¿ªÔÆÌåÓý

ctrl + shift + ? for shortcuts
© 2025 Groups.io
  1. Nanovna-Users
  2. Topics

CMC MEASUREMENTS - PDF'ed & SIZE REDUCED

 

On encouragement from a few posters, I have updated my construction and
measurements of quite a number of CMCs.

My hope and goal in all this is to encourage others to construct something
simple which can greatly reduce coupling of local RFI into the receive path
of their amateur stations and keep the feedline from radiating from their
antennas. I'd also encourage the group to build and use their NANOs.
Hopefully this and the procedures I previously posted will accomplish some
small amount of home brewing and satisfaction of having done it all
yourself - with good measurements thanks to the folks who made the NANOs
possible.

Please have a look at the attachment.

Dave - W?LEV

pdfresizer.com-pdf-resize-2.pdf
pdfresizer.com-pdf-resize-2.pdf

Gary Rondeau
 

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.

Any comments?
Gary AF7NX
 

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
 

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 8:39 AM, Jim Lux wrote:
It's also fairly hygroscopic, but I doubt moisture content drives the dissipation.
 

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.
 

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

On Mon, Feb 8, 2021 at 4:55 PM Jim Lux <jim@...> 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.







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

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

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 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.
 

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 10:34 AM, Jim Lux wrote:
The leakage resistance changes noticeably with humidity.
 

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.
 

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.
 

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

On 2/8/21 11:12 AM, Jim Lux wrote:
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.
 

There is a limited listing of ¦År for a few common dielectrics in any ARRL
Handbook. For a more extensive listing I'd refer you to the CRC Handbook
of Chemistry and Physics published by the CRC Press. About any edition
will do. There are also good online sources.

Dave - W?LEV

On Mon, Feb 8, 2021 at 10:45 PM Dave Cole <dave@...> wrote:

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

On 2/8/21 11:12 AM, Jim Lux wrote:
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.











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

Thanks sir!!!

73, and thanks,
Dave (NK7Z)

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

On 2/8/21 2:55 PM, David Eckhardt wrote:
There is a limited listing of ¦År for a few common dielectrics in any ARRL
Handbook. For a more extensive listing I'd refer you to the CRC Handbook
of Chemistry and Physics published by the CRC Press. About any edition
will do. There are also good online sources.
Dave - W?LEV
On Mon, Feb 8, 2021 at 10:45 PM Dave Cole <dave@...> wrote:

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

On 2/8/21 11:12 AM, Jim Lux wrote:
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.
 

As a non-engineer (of any type) like other nano-users, the simple mission was to wind some coax or wire around a ferrite donut installed on our OCFD and stop the nasty, dreaded, feared, sneaky and often discussed common mode current from creating shack mayhem.

The intended process? Copy the designs of others and validate with our nanos.

The actual process: sink into a putrid pit of RF theory, irreconcilable contradictions, arcania, mystery with misery, and a odd desire to plunge an ice pick in our eye because it would feel better than trying to understand how to (a) build a CMC and (b) measure our newly minted CM choke effectiveness over the lowly average ham bands (say 80-10m) with our beloved nanos.

I, and others, read and study Dave's (W0LEV)'s work like Orthodox study the Talmud - every letter, every grouping of words, even the space between words, has intense meaning to the lost, abandoned and hopeless neophytes.

All this is to say thank you Dave, and others. Your work is light to sheep lost in inky blackness.

My favorite professor would say at the end of every lecture to a class clearly dazed, befuddled and confused: ..."okay, clear as mud?" Followed by "Good, see you next week!"

Don
Km4udx
 

Don, I absolutely love your tongue-in-cheek (well, I do understand)
response! I really do!!!

Some of us had to suffer through learning calculus and then applying it to
electromagnetic theory and antennas and transmission lines. My references
now are graduate level. Yes, we suffered, but at this end of life, I'm
glad I *once* (stress: ONCE) did.

You are to be commended in your response which reflects the audience to
which I attempted to target. If I can't pass along some of the knowledge I
gained in formal education followed by decades of seat-of-the-pants
learning during my career applying what I learned while suffering, I'm
ready for the pine box.

Thank you for your reply!!!!!! You should take up satirical writing.

Dave - W?LEV

On Mon, Feb 8, 2021 at 11:53 PM Don - KM4UDX <dontay155@...> wrote:

As a non-engineer (of any type) like other nano-users, the simple mission
was to wind some coax or wire around a ferrite donut installed on our OCFD
and stop the nasty, dreaded, feared, sneaky and often discussed common mode
current from creating shack mayhem.

The intended process? Copy the designs of others and validate with our
nanos.

The actual process: sink into a putrid pit of RF theory, irreconcilable
contradictions, arcania, mystery with misery, and a odd desire to plunge an
ice pick in our eye because it would feel better than trying to understand
how to (a) build a CMC and (b) measure our newly minted CM choke
effectiveness over the lowly average ham bands (say 80-10m) with our
beloved nanos.

I, and others, read and study Dave's (W0LEV)'s work like Orthodox study
the Talmud - every letter, every grouping of words, even the space between
words, has intense meaning to the lost, abandoned and hopeless neophytes.

All this is to say thank you Dave, and others. Your work is light to
sheep lost in inky blackness.

My favorite professor would say at the end of every lecture to a class
clearly dazed, befuddled and confused: ..."okay, clear as mud?" Followed by
"Good, see you next week!"

Don
Km4udx





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

OMG!!

No topic drift, no politics, no name calling, just chat about our favorite topics!

There _are_ some reasonable people left alive!! I have found my nirvana on groups.io... What a wonderful group to have found, thank you all!


73, and thanks,
Dave (NK7Z)

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

On 2/8/21 4:02 PM, David Eckhardt wrote:
Don, I absolutely love your tongue-in-cheek (well, I do understand)
response! I really do!!!
Some of us had to suffer through learning calculus and then applying it to
electromagnetic theory and antennas and transmission lines. My references
now are graduate level. Yes, we suffered, but at this end of life, I'm
glad I *once* (stress: ONCE) did.
You are to be commended in your response which reflects the audience to
which I attempted to target. If I can't pass along some of the knowledge I
gained in formal education followed by decades of seat-of-the-pants
learning during my career applying what I learned while suffering, I'm
ready for the pine box.
Thank you for your reply!!!!!! You should take up satirical writing.
Dave - W?LEV
On Mon, Feb 8, 2021 at 11:53 PM Don - KM4UDX <dontay155@...> wrote:

As a non-engineer (of any type) like other nano-users, the simple mission
was to wind some coax or wire around a ferrite donut installed on our OCFD
and stop the nasty, dreaded, feared, sneaky and often discussed common mode
current from creating shack mayhem.

The intended process? Copy the designs of others and validate with our
nanos.

The actual process: sink into a putrid pit of RF theory, irreconcilable
contradictions, arcania, mystery with misery, and a odd desire to plunge an
ice pick in our eye because it would feel better than trying to understand
how to (a) build a CMC and (b) measure our newly minted CM choke
effectiveness over the lowly average ham bands (say 80-10m) with our
beloved nanos.

I, and others, read and study Dave's (W0LEV)'s work like Orthodox study
the Talmud - every letter, every grouping of words, even the space between
words, has intense meaning to the lost, abandoned and hopeless neophytes.

All this is to say thank you Dave, and others. Your work is light to
sheep lost in inky blackness.

My favorite professor would say at the end of every lecture to a class
clearly dazed, befuddled and confused: ..."okay, clear as mud?" Followed by
"Good, see you next week!"

Don
Km4udx
 

On 2/8/21 1:51 PM, Manfred Mornhinweg wrote:

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.
Indeed - so it would be an issue in laminated steel cores, but nobody is going to be using that for RF.



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.
This is all true, but in practice, we rarely see "pure" plastics, except in applications like clear windows, or high end applications like spaceflight, where they want "traceability to sand". They're formed in various ways, with fillers and additives, as well as scrap from earlier batches or recycling.? So you can have a plastic that is normally quite low loss, in the pure form, but quite high loss in the form used. This is especially true in price sensitive applications - plumbing and wire insulation strike me as in that bucket.? I would think that coax has fairly well controlled properties, and the inner dielectric is quite pure. But hookup wire, machine tool wire, THHN (which is PVC, with a nylon coating on the outside), and plastic pipe are not so well controlled, except for manufacturing ease and price. As long as the THHN passes the voltage breakdown test, it will pass, lossy or not.? Even wire procured to MIL standards might not be particularly well controlled - they test to the spec, and if the spec doesn't call out RF properties, it might be variable.

1 - 18 of 18