ELLIPTICAL COIL QUESTION

[baum7154]

Yes, in a transmitter with a constant carrier an antenna is tuned ideally to the operating frequency so that a standing wave is set up with minimal reflected power at an SWR of 1:1. The difference is in a PI detector there is a pulse or string of pulses that energize the coil and, hopefully, the coil decays quickly enough to see a target in the short measurement period between pulses.

Dan

[chemelec]

These are just 3 Round Coils, all wound to Simular Diameters and Inductance Values.
Check out the Free Air Resonance and InterWire Capacitance of each one.

[green]

These are just 3 Round Coils, all wound to Simular Diameters and Inductance Values.
Check out the Free Air Resonance and InterWire Capacitance of each one.

How do you shield the flat radial coil and what is the shielded resonance? I've been winding spiderweb coils that I think should be similar to the flat radial. Didn't come up with a jig to wind a flat radial.

[chemelec]

I have jigs and have made hundreds of these coil, but I don't Shield them.
Never had a reason to.

But I imagine you could, the same way as a bundled coil.

Maybe I'll do one and test it for you.

EDITED
This is that Same 388.3 uh Radial Coil with an Aluminum Foil Shield on it.
As a Quick Test, it Seems to work OK on my detector.

Free air Resonance is now Just 410 Khz.
Capacitance is now 392 pF.
WOW!

I have some MG Chemical, Spray Nickel Coating, But I am Reluctant to do a coil with it.
Can't be Easily Removed, if I don't like it.

[green]

Thanks for the reply. If they work for you without shield don't bother. Was just wondering if you had a way to shield without lowering resonance a lot.

[Chet]

Hi Green

I ran three of your coils (from one of your Posts) through a LC capacitance calculator at this site; http://www.1728.org/resfreq.htm the results are as follows:


Coil Description Inductance Self Resident Frequency Capacitance


10.13” Round 310 uH 2.15 mHz 17.67 pF


6 x 12.4” Round ends 292 uH 2.22 mHz 17.6 pF


4.5 x 13.3 Round ends 280 uH 2.27 mHz 17.55 pF


From one of your Posts you were using a MOSFET with a series diode with possibly a short twisted cable to measure the self-resonance frequency of your coils. If this is the case for the above three coils then the capacitances are the above values minus your test set output circuit value. Thus the actual coil capacitances are probably less than half of the listed values.


You are building very fast coils. Keep up the good work.


And have another great day,
Chet

[Tinkerer]

The self resonance of a coil by itself, is interesting to know, but in real life, what matters most, is the coil in it's circuit.
A high capacitance cable can do a lot to spoil a good coil.
The same happens with shielding.
And then, when we have several coils in an IB coil assembly, the mutual coupling has a considerable effect on the self resonance of each individual coil of the assembly.
All in all it is very complicated, as the individual parasitic capacitances are sometimes in parallel and sometimes in series, making the end result difficult to predict.
Of course, the ideal would be if we could find a way to put most of the capacitances in series, that would be the "Golden Fleece".
In the meantime, we can look at simpler ways to reduce the parasitic capacitances.

The best way to reduce capacitance, is to increase the distance between the 2 conductors. Adding ample space between the wire and the shield is one way.
Parallel wires (conductors) give high capacitance.

There are many more factors coming in to play. It would be interesting to analyse each one and look for solutions. A single person has only so much time, If a task is split up between many persons, much more can be achieved.

http://www.mogami.com/e/cad/electrical.html This link connects to an online calculators for calculating cable capacitance. It also gives a good idea on the inter-wire capacitance of a coil.

Another thing to look at, is the way capacitors are built. How do you make the best capacitor? This is the way WE DO NOT WANT to build our coils.

EVERY PROBLEM IS ALSO AN OPPORTUNITY

[chemelec]

Hi Green From one of your Posts you were using a MOSFET with a series diode with possibly a short twisted cable to measure the self-resonance frequency of your coils. If this is the case for the above three coils then the capacitances are the above values minus your test set output circuit value. Thus the actual coil capacitances are probably less than half of the listed values.


You are building very fast coils. Keep up the good work.


And have another great day,
Chet

I have Never used a Mosfet with a series diode.

This is my Test Circuit for Measuring the Free Air Resonance.
And the Coil must be Suspended in FREE AIR, Not just laying on the bench.

[green]

I have Never used a Mosfet with a series diode.

This is my Test Circuit for Measuring the Free Air Resonance.
And the Coil must be Suspended in FREE AIR, Not just laying on the bench.

Don't have a signal generator. This is what I use. Measure the resonance with the scope. Coil inductance = 2.533 million/resonance squared with the 10nf cap. jumper across the test coil. Free air resonance is without the 10nf cap. The coil resonance is close with a Mosfet and MUR460 diode in series. Think the answer should be close to the same as the signal generator.

[Chet]

Hi Tinkerer, Chemelec, Green,

Thank you for the great information. Green and Chemelec I think most of your coils are wound with enameled wire. Have you wound any in the same maner with the same dimensions and inductance with Teflon or some other thick insulation and compared the self-capacitance?

Thank you,
Chet

[green]

Hi Tinkerer, Chemelec, Green,

Thank you for the great information. Green and Chemelec I think most of your coils are wound with enameled wire. Have you wound any in the same maner with the same dimensions and inductance with Teflon or some other thick insulation and compared the self-capacitance?

Thank you,
Chet

Hi Chet, I haven't. Have a large spool of enamel coated, so my goal was to see if I could wind a fast coil with it. Wanted to try the flat radial but didn't have the tools to make a jig. The form for the spiderweb coil is fairly easy to make. Allows my to make round, DD, race track, different shape and size coils. Takes a little while to wind, so not for production.

[Chet]

Hi Green
Thank you,
Chet

PS check your PM

[chemelec]

Previously I Wound a Radial Coil with Tuflon coated wire, But to get the Same Inductance Using Tuflon wire, Requires a LOT more Turns.
That Coil was also quite HEAVY.

I Also wound a Radial coil with Litz Wire, Turned out OK, But Not much difference in Measurements or Performance to the Enamel wire ones I make.

Just for Curiousity, Today I wound another Radial Coil, See Pictures and the comparative Measurements.
Than Coated it with a Conductive Nickel Spay.
I Not Impressed!

The Shielding would probably be OK if it were in a Coil housing and the Shield was NOT Directly on the Coil.
But Some Distance Away, Sprayed on the Coil housing.

[CAS]

Previously I Wound a Radial Coil with Tuflon coated wire, But to get the Same Inductance Requires a LOT more Turns.
The Coil was also quite HEAVY.

I Also wound a Radial coil with Litz Wire, Turned out OK, But Not much difference in Measurements or Performance to the Enamel wire ones I make.

Just for Curiousity, Today I wound another Radial Coil, See Pictures and the comparative Measurements.
Than Coated it with a Conductive Nickel Spay.
I Not Impressed!

Would probably be OK if it were in a Coil housing and the Shield was NOT Directly on the Coil.
But Some Distance Away, Sprayed on the Coil housing.

So you dont usually use any shielding at all and have no issues with rf noise?

[chemelec]

No I Don't have any noise Problems, Or None that I am aware of.

I Threw out the Tuflon Coil that I made, But here is a picture of the Litz Wire Coil that made and I still have, just keeping it for reference.