On Feb 27, 3:10 am, Ian White GM3SEK wrote:
Cecil Moore wrote:
My favorite quotation by an antenna guru on this
newsgroup is that "a 50 ohm antenna can be replaced
by a 50 ohm resistor without changing anything".
If that were true, we don't need antennas. :-)

That sounds like a direct misquotation of me.

Nope, it wasn't you, Ian. You are usually more careful than that.

For a mismatched load, the meter will read higher in the forward
direction than in the reverse - but that is purely a feature of the
instrument. It all looks so plausible on the meter scale, but those are
not genuine power waves flowing in opposite directions.

But they are genuine energy waves flowing in opposite directions.
Standing waves require two coherent energy waves flowing in opposite
directions. Can you explain how to create a standing wave without
two energy waves flowing in opposite directions? And remember, the
two EM wave components in the standing wave cannot stand still.

There's truth in what you say, Ian, but it is not the whole truth.
There's
no difference except frequency (and all that frequency implies)
between
an RF electromagnetic wave and a visible light electromagnetic wave.
In fact, RF waves are covered in many physics textbooks whose subject
is light. There is a wealth of information available from the field of
optics
that is applicable to RF waves.

Visible light physicists don't have the luxury of measuring voltage or
directly measuring phase. They have to rely on a power measurement
of irradiance. As a result, visible light measurements are actually
power measurements so we indeed do know how EM waves behave
at the joules/second level.

Visible light physicists found that when they superpose two coherent
light waves, Ptotal = P1 + P2 + 2*SQRT(P1*P2)cos(A)
where 'A' is the angle between the electric fields of the two waves.
That exact same equation applies to coherent RF waves. Phasor
addition is used for the superposition of two coherent RF voltages.
The power equation is used to find out what happens to the power
during that voltage superposition. P1 = V1^2*Z0 and P2 = V2^2*Z0
The last term in the power equation is known as the interference term
and is either constructive, destructive, or zero.

Since antenna radiation patterns depend upon constructive and
destructive interference of EM waves in space, we hams could
learn a lot from the field of physics known as optics.

I'm going to be away from my computer for 48 hours.

But you'll be back... :-)

Yep, I'm posting from my sister's computer through my Google account.
--
73, Cecil, w5dxp.com