"gareth" wrote in message
...
"Brian Morrison" wrote in message
...
On Thu, 10 Sep 2015 11:27:38 +0100
gareth wrote:
What is the nature of free space such that it requires antennae to be
at least 1/4 wave
before accepting radiation efficiently?
There is no requirement of this nature, the only reason for relatively
large antennas is to achieve an input impedance that makes power
transfer into the antenna relatively efficient.
That is a side issue of the phenomenon

Convince yourself by calculating the retarded potential from a far field
when
the same field strength comes from a dipole and when it comes from a short
antenna.
You find that there has to be more power fed into the short antenna
(irrespective of
feed impedances and ohmic resistances) to achieve that same field strength.

"gareth" wrote in message
...
"gareth" wrote in message
...
"Brian Morrison" wrote in message
...
On Thu, 10 Sep 2015 11:27:38 +0100
gareth wrote:
What is the nature of free space such that it requires antennae to be
at least 1/4 wave
before accepting radiation efficiently?
There is no requirement of this nature, the only reason for relatively
large antennas is to achieve an input impedance that makes power
transfer into the antenna relatively efficient.
That is a side issue of the phenomenon

Convince yourself by calculating the retarded potential from a far field
when
the same field strength comes from a dipole and when it comes from a short
antenna. You find that there has to be more power fed into the short
antenna (irrespective of
feed impedances and ohmic resistances) to achieve that same field
strength.


It's all about capture area. A big antenna will radiate more effectively
than a small one, provided you can match the feed point impedance.
--
;-)
..
73 de Frank Turner-Smith G3VKI - mine's a pint.
..
http://turner-smith.uk

On 2015-09-10, Brian Morrison wrote:
On Thu, 10 Sep 2015 11:27:38 +0100
gareth wrote:

What is the nature of free space such that it requires antennae to be
at least 1/4 wave
before accepting radiation efficiently?

There is no requirement of this nature, the only reason for relatively
large antennas is to achieve an input impedance that makes power
transfer into the antenna relatively efficient.

Also, short antennas have a very reduced broadband.
Alejandro Lieber


--

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