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Author Topic: Coaxial Cable Coil Induction  (Read 577 times)

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Guys,

What do you expect to be induced in the inner solid helical coil when the outer helical coil is wound with an ideal coaxial cable, which is driven symmetrically from an isolated secondary of an RF transformer from one end, and terminated with a resistance R on the other end ?
Both helical windings are coaxial with respect to each other.

1) When R=0
2) When R= the characteristic impedance of the coaxial cable
3) When R=∞

A) When the coaxial cable is driven by a CW signal having a period equal to an integer multiple of halfwavelengths of this piece of cable.
B) When the coaxial cable is driven by a CW signal having a period equal to the quarter wavelength of this piece of cable.
C) When the coax is pulsed with a very short pulse (e.g. 1/100 of its wavelength).

   

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It's not as complicated as it may seem...
For condition 2), I would expect nothing to be induced in all three cases A-C.

1A): I would expect to see an induced signal on the secondary of amplitude 1 and phase A.
1B): I would expect to see an induced signal on the secondary of amplitude 2 (2<1) and phase A.
1C): I would expect to see an induced pulse on the secondary of amplitude C and Phase D.

3A): I would expect to see an induced signal on the secondary of amplitude 1 and phase B.
3B): I would expect to see an induced signal on the secondary of amplitude 2 (2<1) and phase B.
3C): I would expect to see an induced pulse on the secondary of amplitude C and Phase E.

Phase A/D is 180 degrees of Phase B/E.

You ask good hard questions! Not sure I got any correct (I did not cheat). :-[


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Aye, very difficult questions indeed.

One can only wonder if they have any resemblance
to the shielded loop antenna.


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I suppose:
- the coils length is negligible compared to the wavelength (only the wire and coaxial cable length is not).
- the generator is not ideal, it has an output resistance.

The induced signal being proportional to the current in the coaxial cable, i.e the sum of I in inner+outer conductors integrated along the line, I would say it is zero in all cases otherwise current would accumulate somewhere. In other words all the current that flows from the generator in the inner conductor must return to the generator in the outer conductor, canceling any flux across the coaxial coil.

Difficult to realize a convincing experiment, too many biases (capacitive coupling, coils length not negligible, coaxial cable not perfectly symmetrically driven...)



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When R=0

case A, the RF transformer will see hence drive a short circuit -- induction can happen in the inner coil
case B, the RF transformer will see hence drive an open circuit -- induction can happen in the inner coil
case C, the RF transformer will see hence drive either an inductive or capacitive reactance -- induction can happen in the inner coil   

all such inductions can happen due to standing waves on the coax cable

When R= the characteristic impedance of the coax

In all cases the RF transformer will see and drive the R resistance -- no induction in the inner coil

when R= ∞

Case A, the RF transformer will see hence drive an open circuit -- induction can happen in the inner coil
Case B, the RF transformer will see hence drive a short circuit -- induction can happen in the inner coil
Case C, the RF transformer will see hence drive either an inductive or capacitive reactance -- induction can happen in the inner coil

An addition: in all cases of A and B at R=0 or R=∞, the induced voltage in the inner coil can only be small due to the coax's short or open circuit behaviour

Gyula
« Last Edit: 2019-02-20, 11:18:05 by gyula »
   
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When R=0

case A, the RF transformer will see hence drive a short circuit -- induction can happen in the inner coil
case B, the RF transformer will see hence drive an open circuit -- induction can happen in the inner coil
case C, the RF transformer will see hence drive either an inductive or capacitive reactance -- induction can happen in the inner coil   

all such inductions can happen due to standing waves on the coax cable

When R= the characteristic impedance of the coax

In all cases the RF transformer will see and drive the R resistance -- no induction in the inner coil

when R= ∞

Case A, the RF transformer will see hence drive an open circuit -- induction can happen in the inner coil
Case B, the RF transformer will see hence drive a short circuit -- induction can happen in the inner coil
Case C, the RF transformer will see hence drive either an inductive or capacitive reactance -- induction can happen in the inner coil

An addition: in all cases of A and B at R=0 or R=∞, the induced voltage in the inner coil can only be small due to the coax's short or open circuit behaviour

Gyula

And what if the coaxial cable is replaced by a symmetrical bifilar line like a twin lead 300 Ω?   :)




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And what if the coaxial cable is replaced by a symmetrical bifilar line like a twin lead 300 Ω?   :)

Induction will occur in a helical secondary from a pulsed symmetrical transmission line with a resistive load equal to the tline Z.  The pulse period ideally would be td/2 but certainly not limited to this.

Other criteria not stated in the original post which is important to the performance of a loaded helical secondary in this case, is the secondary's characteristic unit inductance and capacitance relative to the symmetrical tline.  From this info, the secondary is then treated as an independent tline with it's own characteristics and performance can be enhanced.

Regards,
Pm 
   
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And what if the coaxial cable is replaced by a symmetrical bifilar line like a twin lead 300 Ω?   :)

I think all the answers I made remain still valid, except the R=300 Ohm (i.e. matched) case when the twin line will induce a certain amount of voltage in the inner coil, I agree with Partzman.

Gyula
   
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With a bifilar line such as twin lead 300 Ω, the symmetry of the circuit implies that the current in each wire flows in the opposite direction and therefore cancels each other out when attempting to create a magnetic flux. So there is no induction.
A coaxial cable does the same thing, only its appearance differs, not the currents flowing through the two conductors.

What we see in "real life" are the effects of
  • capacitive coupling
  • not perfect cancellation of the effects of opposite currents due to their spatial separation leading to an electric field detectable between the two conductors outside (not only for a bifilar line or a twisted pair like a networking cable, but also for a coaxial cable rarely perfectly shielded)
  • propagation effects due to the significant length of the coil in comparison with the wavelength, meaning that the coil doesn't act as a whole, i.e. each winding section does not perfectly share the same magnetic flux, so that part of the current, spatially non-constant along the line due to the propagation, also generates a non-constant flux along the coil
  • etc...

There are so many possible artifacts that when I see a TPU or the Kapanadze device with wires in all directions wound without any precaution or control of the parameters, unless it is at really low frequencies, the result can be anything even if people started from the same schematic.


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Guys,

What do you expect to be induced in the inner solid helical coil when the outer helical coil is wound with an ideal coaxial cable, which is driven symmetrically from an isolated secondary of an RF transformer from one end, and terminated with a resistance R on the other end ?
Both helical windings are coaxial with respect to each other.

1) When R=0
2) When R= the characteristic impedance of the coaxial cable
3) When R=∞

A) When the coaxial cable is driven by a CW signal having a period equal to an integer multiple of halfwavelengths of this piece of cable.
B) When the coaxial cable is driven by a CW signal having a period equal to the quarter wavelength of this piece of cable.
C) When the coax is pulsed with a very short pulse (e.g. 1/100 of its wavelength).
From my knowledge of transmission lines and especially coaxial ones, any magnetic field is confined to the inside,  i.e. there is always equal and opposite current flowing along the inner and the outer.  Because of the cylindrical symmetry the magnetic fields from those currents cancel outside the cylinder.  This is true whether the line is driven balanced or unbalanced.  Thus the only magnetic coupling to the inner helix comes from the wires feeding the line and those connected to the terminating  resistor.   Ignoring those wires, my answer is zero magnetic coupling in all cases.  There will be capacitive coupling to the helix but since you have not specified what is connected to the helix to measure anything I can't give a definitive answer, except to say that for cases 1 and 3 where there will be standing waves that coupling will be greatest.
Smudge
   
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With a bifilar line such as twin lead 300 Ω, the symmetry of the circuit implies that the current in each wire flows in the opposite direction and therefore cancels each other out when attempting to create a magnetic flux. So there is no induction.
A coaxial cable does the same thing, only its appearance differs, not the currents flowing through the two conductors.
...

Well, I agree with what you termed and wrote as "real life" situation and I did not quote that part.

Speaking of twin leads, as an addition I note that EM field cancellation is mostly correct for the space between the two conductors. But in the space around them, for instance at the top or bottom or at the left and right sides of the conductors, there is EM field, a near field if you like  and the some mm or even 1-2 cm closeness of the cylinder coil wound inside as Verpies drew in his figure (but he drew coax while you asked the twin lead case) can pick up those fields.  Cancellation cannot be as "perfect" outside as between the two conductors. See the attached pictures I took from the web to illustrate what I mean.
This means that in the R=300 Ohm case (matched twin lead), there will be RF voltage induced in the secondary coil. The induced voltage will of course be relatively smaller than in any of the unmatched cases.

Gyula
   
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Kapanadze had an arrangement on one of his devices that resembles the first pic verpies posted. Rumor is he was trying to find a length of a certain type of coax at one point (impedance or low loss). I can't find the schematic right now, but it was a  length of coax shorted at one end,   wound around another normal conductor, and the coax was driven by a DRSD.

Anybody else remember this or have the schematic? Is this why verpies posted it?

https://www.overunityresearch.com/index.php?action=dlattach;topic=3735.0;attach=31303
« Last Edit: 2019-02-22, 15:35:42 by ion »


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Believing something false does not make it true.
I owned and operated a TV repair business for a little over 6 years.  Twin lead does have a field around the outside that is very susceptible to outside influence.  And conversely can affect the area around it.

I once made a call where the customer was complaining of very poor reception.  I had sold him a new antenna and twin lead some time before.  When I got to the house I saw he had NOT used the standoff insulators I had told him he needed to use.  He had taped the twin lead to the steel pipe supporting the antenna.  I climbed up on the roof and cut the tape loose so  the twin lead was just hanging from the antenna.  When I went back in the house he was off the couch and standing up in the middle of the room.  He turned around and with a big smile on his face said that the picture was the best he had ever seen on that TV.

I explained that the twin lead next to the pipe was killing the magnetic field around the twin lead and that prevented the signal from reaching his TV.  He said he would immediately install the standoffs I had sold him.

I found over the years that twin lead had less signal loss than coax if installed correctly.  But coax was much less susceptible to interference.



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Hi CITFTA

There was a version of 300 ohm twin lead with most of the plastic insulator punched out to further reduce dielectric loss, of course ultimitely for lowest loss, radio engineers used just a few  insulating spacers between balanced lines feeding the transmitter tower.

I believe with 300 ohm twin lead the recommended installation technique was to provide a certain number of twists along it's length
to cancel incident radiation from an interfering signal, and of course to use the insulating standoffs and never tape the wire to, or allow it near to anything metallic.
 
We have something in common, I started repairing TV's in the neighborhood with my father at around age 12. That was in the vacuum tube era starting with the RCA 630 series chassis in the early 50's. Before that we were repairing vacuum tube radios.

So does anybody remember the Kapanadze coax schematic and rumor?


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It's not as complicated as it may seem...
E,

I may have it at home. I will look later.


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Believing something false does not make it true.
Hi Ion,

Yes we do have something in common.  I also started repairing electronics with my dad but a little later in the late 50's.  And I was also about 12 years old when I started.  Later I opened my own shop after trying several other jobs for many years.  Unfortunately my timing was not too good.  I got into the business as the old tube type tvs were on the way out and solid state was replacing the tubes.  So the amount of repairs dropped considerably.  So I then went into industrial machine repair.

And yes you are correct about the proper installation of the twin lead.  I used to pull it off the end of the spool instead of the side and that gave it a natural twist which seemed to be just right to cancel out any interference.

Take care,
Carroll


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I agree with everyone about the incomplete cancellation of the field of a twin lead, but only at short distances: when you move away from the line, the distances with respect to each wire tends to equalize, with the result that the field of each wire cancels out, because current flows in the opposite direction. That is why twin lead do not radiate significantly.

With a coaxial cable we have more protection but it is not a panacea. I have a coaxial cable that is 2 meters long. When I connect it to the input of my VHF/UHF receiver, all local FM stations are received 10 to 30 dB above the background noise. With another one-metre coaxial, I receive the most powerful.
Only with the 1 meter coaxial supplied with my Siglent generator or with a RG214 cable, 6 meter long, the receiver provides only background noise.

We need high quality coaxial cable if we are dealing with W or kW instead of hundreds µW or some mW from the radio, and are expecting to detect induction from a coaxial instead of artifatcs.


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There are different types of coax, most are flexible ones using braided outers.  Some of the cheaper ones for TV's have minimum braids allowing some leakage.  More expensive ones have double layers of braid to minimise leakage or loss.  The best type use solid outer usually of Al to allow some bending.  My comments applied to the solid ones which I have used extensively for frequencies between 1 and 2 GHz.
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F6FLT said:
Quote
We need high quality coaxial cable if we are dealing with W or kW instead of hundreds µW or some mW from the radio, and are expecting to detect induction from a coaxial instead of artifacts.

I think I may have some short lengths of the very high quality stuff around somewhere. It came out of an engineering lab and used for VHF  experiments where cost was not critical. As I remember, the stuff is fairly stiff and had a brownish outer jacket.


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FWIW, here is just one example of what I described in my reply #6.  Is this a conventional use of a symmetrical transmission line.......no!  This is a view from outside the box of conventional thinking in regards to the use of a transmission line and I won't bore anyone with details.

The schematic is attached with the various probe IDs and the scope pix shows the signals and measurements.  The operating frequency in this example is 15.7MHz so you may discount any and all measurements if you so desire.  The signal source is a Rigol DG4162 and the current probe used is a properly calibrated and de-skewed Tek TCP 0020.  The resistors are 1% Caddock non-inductive film and the device is one of my early TPU structures.

Here we see the Pin is 124.4mw.  The Pout of R1 is 72.3mw and the Pout of R2 is 132.7mw resulting in an apparent COP = 1.65.

Regards,
Pm

     
   

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From my knowledge of transmission lines and especially coaxial ones, any magnetic field is confined to the inside,  i.e. there is always equal and opposite current flowing along the inner and the outer.  Because of the cylindrical symmetry the magnetic fields from those currents cancel outside the cylinder.  This is true whether the line is driven balanced or unbalanced.  Thus the only magnetic coupling to the inner helix comes from the wires feeding the line and those connected to the terminating  resistor.   Ignoring those wires, my answer is zero magnetic coupling in all cases.  There will be capacitive coupling to the helix but since you have not specified what is connected to the helix to measure anything I can't give a definitive answer, except to say that for cases 1 and 3 where there will be standing waves that coupling will be greatest.
Smudge
What could be said about the coax as an A-field source ?
   

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The induced signal being proportional to the current in the coaxial cable, i.e the sum of I in inner+outer conductors integrated along the line, I would say it is zero in all cases otherwise current would accumulate somewhere.
Yes, the integrated current is zero so the only induction should come from any coaxial cable imperfections and capacitive coupling.
Where would the capacitive coupling be the greatest if the coax is symmetrically driven?
   

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When R=0

case A, the RF transformer will see hence drive a short circuit -- induction can happen in the inner coil
case B, the RF transformer will see hence drive an open circuit -- induction can happen in the inner coil
case C, the RF transformer will see hence drive either an inductive or capacitive reactance -- induction can happen in the inner coil   

all such inductions can happen due to standing waves on the coax cable

When R= the characteristic impedance of the coax

In all cases the RF transformer will see and drive the R resistance -- no induction in the inner coil

when R= ∞

Case A, the RF transformer will see hence drive an open circuit -- induction can happen in the inner coil
Case B, the RF transformer will see hence drive a short circuit -- induction can happen in the inner coil
Case C, the RF transformer will see hence drive either an inductive or capacitive reactance -- induction can happen in the inner coil

An addition: in all cases of A and B at R=0 or R=∞, the induced voltage in the inner coil can only be small due to the coax's short or open circuit behaviour

Gyula
So many different expectations !
   
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So many different expectations !

So what are your "expectations" ?  I would like to learn.
   

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It's not as complicated as it may seem...
F6FLT said:
I think I may have some short lengths of the very high quality stuff around somewhere. It came out of an engineering lab and used for VHF  experiments where cost was not critical. As I remember, the stuff is fairly stiff and had a brownish outer jacket.
You may be referring to RG400 E.


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