Standing wave on feeders
 

Standard antennas often have a standing wave on them e.g. dipole or
quarterwave vertical.

Antenna is resonant and presenting good impedance. SWR is 1:1.

If feeder is coax, is there a standing wave on coax?

If feeder is open wire feeder, is there a standing wave on the feeder?

Standing wave on feeders
 

On Thu, 17 May 2007 19:27:55 +0100, "David" nospam@nospam wrote:

Standard antennas often have a standing wave on them e.g. dipole or
quarterwave vertical.

Antenna is resonant and presenting good impedance. SWR is 1:1.

If feeder is coax, is there a standing wave on coax?

If feeder is open wire feeder, is there a standing wave on the feeder?


http://www.w2du.com/r2ch21.pdf

Standing wave on feeders
 

David wrote:
Standard antennas often have a standing wave on them e.g. dipole or
quarterwave vertical.
Antenna is resonant and presenting good impedance. SWR is 1:1.
If feeder is coax, is there a standing wave on coax?

Not if the SWR is 1:1 on the coax.

If feeder is open wire feeder, is there a standing wave on the feeder?

Not if the SWR is 1:1 on the open-wire feeder.
--
73, Cecil, w5dxp.com

Standing wave on feeders
 

On May 17, 11:27 am, "David" nospam@nospam wrote:
Standard antennas often have a standing wave on them e.g. dipole or
quarterwave vertical.

Antenna is resonant and presenting good impedance. SWR is 1:1.
so it is either 36 or 72 ohms

If feeder is coax, is there a standing wave on coax?
50 ohm coax = yes

If feeder is open wire feeder, is there a standing wave on the feeder?
not 50 ohm twinline = yes

Standing wave on feeders
 

The last reply says "twinline = yes" i.e. that there is a standing wave on
the open wire feeder, even though SWR is 1:1.

In the book Antenna Theory and Design by Robert S. Elliott, on page 60 there
is figure 2.2 titled "The dipole as a transmission line that is opened out".
The same figure is figure 1.15 on page 18 of Antenna Theory by Constatine
Balanis.

The figure shows that for a transmission line with the conductors close
together,
the fields cancel. The conductors are opened out into a dipole showing the
fields in phase. The diagram shows the current distribution on the dipole as
approximately sinusoidal. The high current area is at centre of dipole. The
diagram shows the current distribution sinewave on the dipole as also going
down the feeder.

Can someone comment on the reason the current sinewave is going down the
feeder? Perhaps in this example, the antenna is not resonant. I would have
expected a resonant antenna to have a flatline voltage and flatline current
on the feeder with only a standing wave on the dipole.

Is the current sinewave on the feeder a standing wave?

What is reason for standing wave not being on feeder? Do the forward and
reflected waves cancel at dipole centre? Are the waves in phase causing
resonance or a build up of signal, like pushing a swing in phase to get the
swing higher and higher?

Standing wave on feeders
 

"David" nospam@nospam wrote in message
...
The last reply says "twinline = yes" i.e. that there is a standing wave on
the open wire feeder, even though SWR is 1:1.

In the book Antenna Theory and Design by Robert S. Elliott, on page 60
there
is figure 2.2 titled "The dipole as a transmission line that is opened
out".
The same figure is figure 1.15 on page 18 of Antenna Theory by Constatine
Balanis.

The figure shows that for a transmission line with the conductors close
together,
the fields cancel. The conductors are opened out into a dipole showing the
fields in phase. The diagram shows the current distribution on the dipole
as
approximately sinusoidal. The high current area is at centre of dipole.
The
diagram shows the current distribution sinewave on the dipole as also
going
down the feeder.

Can someone comment on the reason the current sinewave is going down the
feeder? Perhaps in this example, the antenna is not resonant. I would have
expected a resonant antenna to have a flatline voltage and flatline
current
on the feeder with only a standing wave on the dipole.

Is the current sinewave on the feeder a standing wave?

What is reason for standing wave not being on feeder? Do the forward and
reflected waves cancel at dipole centre? Are the waves in phase causing
resonance or a build up of signal, like pushing a swing in phase to get
the
swing higher and higher?


David

As far as I am aware, the feeder forms part of the antenna system.
Everything from the antenna socket onwards of a transmitter potentially
forms part of the antenna and is potentially radiating signals. With a
resonant dipole, the signals in each half are in antiphase. With two closely
spaced parallel conductors, the signals in each of the conductors cancel
each other out (at least partially) due to them being of equal but opposite
phases. In the case of coax cable, the signal from one conductor in the
centre of the cable is enclosed by the outer sheath and again the signals
are cancelled out. Theory and measurements indicate that the radio frequency
signals actually propagate alone the surface of the conductors, so the
signal propagates along the outer surface of the central conductor of the
coax cable and along the inside of the screen. Some signal also passes along
the outside of the screen of a coaxial cable, upsetting the theoretically
perfect balance, hence the recommendation to use a balun when using coax, it
helps to stop out of balance currents flowing along the outside of the cable
into the radiating part of an antenna system. This is due to the way wire
coaxial cable is constructed. Waveguides and solid screen coaxial cables do
not normally carry any current or voltage on their outer surfaces.

For conventional wire type antenna systems, it is the high voltage at the
antenna ends that determines how much power is radiated from the antenna. A
perfectly resonant antenna could be represented by a standing wave with high
current and zero voltage at the antenna socket of the transceiver, gradually
transforming through the antenna tuning unit, feeder system and antenna to
zero current and very high voltage at the antenna ends.

From the instant the transmit key is pressed, the waves start to propagate
along the feeder to the antenna and if the antenna is resonant, reflections
from the end of the antenna reinforce following waves in phase and power
builds in the system. After quite a few cycles a steady state is approached.
As transmissions are usually made using several MHz of frequency, the 50 or
100 cycles used to establish a steady state are not of any great
consequence. When transmissions are made at very low frequencies, there is a
need to consider the time taken for radiating antenna systems to build to
resonance, hence the very low data rates used in worldwide submarine
transmissions, etc.

Mike G0ULI

Standing wave on feeders
 

David wrote:

In the book Antenna Theory and Design by Robert S. Elliott, on page 60 there
is figure 2.2 titled "The dipole as a transmission line that is opened out".
The same figure is figure 1.15 on page 18 of Antenna Theory by Constatine
Balanis.

The figure shows that for a transmission line with the conductors close
together,
the fields cancel.

Hi David,

Just a small point. The fields cancel
only at some distance from the
transmission line. If you get close
enough to the wires, the field on each
is easily detected.

Chuck

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Standing wave on feeders
 

There are no twinlines with 72 Ohms
no 1;1
= standing wave
On May 20, 5:10 am, "David" nospam@nospam wrote:
The last reply says "twinline = yes" i.e. that there is a standing wave on
the open wire feeder, even though SWR is 1:1.
Swr cannot be 1;1


In the book Antenna Theory and Design by Robert S. Elliott, on page 60 there
is figure 2.2 titled "The dipole as a transmission line that is opened out".
The same figure is figure 1.15 on page 18 of Antenna Theory by Constatine
Balanis.

The figure shows that for a transmission line with the conductors close
together,
the fields cancel. The conductors are opened out into a dipole showing the
fields in phase. The diagram shows the current distribution on the dipole as
approximately sinusoidal. The high current area is at centre of dipole. The
diagram shows the current distribution sinewave on the dipole as also going
down the feeder.

Can someone comment on the reason the current sinewave is going down the
feeder? Perhaps in this example, the antenna is not resonant. I would have
expected a resonant antenna to have a flatline voltage and flatline current
on the feeder with only a standing wave on the dipole.

Is the current sinewave on the feeder a standing wave?

What is reason for standing wave not being on feeder? Do the forward and
reflected waves cancel at dipole centre? Are the waves in phase causing
resonance or a build up of signal, like pushing a swing in phase to get the
swing higher and higher?

Standing wave on feeders
 

On 20 May 2007 12:49:12 -0700, wrote:

There are no twinlines with 72 Ohms

Wrong. There is twinlead that has the characteristic impedance of 72
ohms.

Standing wave on feeders
 

David wrote:
"Antenna is resonant and presenting good impedance. SWR is 1:1.

If the feeder is coax, is there a standing wave on the coax?

If the feeder is open wire, is there a standing wave on the feeder?"

SWR is 1:1 means the line is flat. The line matches the antenna
regardless of configuration. There is no reflection from the load.
Standing waves are produced by a combination of a reflected wave with
the incident wave. No SWR means no reflectiion if coax, open wire, or
any other feedline configuration is in use.

Best rergards, Richard Harrison, KB5WZI