concentric vlf coil frequency

[mykaitch]

Hello.I have spent a few days searching but cannot find the answer. When I have wound my tx coil (done- the osc runs at 10kHz exactly),I then wind an Rx coil.
If I connect it to the osc should it be 10KHz ? (I think so).
When I add the bucking coil to the Tx I expect the osc to drop to around 9.5KHz. Now,some info suggests this is fine. Howver, I could tweak the osc caps back to 10KHz, but that would also affect the Rx tuning. I spent a good hour or so getting the tx right.
So I guess the question is: Can there be a small difference in the inductance of the rx and tx ? I found a good online colpitts cacl so can play around to find best values.
When all is done I will post more pics and the tale of the coil...oh I have been round and round in circles.

Thanks in advance.

[dfbowers]

Hello.I have spent a few days searching but cannot find the answer. When I have wound my tx coil (done- the osc runs at 10kHz exactly),I then wind an Rx coil.
If I connect it to the osc should it be 10KHz ? (I think so).
When I add the bucking coil to the Tx I expect the osc to drop to around 9.5KHz. Now,some info suggests this is fine. Howver, I could tweak the osc caps back to 10KHz, but that would also affect the Rx tuning. I spent a good hour or so getting the tx right.
So I guess the question is: Can there be a small difference in the inductance of the rx and tx ? I found a good online colpitts cacl so can play around to find best values.
When all is done I will post more pics and the tale of the coil...oh I have been round and round in circles.

hanks in advance.

When you design your coil, you must account for the inductance of the bucking coil and subtract that from the inductance of the main Tx coil. Adding the bucking coil should INCREASE your frequency because it is supposed to be wound in the opposite direction of the main Tx coil.

[porkluvr]

mykaitch,

Hard to give you a definite answer (you did not provide important details), but tx and rx tanks are usually NOT tuned to the same frequency. For instance, Tesoro Bandito transmitter works at ~10kHz but the rx tank is tuned closer to 11kHz. Similarly, Golden Sabre tx is ~13.5kHz but rx is tuned to ~15kHz (rough figures).

This avoids the situation where, depending on temperature drift or other factors, rx resonant frequency could bounce above and below tx frequency. Rx phase could be wildly unpredictable.

Some manufacturers go lower in their rx frequency. Some do use the same tx and rx tuning but they are an exception, because of various complications that might best be avoided.

If you have different L values for tx and rx, then of course you would have different values of tx frequency depending on which coil is connected to the oscillator.

[mykaitch]

Thanks for all the info guys. Doh! Already wrapped the tx in tape, time to unwrap then. No point doing it unless I do it right.
I notice 'but the rx tank...' I have been regarding the Rx coil as essentially an antenna, or indeed similar to near field RF tags (lots of exp there). Is not a tank coil a reference to the coil in say a buck regulator or a simple osc ? When I 'test' the rx coil by wiring as the Tx, then it is the tank coil (but I will wind it for 11KHz). This is a query, not a criticism. I listen and heed all advice.

[porkluvr]

An alternative to removing windings on a rx coil may be to use a smaller value parallel capacitor; aren't you are trying to increase rx tank frequency?

I think IB coils, instead of being antenna, are better thought of as loosely coupled transformers and for that reason having more rx turns than tx turns can give a step-up (voltage gain) advantage.

Regarding metal detectors, imho a tank is a search coil/tuning capacitor (or tx coil/tuning capacitor). Usually the tank is parallel tuned.

You still have not provided a picture or good description of what you are trying to accomplish. What is your circuit? maybe that will be useful information.

[mykaitch]

I have built a Tesoro Bandido II uMax. I am making another coil for it. My first one was okay but not great - I nulled it by shaping the tx coil. Not sure if it is a good idea to change the cap- better to follow your other advice and get it right, but thanks a lot. Much appreciated.

[mykaitch]

Hello. I have done a lot of reading but have to come back to you,sorry.
Say we have a colpitts osc
If we add another L in series and phase then the L increses and F decreases ? Must do.
If the second L is in anti-phase (and say not same size) then there is a cancellation effect and so F increases,even though L1+L2 is >L1,
this is because of the phase diff (Iam happy with this) ?
If L1=L2 then in theory, when out of phase both cancel and osc stops ?
thanks.
Been too cold to work in garage but hope to resume coil soon - will post pics etcwhen done. Then going to replace analogue stuff with PIC and LCD..

[golfnut]

[QUOTE=mykaitch;164949]If we add another L in series and phase then the L increses and F decreases ? Must do.
If the second L is in anti-phase (and say not same size) then there is a cancellation effect and so F increases,even though L1+L2 is >L1,
this is because of the phase diff (Iam happy with this) ?
If L1=L2 then in theory, when out of phase both cancel and osc stops ?
QUOTE]

If we add another L in series and phase then the L increses and F decreases ?

YES, series L's (wound in the same) direction ADD. 4mH + 10mH = 14mH Larger L lower Frequency. (whether series LC or Parallel LC)


If the second L is in anti-phase then there is a cancellation effect and so F increases,

Not sure here. Never thought about it.
This statement is true in a transformer secondary. If the scondary had 1/2 of its turns one way and then changed direction for the 2nd 1/2 of its turns. Then, the net output would be zero.

S

[golfnut]

[QUOTE=mykaitch;164949]If we add another L in series and phase then the L increses and F decreases ? Must do.
If the second L is in anti-phase (and say not same size) then there is a cancellation effect and so F increases,even though L1+L2 is >L1,
this is because of the phase diff (Iam happy with this) ?
If L1=L2 then in theory, when out of phase both cancel and osc stops ?
QUOTE]

If we add another L in series and phase then the L increses and F decreases ?

YES, series L's ADD So 4mH + 10mH = 14mH so Larger L lower Frequency.
(whether series LC or Parallel LC same applies)


If the second L is in anti-phase then there is a cancellation effect and so F increases,

This statement is true in a transformer secondary. If the secondary had 1/2 of its turns one way and then changed direction for the 2nd 1/2 of its turns. Then, the net output would be zero.

A concentric is like a xfmer and this is what you do with the buck coil.


For Ls in series I THINK they only ADD, you dont get the subtraction from a reversed subsequent winding.


Try it on an L bridge and turn one around see if halves or stays double..

The magic happens when windings are touching and the same flux goes into both turns, then the direction of the turns influences the net L value

S

[porkluvr]

If L1=L2 then in theory, when out of phase both cancel and osc stops ?
...

It's not that simple. Look at the formulas. (Fortunately, there ARE formulas.) edit: YES that is correct, if both coils are same and perfectly coupled.

When your coils have space between them, something that will very likely prevent you from being able to accurately predict how the coils either add or subtract: coefficient of coupling 'k'.

If you have a good LCR meter and some skill with algebra you could measure some air core coil combinations and work your way back to k, but that's not my idea of fun!


Two identical series aiding coils with perfect coupling, will give you 4x the value of a single coil. That goes along with the fact that doubling the windings in a coil can give you nearly 4x the original value. (The coil must be close wound to obtain 4x.)

A good scientific calculator can come in handy, but if you are trying to calculate an IB coil with a buck winding, you have a largely unknown value of k because of the distance between the two windings. The trial and error of adding or subtracting windings to stay on frequency will be inescapable unless you not bother with frequency change. Maybe you should worry about getting a good null and not worry so much about small frequency changes.

Of course you have seen the new PDF file on this subject? You'll find it here:
http://www.geotech1.com/forums/showt...-coil-for-TGSL

Also, look at " Construction of Coplanar Search Coils "

and US4293816.

edit: I had to remove some material that when evaluated left me baffled. Formula for M was incorrect.
Two indentical coils, reverse wound with perfect coupling (k=1), will indeed give you zero L. (I can see that now that I corrected my formula for M.)

[dfbowers]

Thanks for the detailed explanation of how these coils interact. I never did quite understand it completely. In practice it's not that hard to use the trial and error method, especially after you make one or two of them.

If you get L1 close, then focus on the null with L2. You will have to go back and and add a turn or two to L1 to correct the frequency. Then, just move a singel turn of wire around to fine tune the null. It's not any more difficult than that.

Don

[porkluvr]

Whew. I tripped all over that last post because I was originally using k=sqrt (L1+L2), but k=sqrt(L1XL2) is correct. I only realized that after I came up with a negative value when trying to evaluate the case of two reverse wound coils.

[Aziz]

BTW guys,

in a concentric co-planar IB coil, the coefficient of coupling k is negative. The bucking coil is in opposite windinging direction to the transmit coil. For a half diameter bucking coil, use an approximation for k around -0.1 to -0.15.
Aziz

PS: To avoid confusion:
L = L1 + L2 + 2*k*sqrt(L1*L2), since k is negative due to reversed bucking coil winding direction
->
L = L1 + L2 - 2*|k|*sqrt(L1*L2), |k| = absolute value of k ( abs(k) )

[Davor]

Actually when a transformer is presented as a two port network, and K is less than 1, you may come up to negative inductances in equivalent networks. Because they are overcome by the positive ones, there certainly is no instability in such system, and such negative inductances may come handy for creation of certain filters. Unfortunately Spice does not support negative L-s

[mykaitch]

Hmm. I knew that L1+Ln add to increase inductance. Fine, so I have just done an experiment. I wound my Rx coil and measured the F at 10.6KHz. Then I placed an old buck coil in series, a few inches away. F reduced as I knew it would. Reversed buck coil - no difference.
I have just destroyed one of the foundations of electronics - a nobel prize beckons, or I am a twat (is that okay to say ?). Knowing about EMF and coupling and stuff a placed the secons coil on top of the Rx coil. Now it follows the rules, ie F goes up or down depending on L2 phase (easily done by turning the coil over. Damn - No Nobel prize, again.
So, I think the point missing in the discussions is simple transformer theory and I should have thought of that!
EG; If you take two identical transformers and parallel the secondaries you can double the current - so long as they are in phase. If one is out of phase they don't 'alf get 'ot.
Many many thanks...outside temp is right up to 4 now so T shirt on and back to the winding.