On 6 Mar, 19:44, Fred McKenzie wrote:
I think the easiest way to see this is by looking at the transmitted
power having a value at the receive antenna, of so many Watts per square
meter power density. *The receive antenna presents an "effective" area
of so many square meters. Its value depends on things like frequency,
antenna size, gain and direction.

By using the word "effective", do you suggest that the word area
should not be interpreted in it's literal sense of the size-property
of a surface in physical space? So it is not simply a matter of taking
the product of a height and a width (of a e.g. a patch antenna)? I
understand that the direction is a factor to deal with; that's
obvious. I also understand there might be losses if the frequency is
different than what the antenna designer had in mind. But I don't
understand how the gain makes any difference when receiving. Here I
use gain in the sense of transmission gain (radiation intensity in the
lobe compared to an ideal isotropic antenna).

Multiply transmitter power density at the receive antenna, by the
receive antenna's effective area to get received power.

I would totally agree unless I was confused about the notion of
"effective area". I guess my mental picture of a real physical surface
exposed to incident radiation is naïve and stops me from
understanding, but I simply don't know what image to replace it with.

But how do you determine your antenna's effective area? *That isn't so
easy! *I understand a half wave dipole has an effective area of about
one half wavelength times one quarter wavelength in the direction of the
antenna's major lobes. *

I assume by definition the height of a half wave dipole antenna is one
half wavelength. But what about the width? Why one quarter wavelength?

On 40 Meters, that would be 200 square meters.

This I don't understand. Again, it seems to me to be something that
would only be meaningful when transmitting, not when receiving.

Once all the factors are known, you can reduce the calculations to
Decibels.

Yeah, I look forward to that day!! Thanks for your reply,

Regards,
Daniel