Dr. Slick wrote:

Roy Lewallen wrote in message ...

So, in theory, if you have achieved a perfect match with your

antenna, you will have matched the impedance to the 377 Ohms of free

space, you will not have reflections at the matching point, and the

energy will radiate in whatever pattern you have designed for.

No, an antenna doesn't "match" the impedance of free space. The input

impedance of an antenna is the ratio of V to I. The impedance of free

space is the ratio of the E field to the H field of a plane wave. They

both happen to have units of ohms, but they're different things and

there's no "matching" going on. If you apply 100 watts to an antenna,

resonant or not, 100 watts will be radiated, less loss, regardless of

the antenna's input impedance.

I disagree on this point. You can think of an antenna as a type

of tranformer, from 50 Ohms to 377 Ohms of free space. A tuned

antenna is "matching" 50 to 377 ohms, and will therefore have no

reflections, ideally.
Thinking of it doesn't make it so. All the energy applied to the antenna

is radiated, less loss, regardless of the antenna's feedpoint impedance.

How does that fit into your model.

Have you heard of something called mis-match loss? If the

antenna's input impedance is not matched to the tranmission line (or

the final PA), then the radiated power will be significantly less than

100 watts.

I have indeed, and have posted several times about this

often-misunderstood and misused term. You can find the postings by going

to

http://www.groups.google.com and searching this group for postings by

me containing "mismatch loss".

The funny thing about this, is that you cannot say that the 50

Ohms in the center of the chart is a "resistive" 50 Ohms, as there is

very little real resistance in the average antenna.

Sure you can. You're confusing resistance with resistors. Resistance is

a dimension, like length. There are lots of things which have dimensions

of resistance but aren't resistors, like transresistance, characteristic

resistance of a transmission line, or radiation resistance to name just

a few. People who have a shaky understanding of basic electric circuit

theory seem to have trouble dealing with this, but it becomes easier to

deal with as you learn more about basic electricity. A mechanical

example is torque and work, which have the same dimensions (force times

distance) but are definitely different things.

ok, so perhaps the way to think of it is: when an antenna is

matched, the I and V curves
what curves?

will be in phase (no reactance), and the

product of I*V (integrated) will be the power transmitted.
The average power radiated is always the real part of V*I(conjugate). If

V and I are in phase, this is simply equal to V*I. But what does this

have to do with the confusion between a resistor and resistance?

This "resistive"

50 Ohms is really what people call the "radiation" resistance, which

is something of a misnomer again, because this is trying to equate the

successful impedance matching and subsequent non-reflected EM

radiation with a truly real resistance like an ideal dummy load.

Sorry, that doesn't make a whole lot of sense. Yes, it's called the

radiation resistance, but it's not a misnomer at all. (I suppose it

would be if you called it a "radiation resistor", but nobody I know of

has ever called it that.) If you calculate the power "consumed" by this

resistance (that is, the power flow into it), it's the power being

radiated. If the radiation and loss resistance of an antenna were zero,

no energy would flow into it and consequently none would be radiated.

(If the radiation resistance was zero and the loss resistance wasn't,

then energy would flow into the antenna but none would be radiated -- it

would all be dissipated as heat.)

If both the radiation and loss resistance were zero, i would

expect an ideal short, and therefore full -180 degree reflections.
You're assuming that the antenna is fed with a transmission line, but

that's ok.

I suppose what this all means is that if you have a matched

antenna, it's V and I curves
what curves?

will be IN PHASE and will have the exact

same RMS values as if you had a truly resistive dummy load instead.
Yes and no. If you're feeding the antenna with a 450 ohm line, it's

matched to the line only if its impedance is 450 + j0 ohms, so it looks

like a 450 ohm dummy load, not a 50 ohm one. On the other hand, you can

feed a 50 + j0 ohm antenna with a half wavelength of 450 ohm line, and

get a perfect 50 ohm match to a transmitter at the input end of the

line, while running a 9:1 SWR on the the transmission line. Then either

the antenna or the input of the line looks like a 50 ohm dummy load.

Therefore, you can consider the center of the Smith Chart (or the

entire real (non-reactive) impedance line) as a real resistance like

an ideal dummy load.

Do you agree with this statement Roy?
Yes.

Your use of "non-reflected EM radiation" seems to imply that the

radiation from an antenna is somehow bounced back from space if the

antenna feedpoint impedance is reactive. That's one of the rather

bizarre and very wrong conclusions you could draw from the mistaken idea

that the antenna "matched" the characteristic impedance of free space.

I think i'm correct to think of antennas as impedance matching

transformers.

50 Ohms to 377 Ohms.
I like to think of 'em as sort of potato guns, launching RF potato

photons into the aether. But that doesn't make them potato guns.

Feel free to think of them any way you like, as long as you consistently

get the right answer.

Now tell me, why can't you just make some 377 ohm transmission line

(easy to make) open circuited, and dispense with the antenna altogether?

When you figure out the answer to that one, you might begin to see the

error with your mental model.

. . .

I believe i understand the Chart better than you think, Roy,

enough to know that you do know what you are talking about.

I still think it's ok to consider antennas as impedance

tranformers, but you have brought up some very good points.

Ok by me. I'll be waiting for your patented no-antenna 377 ohm feedline.

Roy Lewallen, W7EL