
The damping resistor prevents "ringing" or extra pulses once the coil has been shut off.
The capacitance defines how fast the coil can shut off. Now what part does the coil resistance play? Other than how many amps you can push through it. How do you measure the capacitance? I do have a capacitance/inductance meter but it doesn't measure the capacitance across the coil.

P.S I do have an oscilloscope.

[Vortxrex]

Now what part does the coil resistance play? Other than how many amps you can push through it. How do you measure the capacitance? I do have a capacitance/inductance meter but it doesn't measure the capacitance across the coil.

http://www.geotech1.com/pages/metdet...s/FastCoil.pdf

[Al1g]

Aha! I overlooked that the first time I read that. My Dad suggested adding a resistor in series with the coil to limit current. Would that help?

[golfnut]

No, you want current.

Id guess you dont have a big inductance maybe < 200uH

Id guess that because the windins are not physically close to each other.


SO your coils mutual inductance is low,.

Id also guess you have relatively high capacitance between your widings with all of that polyproylene insulation from coax.

The R value Id estimate is reasonably low.

My simple model says this coil will do a 4A flyback pulse taking 70us to decay to zero on a SURF PI det

Steve

[porkluvr]

Al1G, you should construct a test apparatus to measure your coil's self resonant frequency. Use that number and shoot for a higher number in your next coil.

Using a series resistor to limit max current can give you a faster response and there are detectors that use that method, but that comes at the expense of depth of detection and it is no substitute a well designed coil.

I remember seeing somebody write about building a coil form for a spiral coil; constructing a narrow spool from some low dc material (polypropylene maybe) to hold the wires in place - not using resin filler. Keep that in mind?

Any material that you use in the vicinity of your coil will absorb energy. You need to try and find materials and a construction technique to minimize that absorbtion.

[Al1g]

Thanks for the advice. I tried to measure the resonance and got ~633 khz does that sound right?
Thanks again.

[porkluvr]

Thanks for the advice. I tried to measure the resonance and got ~633 khz does that sound right?
Thanks again.

That sounds right from the standpoint of confirming that something is wrong.

633kHz is quite low, I think you need to get closer to 1Mhz.

I have never taken time to do such a measurement but what I have read on the subject suggests that 1Mhz is good... and if you can get above that number, even better.

UNFORTUNATELY, the search engine here at Geotech does not appreciate that when somebody wants to research "self resonant frequency", that returning search results for "resonant, frequency" is not the same thing. And a search on "SRF" comes up empty.

In general, the SRF frequency will naturally tend to dip as your coil's inductance is raised. Somebody using a 500uH coil on the Deltapulse, for instance, would probably have to settle on a lower SRF. But for faster sampling the higher numbers are imperative.

edit: Something you can do to circumvent the rather pitiful Geotech1 search engine (I'm sorry Qiaozhi, but that's the truth) is to invoke Google's search engine and enter something like "self resonant frequency geotech1" and let Google do all the work. The results are far from complete, but way better than nothing.

[Qiaozhi]

edit: Something you can do to circumvent the rather pitiful Geotech1 search engine (I'm sorry Qiaozhi, but that's the truth) is to invoke Google's search engine and enter something like "self resonant frequency geotech1" and let Google do all the work. The results are far from complete, but way better than nothing.

Actually, I agree. The searching in vBulletin sucks. Perhaps the new version Carl is planning to install will fix these issues.

[Qiaozhi]

Hmmm... very strange.

It appears that the word "self" is considered too short for the search engine, so I added it to the exclusion list to see what would happen. Also, the minimum word size is apparently set to 2, which means the word "self" should already be allowed. However, if you previously had tried entering "self resonant frequency" into the dialog box, it would have returned zero results. Now it only finds my own posts that mention "self resonant frequency", but none of the older ones.

My conclusion is that I don't really understand how this works, and it seems like the search results are not retrospective.

I'll ask Carl what he thinks.