"Cecil Moore" bravely wrote to "All" (02 Apr 05 09:57:02)
--- on the heady topic of " Antenna wires and ferrite"

CM From: Cecil Moore
CM Xref: aeinews rec.radio.amateur.antenna:27806
[,,,]
CM Adding the insulation increased the feedpoint impedance from
CM 57 ohms to 65 ohms which means the forward and reflected
CM waves on the standing-wave antenna were attenuated more
CM using insulated wire and sure enough, using that particular
CM insulation reduced the EZNEC maximum gain by 0.12 dB.

CM Many people have noticed shifts in resonant frequency when
CM their antenna gets wet. Water has a dielectric constant
CM around 80.


Speaking of odd antennas, how about using a long neon tube as an
antenna and what would eznec give as values then? After all a plasma
behaves like a conductor doesn't it?

A*s*i*m*o*v

.... Email returned to sender -- insufficient voltage.

On Sat, 2 Apr 2005 17:15:03 -0500, "Hal Rosser"
wrote:

interesting.
if coating the antenna with ferrite can reduce its size,
would ferrite sleeves over the ferrite sleeves reduce the size even further?
we're always looking for ways of reducing the size of our dipoles.


And conversely, judging by the number of email offers I receive,
always looking for ways to *increase* the size of our monopoles.

And conversely, judging by the number of email offers I receive,
always looking for ways to *increase* the size of our monopoles.

=============================

A sort of a ferrite-viagra ointment?

Asimov wrote:
Speaking of odd antennas, how about using a long neon tube as an
antenna and what would eznec give as values then?

Back in college, we used to use a florescent bulb
to detect RF electric fields. What's the feedpoint
impedance of a neon tube?
--
73, Cecil http://www.qsl.net/w5dxp

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"Cecil Moore" bravely wrote to "All" (03 Apr 05 08:38:34)
--- on the heady topic of " Antenna wires and ferrite"

CM From: Cecil Moore
CM Xref: aeinews rec.radio.amateur.antenna:27837

CM Asimov wrote:
Speaking of odd antennas, how about using a long neon tube as an
antenna and what would eznec give as values then?

CM Back in college, we used to use a florescent bulb
CM to detect RF electric fields. What's the feedpoint
CM impedance of a neon tube?

I suppose the plasma looks like a DC resistance over a region of its
characteristic and maybe even negative at some point. It depends of
course if we are talking about a glow or an arc. With an arc the
current discharge is somewhat infinite and the huge noise makes it
impractical. I'm not sure about the noise with glow discharge on a
long neon tube but I'm assuming it is very low judging from some brief
measurements I made on a neon indicator bulb. BTW these make good UV
detectors wrapped in aluminium and biased at the conduction threshold.
I'm only guessing the long neon tube has a DC resistance of about 50
ohms per foot. Something 30 feet high would therefore be around 1.5K.
What are typical running voltages and currents for neon signs?
BTW don't know if running 1KW would it make a fabulous light show?
Shades of Nicolai Tesla!

A*s*i*m*o*v

.... Speeding doesn't kill people... Stopping really fast does!

My dear friend Cecil,

It's a waste of time mentioning things like DIP.TL.EZ and EZNEC4.
Hardly anybody has ever heard of whatever they are. I certainly
havn't. And the chances of obtaining them, even if legal, within the
next 12 months is so remote, by then, everybody will have forgotten
what it's all about and will have lost interest in the subject. So
nobody ever takes any notice of references and switches to another
more-interesting thread on the newsgroup.

If you have any facts to say then say them. It's up to you to be
convincing. If you think you need the support of Terman or Kraus then
you lack self-confidence. Bibles are usually misquoted, or taken out
of context anyway. Second-hand, plagiarised, information adds nothing
to reliability.

As usual, you gave only half of the information needed to make sense.

In addition to a thick neoprene layer of 0.1 inches, with a high
permittivity of 6.7, what was the antenna wire diameter and the
approximate height above ground?

Without such details your information is old-wives' waffle.

As things are, your velocity factor reduction of 7.8% does not go out
of the ball park value predicted by my formula.

My formula takes a few milliseconds to calculate. Whereas your method
requires a 4-weeks training course and several hours making the model.
----
Reg,G4FGQ

Reg Edwards wrote:
It's a waste of time mentioning things like DIP.TL.EZ and EZNEC4.
Hardly anybody has ever heard of whatever they are. I certainly
havn't. And the chances of obtaining them, even if legal, within the
next 12 months is so remote, by then, everybody will have forgotten
what it's all about and will have lost interest in the subject.

EZNEC is available in a free demo version from www.eznec.com
You can enter dielectric constant and thickness of insulation.

If you have any facts to say then say them. It's up to you to be
convincing.

I once replaced an uninsulated loop with insulated wire to
try to reduce wind static/noise in AZ. The resonant frequency
went down by about 200 kc on 40m.

As things are, your velocity factor reduction of 7.8% does not go out
of the ball park value predicted by my formula.

Maybe I misunderstood. I thought you were saying insulation
has no effect.

My formula takes a few milliseconds to calculate. Whereas your method
requires a 4-weeks training course and several hours making the model.

Some training is worth it. My training using ELNEC and later
EZNEC has been very valuable. I certainly wouldn't spend "several
hours" on a model only to report the results in one thread on
this newsgroup. That's not enough return on investment. It took
me about seven minutes for that last report.
--
73, Cecil http://www.qsl.net/w5dxp


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Oh my. It is time for a story. My students use superb oscilloscopes that
are actually computers with a D-to-A converter used to display waveforms.
When measuring things such as the peak-to-peak size of a periodic waveform,
students (being students) initially writedown the number exported by the
oscilloscope. Whereupon, I ask the student if they wrote the computer code
that exported the number and, if not, why did they believe the result. The
student is sent back to note the max. and min. value, and to perform the
reliable calculation of subtraction.

NEC source code exists, is understandable, and has been verified many
times by independent persons. I teach my students to avoid using tools that
can not be verified at a fundamental level. Of course, rules-of-thumb are
an important part of error checking, and somethings are only amiable of
being approximated with a heuristic equation.

Mac N8TT

--
J. Mc Laughlin; Michigan U.S.A.
Home:

"Reg Edwards"
snip

My formula takes a few milliseconds to calculate. Whereas your method
requires a 4-weeks training course and several hours making the model.
----
Reg,G4FGQ

Reg Edwards wrote:

Let's start a parallel thread on the effect of coating a 14-gauge
antenna wire with a thick layer of ferrite. Say 1mm thick,
permeability = 100.

Would this have any effect on velocity factor? If so, by how much?
----
Reg

The way it looks to me, the speed of propagation is pretty much the
inverse of the squareroot of the product of mu and epsilon for the
dielectric between conductors.

ac6xg

Jim Kelley wrote:

The way it looks to me, the speed of propagation is pretty much the
inverse of the squareroot of the product of mu and epsilon for the
dielectric between conductors.


That's almost correct, but not quite. You need to modify it by changing
"the dielectric between conductors" to "the medium containing the
fields". Inside a coaxial cable, both are the same, so you can easily
calculate the velocity factor from the dielectric constant (relative
epsilon) of the dielectric. In the case of ladder line, TV twinlead, or
microstrip line, though, part of the field is in the dielectric and part
is in the air. So the velocity factor is a function of the dielectric
constants of both. Often, an "effective" dielectric constant is
calculated that fits the rule you mentioned(*). For the types of line I
mentioned, it's between those of air and the dielectric material. It's
not at all trivial to calculate, so it's usually determined by
measurement or a field-solving computer program.

In the case of an insulated antenna wire or one with a ferrite core on
the outside, the "other conductor" is usually a very great distance away
so the vast majority of the field is in the air. Also, the simple
formula you refer to might not apply when the distance between
conductors is a substantial fraction of a wavelength or more. If you
take a piece of coax with solid polyethylene dielectric and measure its
velocity factor, you'll find it to be around 0.66 (following the formula
you mention). But if you strip off the shield and use the same center
wire and insulation for an antenna, you'll find the insulation slows the
wave on the antenna by only a few percent (almost certainly less than five).

(*) In the case of microstrip line, the field distribution changes with
frequency. This results in an effective dielectric constant, and hence
velocity factor, which changes with frequency. With something like
Teflon dielectric, which has a relatively low dielectric constant, this
change isn't much. But it sure gave me grief when designing time-domain
circuitry using microstrip lines on an alumina substrate (dielectric
constant ~ 10), where the change was much greater.

Roy Lewallen, W7EL