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

Something else to think about that may help you understand:

Realize that if you connect a section of transmission line with one
impedance to a section with a different impedance, you will in general
have very different standing waves on the two sections. So, for
example, if you connect the output of a transmitter to a 50 ohm line
that goes for some distance, and that connects to a 250 ohm line
that's an even number of quarter waves long, and at the end of that
line there's a 50 ohm load, assuming the 250 ohm line is lossless, the
50 ohm line will see a 50 ohm load and have no standing wave. But the
250 ohm line has a 5:1 SWR.

Similarly, though the antenna conductors may have a large standing
wave on them, they may reflect a load to their input, their feedpoint,
that allows a low SWR on the feedline connected to that feedpoint.

You might think of the antenna as a "lossy transmission line," where
its loss is (mostly) radiation. You can then see another example
using just transmission lines...this time the 50 ohm line from the
transmitter connects to a piece of 75 ohm line. The 75 ohm line is
some even number of half-waves long, and it happens to have 7dB of
loss -- maybe it's fairly small line, or is several wavelengths long.
(In this case, the loss is as heating in the conductors of the line,
not radiation as in the antenna.) At the far end, it's shorted, so
the SWR at the short is clearly infinite. But at the end connected to
the 50 ohm line, because of the loss in the 75 ohm line, the 50 ohm
line sees a 50 ohm load, and the SWR on the 50 ohm line is 1:1. Note
that along the 75 ohm line, the SWR went from infinity at the shorted
far end, to 1.5:1 where the line connects to the 50 ohm line.

Cheers,
Tom

On 6/9/2011 4:13 PM, K7ITM wrote:
On Jun 8, 11:06 pm, 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

Something else to think about that may help you understand:

Realize that if you connect a section of transmission line with one
impedance to a section with a different impedance, you will in general
have very different standing waves on the two sections. So, for
example, if you connect the output of a transmitter to a 50 ohm line
that goes for some distance, and that connects to a 250 ohm line
that's an even number of quarter waves long, and at the end of that
line there's a 50 ohm load, assuming the 250 ohm line is lossless, the
50 ohm line will see a 50 ohm load and have no standing wave. But the
250 ohm line has a 5:1 SWR.

Similarly, though the antenna conductors may have a large standing
wave on them, they may reflect a load to their input, their feedpoint,
that allows a low SWR on the feedline connected to that feedpoint.

You might think of the antenna as a "lossy transmission line," where
its loss is (mostly) radiation. You can then see another example
using just transmission lines...this time the 50 ohm line from the
transmitter connects to a piece of 75 ohm line. The 75 ohm line is
some even number of half-waves long, and it happens to have 7dB of
loss -- maybe it's fairly small line, or is several wavelengths long.
(In this case, the loss is as heating in the conductors of the line,
not radiation as in the antenna.) At the far end, it's shorted, so
the SWR at the short is clearly infinite. But at the end connected to
the 50 ohm line, because of the loss in the 75 ohm line, the 50 ohm
line sees a 50 ohm load, and the SWR on the 50 ohm line is 1:1. Note
that along the 75 ohm line, the SWR went from infinity at the shorted
far end, to 1.5:1 where the line connects to the 50 ohm line.

Cheers,
Tom

Well done, Tom. Kudos.

73,
John - KD5YI