On 9 jun, 08:06, 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

Hello Camelot,

Resonance in the antenna is not required for producing RF radiation.
Horn shaped structures and flared transmission lines are good
radiators over frequency ranges that can be over 1:5 (for example 1
GHz to 5 GHz). These wide band antennas rely on travelling waves in
their conductors. When the wave goes from the feed point towards the
end of the structure, it gradually attenuates because of the lost
energy due to (desired) radiation. The distribution of the current in
the antenna's conductors (amplitude and phase) determines the
radiation pattern.

Antennas made from thin wires show inconvenient impedance, except for
some special lengths. The best-known example is the halve wave center-
fed dipole. It gives a good match to coaxial transmission lines.
Though it has a good match to a 50 Ohm line (so no standing waves in
the transmission line), there are standing waves in the antenna. The
sinusoidal voltage and current distribution in a half wave dipole is
because of the large VSWR inside the antenna itself.

So the halve wave dipole fed from a 50 Ohms line shows high SWR in the
antenna, but low SWR in the feed line.

The full wave dipole is also in resonance (two end-fed halve wave
antennas), but its impedance is in the several hundreds Ohms to kOhms
range (depending in wire thickness). It has somewhat more directivity
then the half wave dipole, but gives large mismatch to coaxial
transmission lines. If you drive the full wave dipole directly from a
coaxial line (and do the matching in the shack), you will very likely
lose most of your transmitting power (in the form of heat in the
coaxial feed line).

The full-wave dipole can be fed from an open wire transmission line
(ladder line) directly. There will be mismatch at the antenna-feeder
transition, but it will be less (ladder line has high char.
Impedance). As Tim said, the loss (or attenuation) of the ladder line
is significantly below that of coaxial transmission lines, so even
high VSWR inside the transmission line will not lead to significant
line loss.

Short antennas (length 0.1 lambda) are good radiators, but their
impedance can be so weird that it is impossible to match them to a
convenient impedance without excessive loss. The losses are in the
matching networks, not in the radiating structure itself.

So generally spoken, resonance (that is ohmic antenna behavior) is not
required for an antenna to radiate.



Regarding transmission lines.
Zload = Zcable, is not required for long lines, but gives the lowest
loss. Depending on your requirements, you may accept some mismatch
between load and line (so standing waves in the line and somewhat
increased line loss). Ladder line (parallel wire transmission line)
can be used with significant SWR inside the line with relative low
loss. The same SWR in a coaxial transmission line with same length
may result in too much loss.

Transmission sections are used in matching networks. Well-known
example: quarter wave transformer. It does function because of a
standing wave part inside the quarter wave line.

Regarding the PA.
The PA just wants to see a certain load impedance to operate
properly. With "properly" I mean delivering the stated power without
too much stress on the PA's components.

With kind regards,


Wim
PA3DJS
www.tetech.nl
Please tell your racing pigeon to remove abc in case of PM.