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.

What I HAVE said - and often - is that if you measure the load impedance
presented by an antenna and feedline at the output socket of the
transmitter, and replace it by the same impedance made from lumped R and
L/C components, then the steady-state operating conditions of the
transmitter will not change.

If the transmitter isn't touched, it will deliver exactly the same power
as before - because that happens to be how much power it can deliver
into that particular load impedance.

That's all the RF power there is. In a lossless system, all of that
power will be radiated from the antenna. With the alternative lumped
load, exactly the same power will be delivered into the resistive part
of the load, and dissipated as heat.

Of course the transmitter is under more stress from voltage, current and
heat when it's operating into an incorrect load impedance (not what it
was designed for) but that's all it is. There is no need to invent
reflected power that is being "dumped" back into the transmitter to
cause this stress.

There is also a strong tendency to invent virtual instruments such as
"directional wattmeters" which do not actually exist. An instrument such
as the Bird 43 is calibrated in watts, but it doesn't actually sample
power. As Owen relates (and so have I) these instrument only sample V
and I on the line - they categorically DO NOT sample power.

The power scale is only a meter calibration - literally, only ink on a
meter scale. It indicates the amount of power delivered into a matched
load, when the "reverse" reading is exactly zero [1]. The instrument was
calibrated under those specific conditions, and the "forward power"
reading is only physically meaningful for that specific case.

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.

Everything that's happening inside the instrument can be completely
explained from the new V and I conditions on the line. Waves of power
simply don't come into it. Most people don't want to go that deep into
the theory... but, regrettably, that may be the only way to understand
that the "power" readings on the meter scale are no longer valid under
these conditions.

What IS physically meaningful is the DIFFERENCE between the forward and
reverse "power" readings. That difference will equal the net power
delivered to the load [1]. But those two readings are only meaningful as
a pair - individually they are only "intermediate results" with no
physical meaning of their own.


[1] Ignoring real-life meter errors such as directivity and scale
accuracy.


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

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


--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek