Short Antennae
 

It is sometime since I studied the matter, but there is a formula
that describes the energy assocaited with an antenna, and it
is made up of two parts, for the near field and for the far field.

That part for the far field has the length of the antenna in
terms of fractions of a wavelength.

That part for the near field shows the local stored energy
which is indicative of the antenna system being an oscilatory
store of energy which must dissipate as heat if it is not
reflected back down the feeder.

I cannot, for the moment, locate that formula. Can anyone
else whilst I search through the various EM tomes here?

Short Antennae
 

"gareth" wrote in message
...
It is sometime since I studied the matter, but there is a formula
that describes the energy assocaited with an antenna, and it
is made up of two parts, for the near field and for the far field.

That part for the far field has the length of the antenna in
terms of fractions of a wavelength.

That part for the near field shows the local stored energy
which is indicative of the antenna system being an oscilatory
store of energy which must dissipate as heat if it is not
reflected back down the feeder.

I cannot, for the moment, locate that formula. Can anyone
else whilst I search through the various EM tomes here?

PS. Perhaps Reay with his claimed technical superiority over me
would be so good as to quote it?

Short Antennae
 

On 10/10/2014 04:44 AM, gareth wrote:
It is sometime since I studied the matter, but there is a formula
that describes the energy assocaited with an antenna, and it
is made up of two parts, for the near field and for the far field.

That part for the far field has the length of the antenna in
terms of fractions of a wavelength.

That part for the near field shows the local stored energy
which is indicative of the antenna system being an oscilatory
store of energy which must dissipate as heat if it is not
reflected back down the feeder.

I cannot, for the moment, locate that formula. Can anyone
else whilst I search through the various EM tomes here?




Hello, and are you looking for the formula for the E & H fields for a
small dipole or a small loop or some other antenna? "Bible" antenna
reference books such as Kraus or Jasik provide these formulas. For
example for a small loop (magnetic dipole) we have for the radiated
(far) E and H fields:

E (phi) = (120 * pi^2 *N *A * I * sin(theta))/(r * lambda^2)

H (theta) = (pi * N * A * I * sin(theta))/r * lambda^2)

where

r is the distance from antenna, I is the uniform loop current, N
is the number of loop turns, A is the loop area and lambda is the wavelength

The small loop is assumed to lie in the x-y plane of an x-y-z orthogonal
Cartesian coordinate system where theta and phi are the angles measured
from the z-axis and x-axis, respectively.

Be advised that the above E and H formulas only apply in the far field;
the general expressions for E and H regardless of the distance from the
antenna are somewhat more complicated. Hope this helps and 73s from N4GGO,

--
J. B. Wood e-mail:

Short Antennae
 

"J.B. Wood" wrote in
:

The small loop is assumed to lie in the x-y plane of an x-y-z orthogonal
Cartesian coordinate system where theta and phi are the angles measured
from the z-axis and x-axis, respectively.


Just two axes? I've been asking on and off for advice on something... I was
shown a nice design for a loop a few weeks ago, it was a helix, so in 3 axes.
What advantage if any does that give over a loop in only two axes? As a guess
I'll suggest it offers finer directional selectivity, but I'm hoping someone
who really knows about this can tell me.

Short Antennae
 

On 10/10/2014 10:04 AM, Lostgallifreyan wrote:
"J.B. Wood" wrote in
:

The small loop is assumed to lie in the x-y plane of an x-y-z orthogonal
Cartesian coordinate system where theta and phi are the angles measured
from the z-axis and x-axis, respectively.


Just two axes? I've been asking on and off for advice on something... I was
shown a nice design for a loop a few weeks ago, it was a helix, so in 3 axes.
What advantage if any does that give over a loop in only two axes? As a guess
I'll suggest it offers finer directional selectivity, but I'm hoping someone
who really knows about this can tell me.

Hello, and the small loop of uniform current is assumed to be lying flat
in the x-y plane. Textbooks do this to facilitate/simplify calculation;
the loop could be oriented in any direction (which is what would happen
if we rotate the original xyz orthogonal axes to some new xyz axes but
keep the loop stationary).

The E and H far-field formulas I previously provided give magnitude and
direction of these vectors using spherical coordinates (r, theta, phi)
relative to a Cartesian system. If you don't understand coordinate
systems and vectors then I can see why you might be confused.

Oh yeah, and one other thing that often escapes folks: You can't radiate
either an E or H field by itself. The other component is always
present. That's why we talk about the propagation of electromagnetic
waves/photons. Sincerely,

--
J. B. Wood e-mail:

Short Antennae
 

"gareth" wrote in message
...
"gareth" wrote in message
...
It is sometime since I studied the matter, but there is a formula
that describes the energy assocaited with an antenna, and it
is made up of two parts, for the near field and for the far field.
That part for the far field has the length of the antenna in
terms of fractions of a wavelength.
That part for the near field shows the local stored energy
which is indicative of the antenna system being an oscilatory
store of energy which must dissipate as heat if it is not
reflected back down the feeder.
I cannot, for the moment, locate that formula. Can anyone
else whilst I search through the various EM tomes here?

PS. Perhaps Reay with his claimed technical superiority over me
would be so good as to quote it?

Hullo?

Brian?

Short Antennae
 

"J.B. Wood" wrote in
:

The E and H far-field formulas I previously provided give magnitude and
direction of these vectors using spherical coordinates (r, theta, phi)
relative to a Cartesian system. If you don't understand coordinate
systems and vectors then I can see why you might be confused.


I'll admit to that. :) All I knew was that this loop I saw was a helix,
pointed so the axis of the 'cylinder' would point to the stantion wanted, and
was laid with an open coil on a frame rather than wound like wire on a pully
or other former that would bunch the windings as close as possible. I
imagined that if ether form were pointed not-so-accurately at the right spot,
the helical form might 'blur' the response, weakening it, allowing it to be
more selective of something it WAS accurately pointed at. Other than this, I
don't know why it would be built with this extra spacing per turn, because it
limits easy portability. I assume there is a good reason, I just don't know
it..

Short Antennae
 

On Friday, October 10, 2014 3:06:36 PM UTC-5, Lostgallifreyan wrote:
"J.B. Wood" wrote in

:



The E and H far-field formulas I previously provided give magnitude and

direction of these vectors using spherical coordinates (r, theta, phi)

relative to a Cartesian system. If you don't understand coordinate

systems and vectors then I can see why you might be confused.





I'll admit to that. :) All I knew was that this loop I saw was a helix,

pointed so the axis of the 'cylinder' would point to the stantion wanted, and

was laid with an open coil on a frame rather than wound like wire on a pully

or other former that would bunch the windings as close as possible. I

imagined that if ether form were pointed not-so-accurately at the right spot,

the helical form might 'blur' the response, weakening it, allowing it to be

more selective of something it WAS accurately pointed at. Other than this, I

don't know why it would be built with this extra spacing per turn, because it

limits easy portability. I assume there is a good reason, I just don't know

it..

I don't think it's really too critical. Solenoid or pancake wound, I
doubt you would notice enough difference to worry about.
Also the wire spacing is not very critical either. Some I've built
with insulated wire, I had the wire tightly wound with no real
spacing.
The insulation adds some spacing, and the wires can't short against
each other due to it.
If I use uninsulated wire, I'll usually have a bit of spacing to
make sure the wires won't touch due to movement, etc. I wind those
pretty taut, so it's not a problem as long as you have a slight space
between the wires even with the bigger loops.
Anyway, do it whatever way you want.. There won't be enough difference
to worry about as long as it's tuned.

Short Antennae
 

On Friday, October 10, 2014 3:06:36 PM UTC-5, Lostgallifreyan wrote:
I

imagined that if ether form were pointed not-so-accurately at the right spot,

the helical form might 'blur' the response, weakening it, allowing it to be

more selective of something it WAS accurately pointed at.

Lack of balance is what will cause problems. It can skew the pattern
a bit, and the nulls won't be as deep. But how you wind the coil
won't effect balance too much as long as everything is symmetrical
and no feedline issues.

Short Antennae
 

Short Antennae