Antenna reception theory
 

Richard Clark, KB7QHC wrote:
"In land stations the actual effective height is from 50 to 90% of the
measured height."

If the effective height is 50%, volts at the antenna terminals are no
more than 50% numericcally of the volts per meter in the field strength
when all else is optimum. This could account for Reg`s 2 to 1
discrepancy. I wonder what the speculations of Reg and his Icelandic
correspondent are? Do they have a formula to predict effective height?
Does Roy have such a formula? It`s a factor which won`t go away, even
when ignored..

Best regards, Richard Harrison, KB5WZI

Antenna reception theory
 

Roy Lewallen wrote:
But voltage isn't energy. Or power.

True, but voltage cannot exist without energy.
--
73, Cecil http://www.qsl.net/w5dxp

Antenna reception theory
 

Be carefully, that is not exactly true. Voltage cannot be created without
energy, it is a force. However once it is 'in place' it requires no energy to
sustain. (Given the freshman physics caveats of frictionless pullies, massless
ropes, etc.)

That said in the situation at hand of an AC voltage being induced by an EM field
you are correct. The energy required to produce a voltage is a function of the
impedance.

- Dan

Cecil Moore wrote:
Roy Lewallen wrote:

But voltage isn't energy. Or power.


True, but voltage cannot exist without energy.

Antenna reception theory
 

dansawyeror wrote:
Be carefully, that is not exactly true. Voltage cannot be created
without energy, it is a force.

Dan,

Stop now, before you further embarrass yourself.

73,
Gene
W4SZ

Antenna reception theory
 

dansawyeror wrote:
Be careful, that is not exactly true. Voltage cannot be created
without energy, it is a force. However once it is 'in place' it requires
no energy to sustain.

A voltage cannot be sustained without energy. The
joule of energy used to create a voltage on a capacitor
is stored in the capacitor until something changes.
--
73, Cecil http://www.qsl.net/w5dxp

Antenna reception theory
 

On Wed, 21 Dec 2005 07:22:58 -0600, (Richard
Harrison) wrote:
Does Roy have such a formula? It`s a factor which won`t go away, even
when ignored..

Hi Richard,

For the NIST half wave field antenna:

Leff = (wl / pi) · tan (pi · len / 2 · pi)

whe

len = (wl / 2) · [1 - (.2257 / (ln(wl / d) -1))]
d = dipole rod diameter, meters

Otherwise, for very short antennas it appears it would be the area
under the curve of the current distribution for the vertical element.
You will undoubtedly run across the formula above, or a variant, in
your copy of Kraus.

73's
Richard Clark, KB7QHC

Antenna reception theory
 

I am amazed that this keeps going on.

In the early days of radio, the frequencies were low, receivers were not
very sensitive, and the antennas were vertical - with some understandable
exaggeration. Modern circuit theory - in the sense that things can be
calculated - started about 1920.

In those early days, the concept of "height" was useful. For a given
physical height, one was interested in increasing the "height." Top loading
was one major tool.

A simple analysis was made based on the reasonable assumption that the
current distribution along a short (less than 0.1 WL), thin, rod over a good
ground was linearly distributed from a max. at the base to zero at the end.
Such an analysis has been made an uncounted number of times since. I found
a reference that suggests that Sommerfield might have made the calculation
in a paper published in March 1909. The result is always that the assumed
antenna has an open circuit voltage between its bottom and ground of no more
than 0.5 of the incident, vertically-polarized wave's v/m times the
physical length. (An ideal 0.25 WL antenna was expected to have a "height"
of 2/pi.)

No expert has ever said something to contradict the 0.5 figure. What
many have described is a way to visualize E in free space using a one meter
wire. That is not the subject.

Before Professor Kraus' first edition (1950), "height" had lost most of
its utility. Professor Terman in his 1943 edition of -Radio Engineers'
Handbook- gives it a definitional footnote on page 841. Directivity and
gain had been defined and have been useful ever since.

73 Mac N8TT
--
J. Mc Laughlin; Michigan U.S.A.
Home:

Antenna reception theory
 

"Cecil Moore" bravely wrote to "All" (21 Dec 05 15:19:51)
--- on the heady topic of " Antenna reception theory"

CM From: Cecil Moore
CM Xref: core-easynews rec.radio.amateur.antenna:221636


CM Roy Lewallen wrote:
But voltage isn't energy. Or power.

CM True, but voltage cannot exist without energy.

I wouldn't say that. Voltage is kind of like having a big boulder
sitting on top of a high cliff. It doesn't do anything so it doesn't
expend energy. However, if it rolls off that's splat! Similarly you
can have all the electrical potential you want but as long as no
charges flow then where is the energy? In an EM wave it is the energy
itself that flows in space using voltage and magnetism as a skeleton.

A*s*i*m*o*v

Antenna reception theory
 

Asimov wrote:
Voltage is kind of like having a big boulder
sitting on top of a high cliff.

Yep, it is. A big boulder sitting on top of a high cliff
contains a lot of potential energy. Voltage is literally
potential energy and cannot exist without energy.
--
73, Cecil http://www.qsl.net/w5dxp

Antenna reception theory
 

Cecil Moore wrote:

Yep, it is. A big boulder sitting on top of a high cliff
contains a lot of potential energy. Voltage is literally
potential energy and cannot exist without energy.


Cecil,

Still practicing physics without a license? 8-)

Electrical potential energy has units of voltage multiplied by charge.
Voltage by itself is not potential energy, literally or otherwise.

73,
Gene
W4SZ