On 31 mar, 13:57, "AndyS" wrote:
{text deleted}
Andy writes:

Thanks to all for the discussion....... I have used the results of
these
calculations for many years but wanted to see what explanations I
would
get from others who also have experience in these matters.....

For capture area, I use " Gee lambda squared over four pi ",
which
is the standard definition for a well behaved antenna with a main
lobe, and it has always worked well for me. I don't remember doing
the
derivation for this, but I'm sure I must have done it in the past
(olden times)...

Yet, I was not comfortable with what seemed like a discrepancy
between
"all the extracted power goes to the load" and the proposition that
the
cosmos and antenna acted like a generator and therefore had an
internal
impedance which must be Thevenin matched for max extracted power....
I resolved it, in my mind, by ignoring the latter explanation
(grin)..... and
apparently it was the correct thing to do...

So, thanks again, guys........

Andy W4OAH

Hi Andy,

As you mentioned, Thevenin does apply, and your effective area formula
is correct (gain w.r.t. isotropic radiator). So a thin halve wave
dipole antenna has an internal resistance of about 70 Ohms, and half
the power is lost, but not as heat in the antenna. The driving
(incident) EM-field induces voltage with causes a current to flow in
your 75 Ohms load (and in the antenna).

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. As your dipole delivers 1mW to the load, also 1mW is
reradiated into space. Behind the antenna, the reradiated field
cancels (partially) the incident field (causing a shadow effect).

When you remove the load, the output voltage at the antenna doubles
and the current in the dipole reduces to a low value, hence less power
is reradiated. When you short circuit the dipole, the current will
double (w.r.t. to the matched situation). This will also happen to
the current in the bars forming the dipole. As the current has been
doubled, the reradiated power is 4 times higher (w.r.t. the matched
situation).

The reradiated power can be detected. Radar and passive UHF RFID use
the reradiated power. For example in UHF RFID, the chip on the RF tag
changes the load to the dipole (that receives power from the
interrogator), and controls the reradiated power. In this way the ID
of the tag is transmitted from the tag to the interrogator (the tag
has no battery present).

The more gain you have in your antenna, the higher the reradiated EIRP
in the direction of maximum gain.

I hope this clarifies the antenna internal resistance.

Best Regards,

Wim
PA3DJS