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

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

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.

On Jun 9, 1:06*am, Camelot wrote:
Hello, I have some doubts about standing waves on antennas that I hope
you could clarify!

First of all, one needs to get one's nomenclature correct. Antennas
like single-wire dipoles and full-wave loops are *STANDING WAVE
ANTENNAS*.

On Jun 9, 1:06*am, Camelot wrote:
Hello, I have some doubts about standing waves on antennas that I hope
you could clarify!

Please forgive the unfinished posting above. It happened without any
action on my part.

First of all, one needs to get one's nomenclature correct. Antennas
like single-wire dipoles and full-wave loops are Standing Wave
Antennas. Standing Wave Antennas always have standing waves on the
antenna. They may or may not have standing waves on the transmission
line.

The 50-75 ohm feedpoint impedance of a 1/2WL dipole is a virtual
impedance, the result of forward and reflected waves superposing into
standing waves on the Standing Wave Antenna. The SWR on the 1/2WL
antenna wire is approximately 20:1. Only about 20% of the total energy
in the RF waves on a 1/2WL dipole is radiated. If 100 watts is being
delivered to the antenna and ~100 watts is being radiated, there is
about 500 watts of joules/second stored on the antenna wire during
steady-state.

Maximum power transfer to the antenna occurs when the antenna *system*
is matched but it is not an impedance match. It is instead a
*conjugate match*. A system with an SWR of 10:1 on the ladder-line may
(or may not) be a near conjugate match. If it is a near conjugate
match, maximum *available* power will be delivered to the antenna.

To summarize: Standing Wave Antennas, like 1/2WL single-wire dipoles,
*require* standing waves on the antenna wire.

A flat system, e.g. 50 ohm coax to a 50 ohm antenna, is an impedance
match but it is also a (trivial) conjugate match. In a low-loss
system, a Z0-match guarantees a near conjugate match at the antenna
feedpoint.

A low-loss system with tuned feeders is a (conjugately matched) system
if the impedance looking one direction in the feedline is very close
to the conjugate of the impedance looking in the other direction.

When a Z0-match to 50 ohms is achieved by a tuner (or other network) a
near conjugate match is achieved in a low-loss system. In general, all
low-loss systems that are transferring the maximum amount of available
energy are matched, by definition, i.e. they are a near conjugate
match.

The reason that I say "near" conjugate match is that the conjugate
matching theorem doesn't allow resistive losses in the analysis. Of
course, there is no such thing as a lossless system in reality.
--
73, Cecil, w5dxp.com

'Standing Waves' are not always a 'dirty' word. It just depends on
where they are and why.
- 'Doc

On Jun 9, 7:42*am, "'Doc" wrote:
'Standing Waves' are not always a 'dirty' word. *It just depends on
where they are and why.

Exactly. Standing waves are often our friend. When one feeds a 1/2WL
dipole with 1/2WL of ladder-line on 80m, one is depending upon the
standing waves to transform the impedance looking into the feedline to
the same value as it is at the antenna feedpoint. Without the standing
waves on the 300 ohm matching section, a G5RV would not work. The
transforming characteristics of a 1/4WL matching section depends upon
standing waves. The proper functioning of stubs depends upon standing
waves.
--
73, Cecil, w5dxp.com

On 6/9/2011 4:50 AM, Cecil Moore wrote:

...
Please forgive the unfinished posting above. It happened without any
action on my part.
...
--
73, Cecil, w5dxp.com

Cecil:

Don't even ask such a silly thing, happens all the time, here.

I just highlight, and then right click the post, of mine, which is in
error. If using thunderbird (and I think you are), a menu will pop up,
look near the bottom and find "Cancel Message", choosing that will
create a popup which will ask you to confirm your wish to delete the
message ...

Good luck ...

--

Regards,
JS
“The Constitution is not an instrument for the government to restrain
the people, it’s an instrument for the people to restrain the
government.” -- Patrick Henry

"Camelot" napisal w wiadomosci
...
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.

"VWWR=1 means no doubled voltage.

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?

Standing wave is if the electrons reflects from the end.

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?

You simply do not know Tesla's discovery. Transmitting antennas are the
electrons guns.

In poor antenna the voltage is doubled (at ends) and you have the standing
waves.
In perfect antenna electrons jump off from the ends of a dipole and the
voltage is not doubled.
Of course Tesla's antenna radiate the alternate electric field.
S*

On Jun 9, 10:30*am, John Smith wrote:
I just highlight, and then right click the post, of mine, which is in
error. If using thunderbird (and I think you are), a menu will pop up,

I was previously using Thunderbird but AT&T dropped their news-server
function and I am now using Google Groups which apparently doesn't
allow a posting to be canceled.
--
73, Cecil, w5dxp.com