Antenna Theory Question
 

- an ideal electrical engine (no ohmic loss, no friction) fully
transforming the applied electrical power into mechanical power. It looks
like a resistor, but no heat is generated anywhere

No, it looks like a resistance, not a resistor. There is one
IEEE definition for resistor. There are two IEEE definitions
for resistance. A resistor with current dissipates power. A
resistance may or may not dissipate power. One definition of
resistance in the IEEE describes a dissipationless resistance.
There is no such thing as a dissipationless resistor.

It IS a resistance, but it LOOKS like a resistor because, from the exterior, one
has no means to determine whether energy gets dissipated or transformed in some
other form.

73

Tony I0JX

Antenna Theory Question
 

Antonio Vernucci wrote:
It IS a resistance, but it LOOKS like a resistor because, from the
exterior, one has no means to determine whether energy gets dissipated
or transformed in some other form.

Appearances can be deceiving. It doesn't look
like a resistor to me until I hold it in my hand.
--
73, Cecil http://www.w5dxp.com

Antenna Theory Question
 

Cecil Moore wrote:
There is no such thing as a dissipationless resistor.
--
73, Cecil http://www.w5dxp.com

Andy comments:

Not to make too fine a point on this, Cec, but I , personally,
would consider a perfect 52 ohm antenna to be a dissipationless
resistor......... although, in the cosmic sense, the power that is
fed into it will eventually reside as "heat" in the bowels of the
cosmos.....somewhere......

Actually, .... "heat" is simply EM at a different frequency....
..... Hmmmmm... I see a deep, philosophical discussion brewing
here..... ..

... Me brain hoits......!!!!

Andy W4OAH

Antenna Theory Question
 

On 1 Apr 2007 09:16:46 -0700, "AndyS" wrote:

I see a deep, philosophical discussion brewing
here....

Only if it starts out with:
"Who's on first?"

Antenna Theory Question
 

AndyS wrote:
Not to make too fine a point on this, Cec, but I , personally,
would consider a perfect 52 ohm antenna to be a dissipationless
resistor......... although, in the cosmic sense, the power that is
fed into it will eventually reside as "heat" in the bowels of the
cosmos.....somewhere......

Yep, a common misconception. The feedpoint impedance
of a standing-wave antenna, like a 1/2WL dipole, is
a *virtual* impedance equal to (Vfor+Vref)/(Ifor+Iref).
It may be a resistance, but it is not a resistor. It
is essentially the same entity as the impedance looking
into a stub. For a resonant 1/2WL dipole the equation
becomes:

Feedpoint resistance = (|Vfor|-|Vref|)/(|Ifor|+|Iref|)

With Vfor at a reference angle of zero, Vref is at
180 degrees. With Ifor at zero degrees, Iref is
also at zero degrees. Thus the ability to add and
subtract magnitudes directly.

Some guru once challenged me to make a measurement
at the shack and tell the difference between a 50 ohm
dipole and a 50 ohm resistor. Told him all I needed
was a field strength meter. :-)

The words "impedor" and "resistor" are reserved for
real physical devices. The words "impedance" and
"resistance" have two meanings and can mean either
real devices or virtual V/I's. So says the IEEE
Dictionary.
--
73, Cecil http://www.w5dxp.com

Antenna Theory Question
 

On Sun, 01 Apr 2007 18:24:33 GMT, Cecil Moore wrote:

AndyS wrote:
Not to make too fine a point on this, Cec, but I , personally,
would consider a perfect 52 ohm antenna to be a dissipationless
resistor......... although, in the cosmic sense, the power that is
fed into it will eventually reside as "heat" in the bowels of the
cosmos.....somewhere......

Yep, a common misconception. The feedpoint impedance
of a standing-wave antenna, like a 1/2WL dipole, is
a *virtual* impedance equal to (Vfor+Vref)/(Ifor+Iref).
It may be a resistance, but it is not a resistor. It
is essentially the same entity as the impedance looking
into a stub. For a resonant 1/2WL dipole the equation
becomes:

Feedpoint resistance = (|Vfor|-|Vref|)/(|Ifor|+|Iref|)

With Vfor at a reference angle of zero, Vref is at
180 degrees. With Ifor at zero degrees, Iref is
also at zero degrees. Thus the ability to add and
subtract magnitudes directly.

Some guru once challenged me to make a measurement
at the shack and tell the difference between a 50 ohm
dipole and a 50 ohm resistor. Told him all I needed
was a field strength meter. :-)

The words "impedor" and "resistor" are reserved for
real physical devices. The words "impedance" and
"resistance" have two meanings and can mean either
real devices or virtual V/I's. So says the IEEE
Dictionary.


Amen to that, Cecil

Walt, W2DU

Antenna Theory Question
 

The current that flows into the antenna structure (two bars) radiate.
So part of the incident field is reradiated by the dipole. It doesn't
matter whether the antenna current is caused by a transmitter or
incident field.

Win,

your explanation is very good and convincing. I believe that most of us would
read with interest some further remarks on what happens in the case of a shorted
element (e.g. a director or a reflector), instead of the driven element.

In those cases all received energy must be re-radiated, but more details on that
mechanism would be welcome.

73

Tony I0JX

Antenna Theory Question
 

Antonio Vernucci wrote:
Win,

your explanation is very good and convincing. I believe that most of us
would read with interest some further remarks on what happens in the
case of a shorted element (e.g. a director or a reflector), instead of
the driven element.

In those cases all received energy must be re-radiated, but more details
on that mechanism would be welcome.

The impinging field induces a current on the element. The current
produces a field which radiates. An AC current flowing on a conductor
creates a field regardless of how that current is produced. It doesn't
matter whether the current is due to conduction from a source, by
induction from a field, or any combination of the two -- the field
resulting from the current is exactly the same. Yagis and other
"parasitic" arrays depend entirely on this phenomenon for their operation.

Roy Lewallen, W7EL

Antenna Theory Question
 

In article , Roy Lewallen
wrote:

Antonio Vernucci wrote:
Win,

your explanation is very good and convincing. I believe that most of us
would read with interest some further remarks on what happens in the
case of a shorted element (e.g. a director or a reflector), instead of
the driven element.

In those cases all received energy must be re-radiated, but more details
on that mechanism would be welcome.

The impinging field induces a current on the element. The current
produces a field which radiates. An AC current flowing on a conductor
creates a field regardless of how that current is produced. It doesn't
matter whether the current is due to conduction from a source, by
induction from a field, or any combination of the two -- the field
resulting from the current is exactly the same. Yagis and other
"parasitic" arrays depend entirely on this phenomenon for their operation.

Roy Lewallen, W7EL

Yes, and with the corollary that you can never extract more than half of
the power/energy transported by an EM wave. That maximum (available
power) is given by Voc^2/(4 * Ra) where Voc and Ra are the open circuit
voltage magnitude and real part of the extractor (receive antenna)
impedance, respectively, measured at the antenna terminals at the
frequency of interest. The remaining energy is scattered (reradiated)
from the antenna. Sincerely, and 73s from N4GGO,

John Wood (Code 5550) e-mail:
Naval Research Laboratory
4555 Overlook Avenue, SW
Washington, DC 20375-5337

Antenna Theory Question
 

Roy Lewallen wrote:

The impinging field induces a current on the element. The current
produces a field which radiates. An AC current flowing on a conductor
creates a field regardless of how that current is produced. It doesn't
matter whether the current is due to conduction from a source, by
induction from a field, or any combination of the two -- the field
resulting from the current is exactly the same. Yagis and other
"parasitic" arrays depend entirely on this phenomenon for their operation.

Roy Lewallen, W7EL

Andy asks:
Roy, is the field radiated by the parasitic element shifted in phase
relative to the incident field. If not, what determines the phase
shift ?

Andy W4OAH