"gareth" wrote in message
...
Ignoring, for the moment, travelling wave antenna, and restricting
discussion to standing wave antennae ...

A wave is launched, and radiates SOME of the power, and suffers
both I2R losses and dielectric and permeability losses associated
with creating and collapsing the near field.

Of course, it goes without saying that the wave was already travelling up
the feeder
and it diffracts along the elements of the antenna, rather than being
launched from
the feedpoint!

gareth wrote:
"gareth" wrote in message
...
Ignoring, for the moment, travelling wave antenna, and restricting
discussion to standing wave antennae ...

A wave is launched, and radiates SOME of the power, and suffers
both I2R losses and dielectric and permeability losses associated
with creating and collapsing the near field.

Of course, it goes without saying that the wave was already travelling up
the feeder
and it diffracts along the elements of the antenna, rather than being
launched from
the feedpoint!

Nope; there is an electric field in a feed line (other than wave guide)
but no electromagnetic field.

As a problem for the student, how big would a wave guide have to be to
be able to transfer 7 Mhz?

About the only antenaa where a "wave is launched" is a dielectric lens
antenna with a wave guide feed.

Of course, at the other end of the wave guide is an antenna to which
voltage is applied, which causes current flow in the antenna, which
causes an electromagnetic field to be created in the wave guide which
then flows to the antenna.


--
Jim Pennino

On 01/11/14 21:38, wrote:
As a problem for the student, how big would a wave guide have to be to
be able to transfer 7MHz?

I'll guess at 34.8488m x 15.7988m by scaling the dimensions for 5.85 to
8.2GHz. (C Band)

--
;-)
..
73 de Frank Turner-Smith G3VKI - mine's a pint.
..
http://turner-smith.co.uk
..
Ubuntu 12.04
Thunderbirds are go.

Frank Turner-Smith G3VKI wrote:
On 01/11/14 21:38, wrote:
As a problem for the student, how big would a wave guide have to be to
be able to transfer 7MHz?

I'll guess at 34.8488m x 15.7988m by scaling the dimensions for 5.85 to
8.2GHz. (C Band)

Sounds in the ball park to me.

For further reading enjoyment and why there is no EM field inside of RG-8:

http://en.wikipedia.org/wiki/Cutoff_frequency


--
Jim Pennino