On Jun 8, 11:06*pm, Camelot wrote:
Hello, I have some doubts about standing waves on antennas that I hope
you could clarify!
As far as I understood, in a generic transmission line where we want
only carry power from a source to a load, we need to cancel the
reflected wave by adapting the load with the impedance of the line.
The result of this operation is S11=S22=0 and VWWR=1 that means no
standing waves.
As far as I understood, in an antenna we want to also avoid standing
waves by having *VWWR=1 in order to avoid overloading problem to the
power stage...
From theory I know that the best radiating condition for an antenna is
when it resonates, that is, *when there is a standing wave… is that
correct? How this condition is compatible with a *VWWR=1 (no standing
waves) for a good antenna matching? Is there something that I’m not
catching?

Regards,

Camelot

I don't know that I'd say that "the best radiating condition for an
antenna is when it resonates," necessarily. But certainly a resonant
antenna is one that will have standing waves on it when it's excited.

However, the standing waves on a resonant antenna result in radiation
by the antenna, and the result of the standing waves, the radiation,
and the resonance is that the feedpoint of the antenna looks like a
resistance that absorbs power. The exact resistance depends on many
things, but in general it's a resistance that either directly provides
a good match to a transmission line, or else can be matched to a
transmission line with a few components.

By the way, it's also not necessary to have the antenna matched to the
transmission line: although a particular line with standing waves
delivering a certain amount of power to a load has more loss than the
same line delivering power to a matched load (no standing waves), the
additional loss from the standing waves may not be a big issue. In
fact, it may be better from a lost-power standpoint to use a higher
impedance open-wire line with relatively high standing waves, than to
use a low impedance line (e.g., coax) with low standing waves, because
the loss in the open-wire line can be very low. There are lots of
other factors to consider, too, but the point is that you shouldn't
assume that it's necessary to get the lowest possible SWR on the
transmission line.

Let's take one more step back from this problem. The antenna, whether
resonant or not, presents some impedance at its feedpoint. The
purpose of the transmission line is to couple power between a
transmitter and the antenna feedpoint (or from the antenna feedpoint
to a receiver). You don't need to know what's actually going on in
the antenna, you only need to know that feedpoint impedance, to select
the proper transmission line and possibly a matching network between
the line and the antenna's feedpoint. (Well...you also need to be
aware of how the metallic conductors of the transmission line may
upset the fields around the antenna and possibly provide for
decoupling the feedline from the antenna's fields, but that's separate
from the issue of coupling power between the antenna and the
transmitter/receiver.)

Keep the details of how the antenna does its job--how the current is
distributed on the antenna's conductors--separate from how you get
power to and from the antenna, and you should be OK.

Cheers,
Tom