using coax shield to create a loading coil ?
 

Good morning. I would like to experiment with making a high Q coil for creating
a tuned radial counterpoise. Reg's program predicts a coil of about 70 uH will
create a match. One way to create such a coil would be to wind coax and use the
shield as the conductor. Besides the obvious loss of physical stability due to
lack of a form what are the limitations or drawbacks from using the shield?

Thanks - Dan - kb0qil

using coax shield to create a loading coil ?
 

dansawyeror wrote:
Good morning. I would like to experiment with making a high Q coil for
creating a tuned radial counterpoise. Reg's program predicts a coil of
about 70 uH will create a match. One way to create such a coil would be
to wind coax and use the shield as the conductor. Besides the obvious
loss of physical stability due to lack of a form what are the
limitations or drawbacks from using the shield?

The braided shield will be substantially lossier than a solid wire or
tube of the same diameter. And the relatively poor quality dielectric
used for the outside of the cable will also reduce the Q somewhat. So
coax isn't a good choice for your stated objective of making a high Q coil.

Roy Lewallen, W7EL

using coax shield to create a loading coil ?
 

The braided shield will be substantially lossier than a solid wire or
tube of the same diameter.

Why?

And the relatively poor quality dielectric
used for the outside of the cable will also reduce the Q somewhat.

The sheath is easy to strip.

using coax shield to create a loading coil ?
 

Mike Speed wrote:
The braided shield will be substantially lossier than a solid wire or
tube of the same diameter.

Why?

Two factors. One is surface roughness. The other is caused by the
current having to continually move from one group of wires to another as
it travels. I'm just now doing some research on how significant these
effects are, but so far I've found out they're very noticeable. It's one
of the reasons the loss of typical coax is substantially greater than
predicted with idealized programs or calculations that fail to take
these factors into effect.

And the relatively poor quality dielectric
used for the outside of the cable will also reduce the Q somewhat.

The sheath is easy to strip.

True enough, but my guess is that the resulting water and crud you'll be
getting among the fine wires will be worse than the insulation.

But hey, you don't have to believe me. Make up some coils and measure
their Q -- it's not hard at all. Then stick them outside for a while and
measure them again. Or do like most amateurs do -- make the coils,
discover that you can talk to far away places "barefoot", and declare
that they "work".

Roy Lewallen, W7EL

using coax shield to create a loading coil ?
 

Two factors. One is surface roughness. The other is caused by the
current having to continually move from one group of wires to another as
it travels.

Interesting, but how do you know the current is moving as you say?

I'm just now doing some research on how significant these
effects are, but so far I've found out they're very noticeable.

Ok. I'm curious: What equipment are you using for the research?


But hey, you don't have to believe me. Make up some coils and measure
their Q -- it's not hard at all. Then stick them outside for a while and
measure them again.

What would be a good way to measure Q?

Or do like most amateurs do -- make the coils,
discover that you can talk to far away places "barefoot", and declare
that they "work".


Uugghh, don't I know.

using coax shield to create a loading coil ?
 

You may have better luck by using sone soft copper tubing. Dimensional
stability would be one advantage.
You could use ScotchKote or some insulating paint if you needed it to be
insulated.

"dansawyeror" wrote in message
...
Good morning. I would like to experiment with making a high Q coil for
creating
a tuned radial counterpoise. Reg's program predicts a coil of about 70 uH
will
create a match. One way to create such a coil would be to wind coax and
use the
shield as the conductor. Besides the obvious loss of physical stability
due to
lack of a form what are the limitations or drawbacks from using the
shield?

Thanks - Dan - kb0qil

using coax shield to create a loading coil ?
 

Mike Speed wrote:
Two factors. One is surface roughness. The other is caused by the
current having to continually move from one group of wires to another as
it travels.

Interesting, but how do you know the current is moving as you say?

Skin effect is well known. On a good conductor at high frequencies,
current is concentrated very near the surface. When a bundle of wires
ducks under another in the direction of current flow, the current has to
migrate to the outside again, which means it has to move from one
conductor to another. There's no question that it happens -- what's a
bit harder to pin down is just how much loss typically results.

I'm just now doing some research on how significant these
effects are, but so far I've found out they're very noticeable.

Ok. I'm curious: What equipment are you using for the research?

Books, and to a lesser extent the web. Information about this is
scattered among a number of sources. Quite a few discuss surface
roughness in a general way, but there's a particularly good explanation,
analysis, and something of a quantitative treatment in Johnson &
Graham's _High-Speed Signal Propagation: Advanced Black Magic_. The
effect of weaving is harder to track down -- most authors simply assume
coax shield conductivity loss to be negligible, and don't deal with
woven conductors in any other context. But it really isn't, if you're
interested in good accuracy. And of course when the braided conductor is
the primary conductor, it becomes much more important. I know Tom, W8JI
has done some measurements on braided vs solid strap, and I'll be asking
him for more information before long. I do know that he found a very
significant difference, and I have a great deal of respect for his
experience, measurements, and opinions.

But hey, you don't have to believe me. Make up some coils and measure
their Q -- it's not hard at all. Then stick them outside for a while and
measure them again.

What would be a good way to measure Q?

The way I do it is by resonating the inductor with a parallel air
variable capacitor. It's important to keep it away from just about
everything. I couple in and out with a very small (typically 1 pF at HF)
capacitor, and make sure that the impedances of the source and detector
are either very high or quite low (say 50 ohms) to minimize loss. I use
a signal generator for the source and a scope for the detector. Using a
frequency counter connected to the signal generator, I measure the
resonant frequency and -3dB points. The Q is the ratio of the center
frequency to the 3dB bandwidth. For convenience, I made a 3dB pad I can
switch in an out of the signal generator. With this, you don't even need
a linear detector, and a diode and meter would do. My measurements have
been within about 5 - 10% of readings with a good HP Q meter on the few
occasions when I've compared them. That's close enough for my purposes.

Or do like most amateurs do -- make the coils,
discover that you can talk to far away places "barefoot", and declare
that they "work".


Uugghh, don't I know.

Many people have worked the world with 1 watt, knowing that's what they
were running. A lot more have worked the world with 1 watt, thinking
they were running 100. Ignorance is bliss.

Roy Lewallen, W7EL

using coax shield to create a loading coil ?
 

Roy,

Thank you. It is a quick experiment to build a test coax coil and measure the Q.
That should produce enough evidence to test a counterpoise.

In the mean time the research to build a coil out of copper tubing continues. So
far the only alternative I can conceive is to make a wooden form and wrap the
coil on the outside.

Thanks again - Dan

Roy Lewallen wrote:
Mike Speed wrote:

Two factors. One is surface roughness. The other is caused by the
current having to continually move from one group of wires to another as
it travels.


Interesting, but how do you know the current is moving as you say?


Skin effect is well known. On a good conductor at high frequencies,
current is concentrated very near the surface. When a bundle of wires
ducks under another in the direction of current flow, the current has to
migrate to the outside again, which means it has to move from one
conductor to another. There's no question that it happens -- what's a
bit harder to pin down is just how much loss typically results.

I'm just now doing some research on how significant these
effects are, but so far I've found out they're very noticeable.


Ok. I'm curious: What equipment are you using for the research?


Books, and to a lesser extent the web. Information about this is
scattered among a number of sources. Quite a few discuss surface
roughness in a general way, but there's a particularly good explanation,
analysis, and something of a quantitative treatment in Johnson &
Graham's _High-Speed Signal Propagation: Advanced Black Magic_. The
effect of weaving is harder to track down -- most authors simply assume
coax shield conductivity loss to be negligible, and don't deal with
woven conductors in any other context. But it really isn't, if you're
interested in good accuracy. And of course when the braided conductor is
the primary conductor, it becomes much more important. I know Tom, W8JI
has done some measurements on braided vs solid strap, and I'll be asking
him for more information before long. I do know that he found a very
significant difference, and I have a great deal of respect for his
experience, measurements, and opinions.

But hey, you don't have to believe me. Make up some coils and measure
their Q -- it's not hard at all. Then stick them outside for a while and
measure them again.


What would be a good way to measure Q?


The way I do it is by resonating the inductor with a parallel air
variable capacitor. It's important to keep it away from just about
everything. I couple in and out with a very small (typically 1 pF at HF)
capacitor, and make sure that the impedances of the source and detector
are either very high or quite low (say 50 ohms) to minimize loss. I use
a signal generator for the source and a scope for the detector. Using a
frequency counter connected to the signal generator, I measure the
resonant frequency and -3dB points. The Q is the ratio of the center
frequency to the 3dB bandwidth. For convenience, I made a 3dB pad I can
switch in an out of the signal generator. With this, you don't even need
a linear detector, and a diode and meter would do. My measurements have
been within about 5 - 10% of readings with a good HP Q meter on the few
occasions when I've compared them. That's close enough for my purposes.

Or do like most amateurs do -- make the coils,
discover that you can talk to far away places "barefoot", and declare
that they "work".


Uugghh, don't I know.


Many people have worked the world with 1 watt, knowing that's what they
were running. A lot more have worked the world with 1 watt, thinking
they were running 100. Ignorance is bliss.

Roy Lewallen, W7EL

using coax shield to create a loading coil ?
 

On Fri, 17 Feb 2006 17:25:28 -0800, Roy Lewallen
wrote:

... Or do like most amateurs do -- make the coils,
discover that you can talk to far away places "barefoot", and declare
that they "work".

Roy,

As we dumb amateur radio down to make it attractive to the
disinterested masses in a desperate and mistaken persuit of increasing
the number of licenced hams, this is becoming the new standard of
understanding in the redefined amateur radio.

I wrote comment on an a recent article in Australia's ham radio
magazine "Amateur Radio" that was an example of the declaration of not
just something that works, but "something that really works" though it
looks to be quite inefficient on at least one band. The comment is at

http://www.vk1od.net/blog/index.php?...Id=21&blogId=1
..

Supporters argue "amateur radio is about having QSOs, so if you have
QSOs then the antenna works... QED".

Though antenna systems remains one of the few areas of amateur radio
where hams can cost effectively design solutions specific to their
location and needs, the lower competency standard of the new
"communicator" style amateur does not support a soundly based
understanding of antenna systems. We frequently hear the argument that
there is no need to understand electronics for modern amateur radio
where commercial radios are the norm, but forget electronics for a
moment, how many hams understand a common three component passive
network that is so often employed with variable results, the ATU.

Get used to it! Amateur radio is being transformed to "I just wanna
talk on the radio".

Owen
--

using coax shield to create a loading coil ?
 

When a bundle of wires ducks under another in the direction of current flow, the current has to migrate to the outside again, snip

There's no question that it happens


Books, and to a lesser extent the web. Information about this is
snip
I have a great deal of respect for his experience, measurements, and opinions

Again, interesting, but what's been outlined so far is not scientific.
For something of this nature to be of any utility, it must be grounded
in science.