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



In his book _Ferromagnetic Core Design & Application Handbook_
Doug DeMaw claimed to have put ferrite sleeves on a vhf dipole
which reduced its size without introducing significant loss.
He claimed to have cut the size of the dipole in half.
You'll have to do the same thing yourself if you want to know
whether or not he was right.
73,
Tom Donaly, KA6RUH

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.

"Tom Donaly" wrote in message
m...
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



In his book _Ferromagnetic Core Design & Application Handbook_
Doug DeMaw claimed to have put ferrite sleeves on a vhf dipole
which reduced its size without introducing significant loss.
He claimed to have cut the size of the dipole in half.
You'll have to do the same thing yourself if you want to know
whether or not he was right.
73,
Tom Donaly, KA6RUH

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.

"Tom Donaly" wrote in message
m...

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



In his book _Ferromagnetic Core Design & Application Handbook_
Doug DeMaw claimed to have put ferrite sleeves on a vhf dipole
which reduced its size without introducing significant loss.
He claimed to have cut the size of the dipole in half.
You'll have to do the same thing yourself if you want to know
whether or not he was right.
73,
Tom Donaly, KA6RUH




Balanis, in his book _Antenna Theory, Analysis and Design_, has
a short section dealing with this. Define a parameter
Q = (mu - 1)ln(b/a), where mu is complex permeability of
the ferrite, a is the radius of the conducting wire, and b is
the radius of the conducting wire plus the ferrite. According to
Balanis, increasing the real part of Q "a. increases the peak input
admittance b. increases the electrical length (lowers the resonant
frequency c. narrows the bandwidth." In order to use this formula,
you have to know the complex permeability of the ferrite coating.
I don't know how you'd measure that. Maybe Richard Clark knows.
It would be fun to try. I wouldn't pin any hopes on it being
practical, though, since it doesn't seem to be in general use anywhere.
73,
Tom Donaly, KA6RUH

On Sun, 03 Apr 2005 00:53:34 GMT, "Tom Donaly"
wrote:

In order to use this formula,
you have to know the complex permeability of the ferrite coating.
I don't know how you'd measure that. Maybe Richard Clark knows.

Hi Tom,

I've measured a number of ferrites, but only in the HF region. They
do show a range of values, with most of them not very reactive (in
relation to the R).

73's
Richard Clark, KB7QHC

You can measure the complex impedance of a ferrite core quite easily and
with moderate accuracy using an antenna analyzer. From that reading and
a low frequency impedance measurement, you could calculate the complex
permeability. However, you can find graphs of the values for common
ferrite types at http://www.conformity.com/040spotlight.pdf and other
web sources. But it's not obvious to me why you'd need to calculate or
measure the complex permeability -- all you need to do is measure the
impedance of a short wire with the core slipped over it. When you slip
the core over the antenna, it'll behave just as though an impedance of
that value was inserted in series with the antenna wire at that point.

Different types of ferrites are quite different at HF. Low frequency
ferrites like the Fair-Rite 70 series are primarily resistive at HF, and
would simply add loss to an antenna like adding a series resistor. High
frequency types like the 60 series are inductive with reasonable Q
through the HF range so would behave pretty much like a series inductor
of moderate Q. Type 43, probably the most common type now available, has
a Q on the order of 1 at HF, so it also would primarily just add loss to
an antenna.

But hey, if you use one of the lossy ferrites you'll end up with an
antenna that's really broadband and quiet. That's what we all want,
isn't it?

Roy Lewallen, W7EL

Tom Donaly wrote:

Balanis, in his book _Antenna Theory, Analysis and Design_, has
a short section dealing with this. Define a parameter
Q = (mu - 1)ln(b/a), where mu is complex permeability of
the ferrite, a is the radius of the conducting wire, and b is
the radius of the conducting wire plus the ferrite. According to
Balanis, increasing the real part of Q "a. increases the peak input
admittance b. increases the electrical length (lowers the resonant
frequency c. narrows the bandwidth." In order to use this formula,
you have to know the complex permeability of the ferrite coating.
I don't know how you'd measure that. Maybe Richard Clark knows.
It would be fun to try. I wouldn't pin any hopes on it being
practical, though, since it doesn't seem to be in general use anywhere.
73,
Tom Donaly, KA6RUH

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?

Tom Donaly wrote:
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



In his book _Ferromagnetic Core Design & Application Handbook_
Doug DeMaw claimed to have put ferrite sleeves on a vhf dipole
which reduced its size without introducing significant loss.
He claimed to have cut the size of the dipole in half.
You'll have to do the same thing yourself if you want to know
whether or not he was right.
73,
Tom Donaly, KA6RUH

Yea, but isn't that the same thing as winding a helix to increase the
inductance per unit length to accomplish the same results. I don't
know if Doug was right, cause I have not done either. May try it.
Gary N4AST

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