Roy Lewallen wrote:
What information are you looking for, capture area or effective height?

Since I wasn't even aware that effective height applied to rod
antennas (or exactly what effective height means) I guess I was
thinking of capture area.

Capture area determines how many watts you'll get into a conjugately
matched load connected to the antenna.

That's it.

Effective height determines how
many volts you'll get from an open circuited antenna.

Does that include an antenna that has been brought to resonance with
an appropriate capacitive load?

The two aren't
directly related. For more information about the two topics, do a
groups.google.com search for postings I've made on those topics in this
newsgroup.

Thanks.

As I've posted here quite a number of times before, the capture area of
a lossless infinitesimally short dipole is very nearly as great as that
of a half wave dipole, in their most favored directions. (The difference
is about 10%, and it's due to the slight pattern shape difference caused
by different current distributions). So except for loss the capture area
of a ferrite rod antenna is within 10% of that of a dipole. But loss in
a ferrite rod antenna will reduce the capture area considerably.

So if a very small rod antenna had a lossless core that could handle
any flux level, and was wound with superconductor, it could couple
into the same volume of space as a 1/2 wave dipole? Amazing.

If
you're interested in knowing how much power you can get from a ferrite
rod, then, what you need to know is its efficiency, which is a function
of wire length, number of turns, and the antenna feedpoint impedance. I
don't have the time right now to work it out for you.

The effective height of a ferrite rod antenna is approximately:

(2 * pi * mueff * N * A) / lambda

where

mueff = effective relative permeability of the rod (mainly a function
of rod length)
N = number of turns
A = rod cross sectional area
lambda = wavelength

I can apply this formula directory to what I am experimenting with,
except that I have to approximate mueff. I am making the rod by
stacking ferrite beads, with various gaps between them. Can I
approximate mueff by taking the ratio of coil inductance with and
without the rod?

And, what if the rod area is not constant all along the rod? Since my
rods are assembled from pieces, I have a lot of freedom in this direction.

"John Popelish" wrote in message
...
Caveat Lector wrote:
Here is a site for examples of capture areas of antennas

http://www.sommerantennas.com/gain.html


Have you got a link to a similar site that covers ferrite rod antennas?

Try URL:

http://www.st-andrews.ac.uk/~jcgl/Sc...rt7/page5.html

Some others using Google
--
CL -- I doubt, therefore I might be !

John Popelish wrote:
Roy Lewallen wrote:
. . .
Effective height determines how many volts you'll get from an open
circuited antenna.

Does that include an antenna that has been brought to resonance with an
appropriate capacitive load?

No. "Open circuited" means that there's nothing connected across the
feedpoint.

. . .
So if a very small rod antenna had a lossless core that could handle any
flux level, and was wound with superconductor, it could couple into the
same volume of space as a 1/2 wave dipole? Amazing.

Yes, exactly the same volume of space, although capture area isn't a
measure of this. Capture area is an area, not a volume, and it's
different in all directions, just like gain. In fact, there's a 1:1
correspondence between capture area and gain, they're just different
ways of expressing the same thing. In the absence of loss, all the power
applied to *any* antenna will radiate, so the integral of the power
density over all directions is the same for all lossless antennas -- the
integral of the power density will equal the applied power, since
there's no dissipation (and in the far field the power density is simply
E^2 / Z0 = H^2 * Z0 where Z0 is the impedance of free space and E and H
are the electric and magnetic field strengths respectively). The
reciprocal of this principle is that the integral of capture areas in
all directions (what you're calling the "volume" the antenna is
"coupling" to) is the same for all lossless antennas.

But more directly to the point, your tiny theoretical rod antenna would
have a gain of about 0.45 dB less than a half wave dipole, and its
capture area would be correspondingly smaller -- about 10%. This is
assuming you're looking in the best direction for each antenna. Because
the total radiated power or integral of the capture areas must be the
same for the two antennas, this means that the tiny antenna has to have
more gain or capture area than the dipole in some other directions. And
indeed it does -- the tiny antenna has slightly fatter lobes than the
half wave dipole. This is a good experiment to run with EZNEC or NEC-2.
The EZNEC demo is adequate. Use free space, set wire loss to zero, and
compare the gains and patterns of a half wave dipole to a very short
one. The Average Gain tells you the ratio of total radiated power to
input power, and it should equal one if the program is doing its
calculations correctly.

If you're interested in knowing how much power you can get from a
ferrite rod, then, what you need to know is its efficiency, which is a
function of wire length, number of turns, and the antenna feedpoint
impedance. I don't have the time right now to work it out for you.

The effective height of a ferrite rod antenna is approximately:

(2 * pi * mueff * N * A) / lambda

where

mueff = effective relative permeability of the rod (mainly a
function of rod length)
N = number of turns
A = rod cross sectional area
lambda = wavelength

I can apply this formula directory to what I am experimenting with,
except that I have to approximate mueff. I am making the rod by
stacking ferrite beads, with various gaps between them. Can I
approximate mueff by taking the ratio of coil inductance with and
without the rod?

Yes. That's exactly what it is.

And, what if the rod area is not constant all along the rod? Since my
rods are assembled from pieces, I have a lot of freedom in this direction.

That one I don't know the answer to.

Roy Lewallen, W7EL

On Sat, 25 Mar 2006 01:09:06 GMT, Owen Duffy wrote:

On Fri, 24 Mar 2006 06:57:21 -0800, "Caveat Lector"
wrote:

Here is a site for examples of capture areas of antennas

http://www.sommerantennas.com/gain.html


Are you recommending it?

Unanswered...


Is the following statement from the page correct?

"Note: Antenna B has only half the capture area of antenna A and is
therefore able to "catch" only 50 percent of the electromagnetic
field; e.g., 50mV, compared to 100 mV/50 Ohms. This means 6dB less
gain for antenna B in comparison to antenna A."

Of course it is not.

The article seems based on some typical misconceptions about Capture
Area and the suggestion that you can run a ruler over a dipole (loaded
or otherwise) to measure up and calculate the capture area is
nonsense.

I wonder if that is how Somner derive the gain figures that they
publish for their antennas. (Gain is related to Capture Area, and if
they don't understand Capture Area, do they understand Gain?)

Owen
--

Roy Lewallen wrote:
John Popelish wrote:

Roy Lewallen wrote:
. . .

Effective height determines how many volts you'll get from an open
circuited antenna.


Does that include an antenna that has been brought to resonance with
an appropriate capacitive load?


No. "Open circuited" means that there's nothing connected across the
feedpoint.

But I can design the coil to be self resonant or not, just by
adjusting the surface area of the wire, or the spacing. It is non
intuitive that if I peak the coil this way, and obtain more voltage,
it is a different case than if I peak the coil with cable capacitance,
or an additional capacitor. I guess I really don't comprehend the
point of this value.

(Snip excellent review of basic lossless radiator. Thank you.)

But more directly to the point, your tiny theoretical rod antenna would
have a gain of about 0.45 dB less than a half wave dipole, and its
capture area would be correspondingly smaller -- about 10%. This is
assuming you're looking in the best direction for each antenna. Because
the total radiated power or integral of the capture areas must be the
same for the two antennas, this means that the tiny antenna has to have
more gain or capture area than the dipole in some other directions. And
indeed it does -- the tiny antenna has slightly fatter lobes than the
half wave dipole.

I understand what you are saying.

(snip)
The effective height of a ferrite rod antenna is approximately:

(2 * pi * mueff * N * A) / lambda

where

mueff = effective relative permeability of the rod (mainly a
function of rod length)
N = number of turns
A = rod cross sectional area
lambda = wavelength


I can apply this formula directory to what I am experimenting with,
except that I have to approximate mueff. I am making the rod by
stacking ferrite beads, with various gaps between them. Can I
approximate mueff by taking the ratio of coil inductance with and
without the rod?


Yes. That's exactly what it is.

Well, now I can calculate the effective height of my antennas, even
though I am not sure what it has to do with height.


And, what if the rod area is not constant all along the rod? Since my
rods are assembled from pieces, I have a lot of freedom in this
direction.


That one I don't know the answer to.

I am also experimenting with designs that do not necessarily have a
small, coil, close to the rod. (My interest in the discussion of
extended coils is showing.) One of the possibilities that shows a
significant increase in tuned Q is an hour glass shaped coil (small
diameter in the center, but sweeping to a larger diameter at the
ends). I have been asked to try putting a rod through the center of a
flat spiral coil. It seems to me that, at some extreme, the above
formula will fail, because it assumes that essentially all the signal
energy exiting the coil was collected by the rod, and that the signal
the coil would collect by itself would be insignificant. But if my
coils get large enough, they become loop antennas in their own right,
and the rod, though it may have a significant length and area, is only
a part of what is happening. In other words, the mueff can get pretty
small, even though the rod has significant dimensions. I guess, what
I am asking are what assumptions about coil dimensions (that are not
explicitly referenced in the formula) are being made in the above formula?

John Popelish wrote:
Roy Lewallen wrote:
John Popelish wrote:

Roy Lewallen wrote:
. . .

Effective height determines how many volts you'll get from an open
circuited antenna.


Does that include an antenna that has been brought to resonance with
an appropriate capacitive load?


No. "Open circuited" means that there's nothing connected across the
feedpoint.

But I can design the coil to be self resonant or not, just by adjusting
the surface area of the wire, or the spacing. It is non intuitive that
if I peak the coil this way, and obtain more voltage, it is a different
case than if I peak the coil with cable capacitance, or an additional
capacitor. I guess I really don't comprehend the point of this value.

Sorry, I don't understand what you're "peaking" and how. I believe that
the formula I gave assumes that the coil is well below self resonance
where the shunt capacitance is negligible. It won't be valid near self
resonance, if that's what you mean.

. . .

Well, now I can calculate the effective height of my antennas, even
though I am not sure what it has to do with height.

Do a groups.google.com search of this newsgroup for my postings
containing "effective height" or "effective length" -- I posted quite a
bit about it not long ago.

. . .

I am also experimenting with designs that do not necessarily have a
small, coil, close to the rod. (My interest in the discussion of
extended coils is showing.) One of the possibilities that shows a
significant increase in tuned Q is an hour glass shaped coil (small
diameter in the center, but sweeping to a larger diameter at the ends).
I have been asked to try putting a rod through the center of a flat
spiral coil. It seems to me that, at some extreme, the above formula
will fail, because it assumes that essentially all the signal energy
exiting the coil was collected by the rod, and that the signal the coil
would collect by itself would be insignificant. But if my coils get
large enough, they become loop antennas in their own right, and the rod,
though it may have a significant length and area, is only a part of what
is happening. In other words, the mueff can get pretty small, even
though the rod has significant dimensions. I guess, what I am asking
are what assumptions about coil dimensions (that are not explicitly
referenced in the formula) are being made in the above formula?

I'm sorry, I don't know. The reference it came from doesn't say. It in
turn references Laurent, H.J. and Carvalho, C.A.B., "Ferrite Antennas
for AM Broadcast Receivers", an application note from Bendix
Corporation. That app note, if you can find it, might or might not tell.
I'm sure it's not valid anywhere near self resonance, though.

Your concept of signal energy getting collected separately by the rod
and the coil, or exiting the coil and being collected by the rod doesn't
fit at all with what I know of the behavior of electromagnetic fields,
so I won't even try to comment on that aspect or the conclusions
resulting from it. Maybe it makes sense to Cecil or art -- they seem to
view things in a different way.

Roy Lewallen, W7EL

(My information for this comes from a number of websites including
commercial antenna sites. Needless to say, the accuracy of any
information, especially that of commercial sites is suspect.)


I have been reading about reduced size antennas using capacity hats
instead of, or in addition to, inductor loading. Most sites claim
that capacity hats reduce size with much less (or no) signal loss
compared to a full size antenna. At least one site claims a vertical
1/4 dipole using cap-hats has gain over a 1/4 vertical ground plane.
The consensus seems to be that size for size, the antenna shortened by
capacity hats has less loss than the same size antenna shortened by
inductors as per http://www.sommerantennas.com/gain.html (taken from
another thread in this newsgroup.)

Some claims are that the capacity hat antennas have equal signal
strength to their full-sized counterparts.

My research, thus far, my theory is:

1) any shortened antenna will have some loss compared to its
full-sized counterpart. (i.e. an 80 meter dipole shortened by one
foot using capacity hats will not be as efficient as the full length
version, even though one might be hard pressed to find the instrument
that could measure it.)

2) antennas shortened with capacity hats have less loss than those
shortened by inductors

3) capacity hat antennas exhibit slightly more bandwidth than inductor
loaded antennas

4) given equal length, a cap-hat vertical dipole will exhibit equal,
(or according to some sites, greater) signal strength to a vertical
monopole either reduced or full-size.



Size Loss vs efficiency of a cap-hat dipole.

Assuming my first point of theory is correct, there must be a point in
which the reduced size of a dipole using only capacity hats is
noticeable. Continued reduction finds additional noticeable points of
loss.

What I would like to know is approximately where those points might be
so the 'value' of a cap-hat dipole antenna can be determined given
some acceptable size or loss.

An example might be I have 25 feet of antenna pole. I can build an
antenna with what I have. However, it may be that for ten more feet
of pole, I can have a much better signal. Should I use what I have,
or order the additional ten feet of aluminum?

Another example would be to estimate the maximum power I can run on 60
meters using a given length antenna. If a 1/4 wave dipole will
radiate almost as effectively as a 1/2 wave, then I would not worry
about adding the extra 5 or ten watts I would need to max the ERP out,
but if the loss were significant, I would know I can leave my radio on
full power (100 watts) without committing a violation.

Thank you for your thoughts.







--
73 for now
Buck
N4PGW

"Buck" wrote in message
...
(My information for this comes from a number of websites including
commercial antenna sites. Needless to say, the accuracy of any
information, especially that of commercial sites is suspect.)


I have been reading about reduced size antennas using capacity hats
instead of, or in addition to, inductor loading. Most sites claim
that capacity hats reduce size with much less (or no) signal loss
compared to a full size antenna. At least one site claims a
vertical
1/4 dipole using cap-hats has gain over a 1/4 vertical ground plane.
The consensus seems to be that size for size, the antenna shortened
by
capacity hats has less loss than the same size antenna shortened by
inductors as per http://www.sommerantennas.com/gain.html (taken from
another thread in this newsgroup.)

Some claims are that the capacity hat antennas have equal signal
strength to their full-sized counterparts.

My research, thus far, my theory is:

1) any shortened antenna will have some loss compared to its
full-sized counterpart. (i.e. an 80 meter dipole shortened by one
foot using capacity hats will not be as efficient as the full length
version, even though one might be hard pressed to find the
instrument
that could measure it.)

2) antennas shortened with capacity hats have less loss than those
shortened by inductors

3) capacity hat antennas exhibit slightly more bandwidth than
inductor
loaded antennas

4) given equal length, a cap-hat vertical dipole will exhibit equal,
(or according to some sites, greater) signal strength to a vertical
monopole either reduced or full-size.



Size Loss vs efficiency of a cap-hat dipole.

Assuming my first point of theory is correct, there must be a point
in
which the reduced size of a dipole using only capacity hats is
noticeable. Continued reduction finds additional noticeable points
of
loss.

What I would like to know is approximately where those points might
be
so the 'value' of a cap-hat dipole antenna can be determined given
some acceptable size or loss.

An example might be I have 25 feet of antenna pole. I can build an
antenna with what I have. However, it may be that for ten more feet
of pole, I can have a much better signal. Should I use what I have,
or order the additional ten feet of aluminum?

Another example would be to estimate the maximum power I can run on
60
meters using a given length antenna. If a 1/4 wave dipole will
radiate almost as effectively as a 1/2 wave, then I would not worry
about adding the extra 5 or ten watts I would need to max the ERP
out,
but if the loss were significant, I would know I can leave my radio
on
full power (100 watts) without committing a violation.

========================================
Buck,

For the same reduction in height a top hat has greater efficiency than
a loading coil. It can amount to 3 or more decibels.

But a top hat gets in the way and is more unsightly than a coil.

Have you seen program TOPHAT available from website below.
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

Roy Lewallen wrote:
(snip)

Do a groups.google.com search of this newsgroup for my postings
containing "effective height" or "effective length" -- I posted quite a
bit about it not long ago.

Will do. Thanks.

Is "height" in "effective height" really referring to effective
radiating length, rather than something to do with elevation above
ground (assuming that one is talking about a monopole above a ground
plane)?

In other words, is the "effective height" of a horizontal dipole
actually related to its end to end length, not its distance above ground?

I am just trying to get started on the right foot in my reading.