开云体育

Hi,

Thanks for the detail!!! If you find that study on various dialectic values for various materials, I would love a copy... I am making some VLF and HF ferrite bar antenna, and using CPVC to protect the ferrite, and as a coil form.

73, and thanks,
Dave (NK7Z)

ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources

Gary,

I ask because I made a few measurements with a thermal probe on the side of a
single core FT-240-31 with 8T RG-11 that is a choke I have for my 10m antenna.
What I discovered is the the temperature rises very quickly on the core
surface when you turn on the power and then more slowly diffuses into the
center of the core.

Strange. Flux density, and thus heat production, happens in the bulk of the ferrite, not just its surface. They are strongest where the magnetic path is shortest, and that's along the inside surface of a toroid, while along its outside surface flux density and heat production are minimal. In between there is close-to-linear distribution through the bulk of the core.

Do you happen to know how much end-to-end voltage you had on that choke, when it got warm? According to my measurements and calculations, with 8 turns on that core, at 28MHz, it takes 194V end-to-end to cause a volumetric loss of 300mW/cm?, which equates to 8W total loss in that core, enough to heat it up reasonably fast. So that could be seen as a not-to-exceed value for ICAS service. In a 50? system where antenna balance is such that half of the signal voltage appears end-to-end, that's enough to handle legal limit power. But if the choke is used in a high impedance system, its power limit will be much lower.


Jim,

Skin depth is a thing, too. And since it's proportional to 1/sqrt(mu), higher mu makes the skin depth smaller. Fortunately it's proportional to sqrt(resistivity),
and the resisitivity of ferrites is very high

Let's not forget that skin effect is a phenomenom that affects conductors. There is no such thing as skin effect in ferrites, which are essentially insulators. The reduction of skin effect with ? happens when the electric conductor is also a magnetic material, such as steel wire. I hope nobody here is using steel wire to wind CMCs! But copper-clad steel wire is OK, as long as the copper layer is plenty thick enough to accomodate the skin depth at the frequency of operation.


About the dielectric loss in plastics:

There are two families of plastics: Polar and non-polar ones. In polar plastics each molecule is electrically asymmetric, making it react strongly to electric fields and thus absorbing a lot of energy at RF, turning it into heath. In non-polar plastics each molecule is electrically balanced, drastically reducing the absorption of energy from RF electric fields. PVC and nylon are polar plastics, polyethylene and teflon are non-polar ones.

Water is a polar molecule, and that's why water absorption in plastics increases their dielectric loss. But polar plastics have such high dielectric loss anyway that water absorption in them is probably of pretty low importance on their total dielectric loss.

RG-58 is not appropriate for US amateur legal power. I don't use it.
This is one of the main reasons I have chosen bifilar wound CMCs.

I have enough RG-142 (Teflon silver coated and double shielded 50-ohm coax
that will take power) to try one good choke wound in the manner of your
referenced presentation. I can try that and present the results.

Dave - W?LEV

On Mon, Feb 8, 2021 at 7:33 PM Max via groups.io <kg4pid@...>
wrote:

Your finding don't compare well with the data found here.



Look at the 12 and 17 turn on RG58.
Max KG4PID


On Sunday, February 7, 2021, 01:16:30 PM CST, Mel Farrer via groups.io
<farrerfolks@...> wrote:

Hi, Dan,
My experience with the 240-31 core with 14 T RG 303 gives me ~7K at 160,

13K on 80/40, slowly rolling off to >4 K at 28 MHz Still >5 K on 12 .

Mel, K6KBE
On Sunday, February 7, 2021, 10:53:04 AM PST, Dan Schaefer W3BU <
clancy.987@...> wrote:

Don
Still curious about the 5k across 10 to 160?
Been reading the mail and has been fun but it isn’t obvious to me you get
5k across the whole band? When you get time it might be fun to discuss
your definitions for the 5k.
Have been enjoying the discussion threads but currently a bit time limited
to engage.
Enjoy.
Dan. W3BU

On Jan 17, 2021, at 9:49 AM, Don - KM4UDX <dontAy155@...> wrote:

groups.io/g/nanovna-users/wiki
-=-=-

--
*Dave - W?LEV*
*Just Let Darwin Work*

The CMCs I have constructed and measured are NOT coaxial cable wound on the
toroids which are presented in your reference. That presentation simply
winds and measures (for the most part) RG-58 wound on the toroids. That's
why I included the picture at the end to illustrate specifically what I was
dealing with. That's the green PVC-insulated wire that heated badly with
400-watts. I have wound and measured and posted bifilar-wound chokes, NOT
coax. These are NOT the same.

Coaxially wound CMC are not true common mode chokes. They offer a
reflective function (inductive) for the RF energy on the outer surface of
the coaxial cable: a large +jX. As such, they function as a current
'balun' reflecting the energy with a bit of absorption as well. A true CMC
accepts CM energy (coaxial cable, for example) at one port and 'outputs' DM
energy (equal amplitude with opposite phases) at the opposite port. It is
a bilateral device. A bifilar wound CMC accomplishes the function of a
coaxially wound choke PLUS ensuring the DM required balance in amplitude
and phase at the DM side of the choke. This is accomplished by a
'feedback' mechanism between the transmission line on the toroid and the
induced magnetic currents within the core. This 'feedback' mechanism which
functions in both directions ideally cancels core magnetic currents induced
by each conductor of the bifilar windings. A coaxial 'balun' or current
balun such as coaxial cable wound on the core does not offer this
additional benefit of a bifilar wound CMC.

The true CMC works on the same principle as parallel wire transmission
line, but without any added and lumped magnetic material. The interaction
between the two conductors of the oscillating RF field consisting of both
electric and magnetic fields on the line cancel eachother, resulting in no
radiation from the transmission line but only transmission of the RF energy
along the line. That's the physics (without the math) of the workings of
a true CMC. The presence of the magnetic material - the toroid -
'concentrates' the magnetic field produced by the bifilar windings much
like a dielectric 'concentrates' the electric field (in the case of a
capacitor), both of which allow for application of a lumped circuit
function instead of a distributed circuit function.

Dave - W?LEV

On Mon, Feb 8, 2021 at 7:33 PM Max via groups.io <kg4pid@...>
wrote:

Your finding don't compare well with the data found here.



Look at the 12 and 17 turn on RG58.
Max KG4PID


On Sunday, February 7, 2021, 01:16:30 PM CST, Mel Farrer via groups.io
<farrerfolks@...> wrote:

Hi, Dan,
My experience with the 240-31 core with 14 T RG 303 gives me ~7K at 160,

13K on 80/40, slowly rolling off to >4 K at 28 MHz Still >5 K on 12 .

Mel, K6KBE
On Sunday, February 7, 2021, 10:53:04 AM PST, Dan Schaefer W3BU <
clancy.987@...> wrote:

Don
Still curious about the 5k across 10 to 160?
Been reading the mail and has been fun but it isn’t obvious to me you get
5k across the whole band? When you get time it might be fun to discuss
your definitions for the 5k.
Have been enjoying the discussion threads but currently a bit time limited
to engage.
Enjoy.
Dan. W3BU

On Jan 17, 2021, at 9:49 AM, Don - KM4UDX <dontAy155@...> wrote:

groups.io/g/nanovna-users/wiki
-=-=-

--
*Dave - W?LEV*
*Just Let Darwin Work*

Your finding don't compare well with the data found here.


Look at the 12 and 17 turn on RG58.
Max KG4PID

ALL the modules are 3.3 volt devices but many have low output regulators in front.
Get the schematic for the one you have and bypass the regulator to run it directly from 3.3v

I can not find a low voltage HC-05 module such as the 3.3v. Where did you purchase your module? Everyone I've found on line are of the 3.6-6v modules.

-Thanks

On 2/8/21 10:56 AM, Dave Cole wrote:

Hi,
Thanks for the clarification, do you think this is due to the PVC itself absorbing water, or just moisture in the air?

A little of both - moisture adsorbing into the surface will increase surface leakage for HV, but after wiping it down with alcohol, the leakage resistance was still higher, so I think there's some bulk absorption into the plastic.? Another indication is that blowing warm dry air on it takes a while to get the leakage back down.? This is in low current HV applications (Electrostatic machines like Van de Graaff generators or Kelvin water droppers), where 1 microamp is a lot of current, so driving leakages low is important.


I'll have to go look, but someone did a study measuring the Q of tesla coil secondaries on various form materials as the weather varied. Cardboard (sonotube) varies a lot, but I can't remember how PVC pipe worked.


My "take home" from this was "choose a different kind of plastic for those applications"

For the "plastic pipe in a microwave" I think it's more bulk absorption into the porosity - plastic pipe is not pure PVC, after all. It's PVC + fillers + dye, and with no attention to bulk dielectric properties.

In most RF applications (e.g. antennas) using PVC pipe is probably fine - there's a lot of other things that will dominate over any small dielectric losses, and whether the leakage is microamps or 10s of microamps is probably not worth worrying about.? I've not worried about it much. I don't think I'd use it as a substrate for a conformal patch antenna, though.? It's too easy and cheap to get something you know will work better.

73, and thanks,
Dave (NK7Z)

ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources

On 2/8/21 10:34 AM, Jim Lux wrote:

The leakage resistance changes noticeably with humidity.

On Sun, Feb 7, 2021 at 09:21 AM, Peter Ivanooff wrote:

.I started tests with several noname ferrite toroids

Chances are that I'm not interpreting your setup correctly, but based on the picture with choke and nanoVNS, you were only testing the DM mode - impact of your choke on power sent from one side to the other side of your choke.
You need to test the CM (common mode) suppression, and that get's done differently - for example, you short both sides

Would be great if you can clarify your test setup and then mention what attached screens shots are for :)

On Sun, Feb 7, 2021 at 09:21 AM, Peter Ivanooff wrote:

the toroides turned out to be MgZn

I suppose you meant MnZn (Manganese-Zinc)

On Sun, Feb 7, 2021 at 09:21 AM, Peter Ivanooff wrote:

Full House with Data ...I've made a try to burn the choke with my PA and 250W
noninductive dummy load (with some pauses).KEY Down on CW and 175 volts p/p on
Tektronix osci.
That is 612 watts on the peaks....The core and windings were cold as before
the test ...I know the real test is up on the feedpoint of the antenna....

You did not get power correctly :) P=Vrms^2/R, so if you have Vpp=175V (peak to peak), Vmax=Vpp/2=87.5V and Vrms=Vmax/sqrt2=87.5/1.41=62V and P=77W on 50ohm load

Hi,
Thanks for the clarification, do you think this is due to the PVC itself absorbing water, or just moisture in the air?

73, and thanks,
Dave (NK7Z)

ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources

On 2/8/21 9:07 AM, Dave Cole wrote:

I am referring to the line:

"It's also fairly hygroscopic..."
Is this referring to PVC?

Yes - I think probably more due to porosity and filler than the actual PVC itself.

If you go look up a plastics table, PVC is in the "generally non-hygroscopic" bucket (like PE, PP and PS)

However, in practice, particularly with pipe in HV gear, I've not found that the case. The leakage resistance changes noticeably with humidity.

The hygroscopic plastics (Nylon, polycarbonate, PET, Acrylic) are just a lot worse <grin>

73, and thanks,
Dave (NK7Z)

ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources

On 2/8/21 8:55 AM, Jim Lux wrote:

On 2/8/21 8:45 AM, Dave Cole wrote:

Are you referring to PVC here?

yes, it's kind of like nylon in that way, and unlike, say polyethylene, PTFE, or polypropylene, which "as delivered" tend to be fairly hydrophobic.

?From what I understand, depending on what kind of PVC you get, it might have contaminants in it (from remelting other batches), fillers, and the porosity can vary.

For example you can take 4 or 5 pieces of PVC pipe and put them in a microwave oven and they will not necessarily heat the same (a rough and ready RF absorption test, granted at a higher frequency than HF, but easier to do)

I've had the interesting experience of putting a piece of white PVC pipe on a lathe to cut grooves in it, and found that only the surface was white, and that there were black streaks in other parts.

I guess the story here is that if you go look up plastic dielectric properties, that's typically for a lab sample, not necessarily what's extruded on your hookup wire or plastic tubing.

That might also explain the varying results people get with stuff not designed for RF (or at least with properties controlled), like zipcord. For hookup wire insulation, all they care about is that it passes the breakdown voltage test, not that it has low dissipation.

On Sun, Feb 7, 2021 at 04:55 PM, AG6CX wrote:

Do you have in your stash a piece on conversion of dB attenuation to Zs= Rs +
Xs ?

Ed, if you have S21 in terms of magnitude and phase, calculation of the series impedance of the device being measured is straight forward:

Zdut = Zo*((2/S21) - 2), where Zo = 50 ohms for a 50-ohm VNC.

- Jeff, k6jca

I am referring to the line:

"It's also fairly hygroscopic..."

Is this referring to PVC?

73, and thanks,
Dave (NK7Z)

ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources

Jim, it was quite interesting to watch the PVC-wound CMC 'do its thing'. I
certainly didn't need 2.4 GHz, just 40-meters at 400-watts. It was really
rather fascinating to watch the SWR climb with applied RF energy. I
chickened out and terminated that at 3:1 SWR to save my amp..... The
microwave oven test is quite effective in revealing bad RF materials.

Thanks for the post!

Dave - W?LEV

--
*Dave - W?LEV*
*Just Let Darwin Work*

On 2/8/21 8:45 AM, Dave Cole wrote:

Are you referring to PVC here?

yes, it's kind of like nylon in that way, and unlike, say polyethylene, PTFE, or polypropylene, which "as delivered" tend to be fairly hydrophobic.

From what I understand, depending on what kind of PVC you get, it might have contaminants in it (from remelting other batches), fillers, and the porosity can vary.

For example you can take 4 or 5 pieces of PVC pipe and put them in a microwave oven and they will not necessarily heat the same (a rough and ready RF absorption test, granted at a higher frequency than HF, but easier to do)

I've had the interesting experience of putting a piece of white PVC pipe on a lathe to cut grooves in it, and found that only the surface was white, and that there were black streaks in other parts.

I guess the story here is that if you go look up plastic dielectric properties, that's typically for a lab sample, not necessarily what's extruded on your hookup wire or plastic tubing.

That might also explain the varying results people get with stuff not designed for RF (or at least with properties controlled), like zipcord.? For hookup wire insulation, all they care about is that it passes the breakdown voltage test, not that it has low dissipation.

Are you referring to PVC here?

73, and thanks,
Dave (NK7Z)

ARRL Volunteer Examiner
ARRL Technical Specialist, RFI
ARRL Asst. Director, NW Division, Technical Resources

On 2/8/21 8:04 AM, Gary Rondeau wrote:

Very nice compilation!

So what is up with the PVC insulation? I know that PVC is particularly bad in terms of dissipation factor - but is it insulation self heating or just getting melted by a hot ferrite core?
I ask because I made a few measurements with a thermal probe on the side of a single core FT-240-31 with 8T RG-11 that is a choke I have for my 10m antenna. What I discovered is the the temperature rises very quickly on the core surface when you turn on the power and then more slowly diffuses into the center of the core. Similarly, when the power is shut off, the temperature falls relatively quickly on the surface, as the surface heat conducts inward or is lost to convection with the air. I can imagine that extreme non-uniform heating could lead to fracturing of the brittle core material.

PVC has a low melting/softening point, so it's sensitive to these kinds of effects.

It's also fairly hygroscopic, but I doubt moisture content drives the dissipation.

Skin depth is a thing, too. And since it's proportional to 1/sqrt(mu), higher mu makes the skin depth smaller. Fortunately it's proportional to sqrt(resistivity), and the resisitivity of ferrites is very high

Any comments?
Gary AF7NX

Great!
...Larry