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