Advanced PI coils

[Bill512]

I was idlely thumbing through a 1944 volume of Wireless World when I saw something that looked a bit familiar. It was a schematic for a frame aerial that was balanced, i.e. it had a centre tap and could be connected differentially. One benefit, according to the article was that it did not need to be shielded, unlike a single ended winding. The article pointed out the benefits of a frame aerial, either shielded or balanced, as reducing capacitive noise, or the electric field component. Having a restored and working 1941 RAF Bomber Command radio, I thought I would give a wrap around lead tape shielded single ended 300uH mono a try as a radio aerial. Wow, it works much better than the internal 20ft wire aerial I have in my workshop. It is easily turned for optimum alignment to the transmitting station too. The electric noise, which seems to come from wall plug switched mode power supplies, solder station, fluorescent lighting, central heating controller etc is way down. Clear signals are heard right up to 18MHz, which is surprising. When I get a bifilar wound differential coil of similar diameter it will be interesting to compare - with and without shielding.

Eric.

Hi Eric, just before a few days I was trying a 11" minelab DD (TX winding), as receiver antenna on the lower part of LW band. Amazing and clear reception.

[Davor]

No, the comparison is fair regards to SNR. All coils rival the good old mono coil with 10 inch diameter.

Not exactly. The tandem or top hat configurations use doubled the area with the same radius, but doubled nevertheless. IMHO all the AI coils should be given the same treatment as they all are using double the geometry this way or another.

[Ferric Toes]

Hadn't thought about that... I like this test, I'll add it next time I'm looking at OVR.

I add a 470K resistor from either pin 2 or pin 3 of the first amp., depending which one is active, to a test pin on the board. Inject 5Hz sine wave to give 1V pk - pk on preamp output and observe integrator o/p. If properly balanced the differential integrator should show little or no movement even when scope sensitivity set to 5mV. Then put up the frequency to between 10 - 100kHz to check that the gain recovers at preamp output to full closed loop value before the start of sampling. Dual amplifier arrangements should be fine.

Eric.

[Ferric Toes]

New Life for Old Monos

I was idlely thumbing through a 1944 volume of Wireless World when I saw something that looked a bit familiar. It was a schematic for a frame aerial that was balanced, i.e. it had a centre tap and could be connected differentially. One benefit, according to the article was that it did not need to be shielded, unlike a single ended winding. The article pointed out the benefits of a frame aerial, either shielded or balanced, as reducing capacitive noise, or the electric field component. Having a restored and working 1941 RAF Bomber Command radio, I thought I would give a wrap around lead tape shielded single ended 300uH mono a try as a radio aerial. Wow, it works much better than the internal 20ft wire aerial I have in my workshop. It is easily turned for optimum alignment to the transmitting station too. The electric noise, which seems to come from wall plug switched mode power supplies, solder station, fluorescent lighting, central heating controller etc is way down. Clear signals are heard right up to 18MHz, which is surprising. When I get a bifilar wound differential coil of similar diameter it will be interesting to compare - with and without shielding.

Eric.

[Aziz]

HI Aziz,

Well I had a little bit of a win! Wound up the coils and the resonant frequency was a bit faster than 500khz, and with a damping resistor installed the coils are not seen by the tx coil. However, I am about 100 turns short I was only able to get 40 turns. The inductance was about 360uh. Now if I were to have enough wire to do the 144 turns, the inductance will be too high and the capacitance too high and the resonant frequency too low..... Hmmmm! Damn it, I want this coil to work! I must be missing something, right?

Cheers Mick

Mick,

use thicker insulation wire (teflon if required, but not magnet wire), use more space in the coil bundle, use less loop turns and increase the gain instead, use 0.5 diameter factors (or slightly bigger), use special weave techniqes, ...

I don't have much problems with my low bandwidth detectors (< 50 kHz).
Aziz

[Mechanic]

HI Aziz,

Well I had a little bit of a win! Wound up the coils and the resonant frequency was a bit faster than 500khz, and with a damping resistor installed the coils are not seen by the tx coil. However, I am about 100 turns short I was only able to get 40 turns. The inductance was about 360uh. Now if I were to have enough wire to do the 144 turns, the inductance will be too high and the capacitance too high and the resonant frequency too low..... Hmmmm! Damn it, I want this coil to work! I must be missing something, right?

Cheers Mick

[Aziz]

ML DD PI Coil vs. Mono Loop PI Coil

Hi all,

this is the long awaited performance comparision between the 10 inch ML DD PI Coil vs. mono loop PI coil, showing each Rx mode (Double D, Monoloop, Cancel). Notice, the comparison (of Double D, Monoloop and Cancel mode) isn't compensated regards to SNR. It is a direct comparison to 300 µH TX, 300 µH RX coil (mono loop 300 µH either). The not compensated DD coil in Double D mode is picking up 1.6 times less EMI noise and therefore 1.6 times less response signal compared to the compensated DD IB coil (I'll show this once more here). The max. response at 1" and 2" for the mono loop coil is corrected now (high sensitivity on its edge).

Notice, that the Cancel mode isn't going deep. Crank up your gain in your detector to compensate for the depth loss.

Coil Comparison (Calculated Response Ratio to Reference Coil)
© 2012 by Aziz Ögüt. All Rights Reserved. 20-Dec-2012
Reference Coil: 300 µH Round Mono Loop Coil
		10" Double-D Induction Balanced Round Coil	10" ML DD PI Coil (Double D Mode)	10" ML DD PI Coil (Monoloop Mode)	10" ML DD PI Coil (Cancel Mode)
Target Depth [inch]	Target Depth [cm]	Relation to Reference	Relation to Reference	Relation to Reference	Relation to Reference
1	2,5	1,195	0,741	1,348	1,684
2	5,1	0,930	0,577	0,982	1,317
3	7,6	0,771	0,478	0,695	0,763
4	10,2	0,703	0,436	0,658	0,475
5	12,7	0,673	0,417	0,658	0,321
6	15,2	0,657	0,407	0,653	0,270
7	17,8	0,647	0,401	0,648	0,251
8	20,3	0,641	0,397	0,644	0,235
9	22,9	0,636	0,395	0,642	0,223
10	25,4	0,633	0,393	0,640	0,213
11	27,9	0,631	0,391	0,638	0,205
12	30,5	0,629	0,390	0,637	0,198
13	33,0	0,628	0,389	0,636	0,192
14	35,6	0,627	0,389	0,636	0,187
15	38,1	0,626	0,388	0,635	0,182
16	40,6	0,625	0,388	0,635	0,178
17	43,2	0,625	0,387	0,635	0,175
18	45,7	0,624	0,387	0,635	0,172
19	48,3	0,624	0,387	0,634	0,169
20	50,8	0,624	0,387	0,634	0,167
21	53,3	0,623	0,386	0,634	0,164
22	55,9	0,623	0,386	0,634	0,163
23	58,4	0,623	0,386	0,634	0,161
24	61,0	0,623	0,386	0,634	0,159
25	63,5	0,622	0,386	0,634	0,158
26	66,0	0,622	0,386	0,634	0,156
27	68,6	0,622	0,386	0,634	0,155
28	71,1	0,622	0,386	0,634	0,154
29	73,7	0,622	0,386	0,634	0,153
30	76,2	0,622	0,386	0,635	0,152

Cheers,
Aziz

[Mechanic]

Hi Aziz,

Well while you were sleeping I've been busy(in between eating Christmas dinner leftovers!) Went and got myself a couple of rolls of .25mm enameled copper wire and made myself a form to wind a 14cm dia coil for an 18"tx. Calculated that I would need 144 turns, the tx is 16 turns. Unfortunately only the rolls of copper I could get were enough to do 105 turns. The self resonance of one of my commercial coils is 500kHz fully assembled with shield and coax, but the one I wound up only has a self resonance of 200kHz without shielding or coax. Upon closer inspection the inductance is just a tad high, 3.26mH!!!!! So I would need to run a lot more strands in parallel to lower the inductance, however the .25mm thickness is not ideal either as it should be .2mm or less. Ho hum! Back to the drawing board!

So just to prove that it can be done, I took 2 of my commercial coils, connected one to my detector and the other I put a 680r damping resistor across it. To my delight, the other coil cannot be seen by my detector So there is hope yet I have some litz here harvested from 2 old coils, however I don't think it will be long enough to get the required turns, but I think I will try winding 2 identical coils with it just to see what happens.....

Cheers Mick

[Aziz]

Hi Aziz,

The coil I measured from is an old 11"DD before the cancel feature was implemented. Though in saying that, this coil does work well in cancel mode. I do have a later DD coil, though it is not with me at the moment, but from memory the inductance was similar to the old coil. I think the tx coil was closer to 300uh and the rx side measured higher than the tx coil as with the old coil I measured from. Perhaps the rx coil has the same turn count as the tx coil and the inductance is higher due to the higher rx winding resistance.
Yes 300uH~310uH seems to be the standard range for the ML coils. Just measured some mono coils I have with me and they were all 303uH.

Cheers Mick

Thanks Mick

It wouldn't make very much sense regards to EMI noise cancel, if the RX coil would have more loop turns of course. The RX coil bundle is probably more compact (thin wire), which could explain, why it has more inductance value.

I'll take TX=RX=300 µH (same loop turns).
Ok, I've made all three coil models in the right way and have to run the simulation yet.
Expected publishing date: tomorrow (it's late here).

Thanks & cheers,
Aziz

[Mechanic]

Is it really implemented actually in the coil?
Is RX = TX inductance? Is this 300 µH (standard)?

Cheers,
Aziz

Hi Aziz,

The coil I measured from is an old 11"DD before the cancel feature was implemented. Though in saying that, this coil does work well in cancel mode. I do have a later DD coil, though it is not with me at the moment, but from memory the inductance was similar to the old coil. I think the tx coil was closer to 300uh and the rx side measured higher than the tx coil as with the old coil I measured from. Perhaps the rx coil has the same turn count as the tx coil and the inductance is higher due to the higher rx winding resistance.
Yes 300uH~310uH seems to be the standard range for the ML coils. Just measured some mono coils I have with me and they were all 303uH.

Cheers Mick

[Aziz]

..
@Aziz, I have an odd notion that comparing the AI IB coils is going to be somewhat more informative if they are compared to a monocoil of half the area. All AI coils are in fact comprised of two mutually competing coil systems that result in cancellation of distant signals, ground and :sigh: distant targets, but a fair comparison would be with doubled the total area of AI coils. Somehow all the AI have nice pinpointing abilities, and all of them have waning sensitivity with distance that is in fact expected. But how they compare to monocoils of half the area will IMHO be a much more comparative challenge.

No, the comparison is fair regards to SNR. All coils rival the good old mono coil with 10 inch diameter.
I haven't taken the near "pin-point" response of the mono coil into account yet as the software didn't search for the maximum response in the former version. This is the edge of the coil but the mono coil response has been taken at the center axis of the coil. Only 1 - 2 inch detection depth is affected.
The mono coil is giving at its edge region at:
@1 inch: 1.83 times more response compared to the center axis
@2 inch: 1.05 times more response compared to the center axis

Divide the calculated ratios by 1.83 for 1" detection depth and by 1.05 for 2" detection depth and you have a comparable result with all coil types regards to pin-pointing performance.

I'll provide the corrected responses later in a very very large excel table (zipped file).

Cheers,
Aziz

[Aziz]

...
So you need to know the exact inductance of the rx coil and number of turns and same for the tx coil? Your a tricky fellow, I think you could easily calculate what the rx coil specs should be to get a good emi cancel using the tx coil specs I hope the answers above are what you needed!

Cheers Mick

Thanks Mick,

you have answered almost all missing details.

For optimal emi rejection in the cancel mode RX would be = TX inductance.
Is it really implemented actually in the coil?
Is RX = TX inductance? Is this 300 µH (standard)?

Cheers,
Aziz

[Mechanic]

Thanks Mick,

nevertheless, I'm totally confused.

Open questions:

How many coil bundles in a DD coil? 2 or 3

2 coil bundles, tx coil and rx coil.

Is the TX coil shared as RX (RX1=TX) in the receive period? Or is the RX1 separate from the TX?

On the ML's, when the rx switch is set to DD only the response from the RX coil is used. When the rx switch is set to mono, both tx and rx coil responses are summed after their preamps. When the rx switch is set to cancel the tx coil response is subtracted from the rx coil response after the preamps.

Is the left TX coil (RX1) in induction balanced position to the right RX2 coil? (Overlapped TX, RX2 coil bundles?)

Yes it is induction balanced.

I can't make the comparison at the moment until I know the exact specifications.

So you need to know the exact inductance of the rx coil and number of turns and same for the tx coil? Your a tricky fellow, I think you could easily calculate what the rx coil specs should be to get a good emi cancel using the tx coil specs I hope the answers above are what you needed!

Cheers Mick

[Davor]

Yes, spice models tend to account for input and output setup of an op amp only and completely disregard all the tripe in between. My spice sims show that NE5532 recovers in less than 2 us after saturation, but that may also be very misleading. For that particular reason it is good to double check the op amp in real life scenario first. It is true that simulations of detailed spice models take ages to converge, but for this particular reason it would be very nice to have a set of real life models at hand.

@Aziz, I have an odd notion that comparing the AI IB coils is going to be somewhat more informative if they are compared to a monocoil of half the area. All AI coils are in fact comprised of two mutually competing coil systems that result in cancellation of distant signals, ground and :sigh: distant targets, but a fair comparison would be with doubled the total area of AI coils. Somehow all the AI have nice pinpointing abilities, and all of them have waning sensitivity with distance that is in fact expected. But how they compare to monocoils of half the area will IMHO be a much more comparative challenge.

[Carl-NC]

One thing to watch out for, particularly with the 5534 used as a single gain stage, is that the gain does not recover quite as fast as you would think by observing the dc recovery trace on the scope. This can be checked by summing a sine wave signal (anywhere between 10kHz and 100kHz) into the input when the TX and coil are running; just enough to give about 1V pk-pk on the preamp output.

Hadn't thought about that... I like this test, I'll add it next time I'm looking at OVR.