Content-Transfer-Encoding: 8bit
Message-ID: <65AECDF1F89AD411900400508BFC869F0D752C@pdw-mail-1.dera.gov.uk>
From: "Talbot Andrew" <ACTALBOT@dera.gov.uk>
To: rsgb_lf_group@blacksheep.org
Subject: RE: LF: Dipole antenna at LF
Date: Thu, 18 Jan 2001 15:12:46 -0000
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1; format=flowed
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
Sender: <majordom@post.thorcom.com>

Yes, thought someone had already tried a dipole up a hill. In that case
I won't try dangling a long wire down from Portsdown Hill.  If I wanted
to install an (amateur) LF antenna at the works QTH (just an idea for a
low power beacon) the very poor ground is in complete contrast to the
clay in my garden at home.  Something else is needed as a radial system
is not viable and earth rods absolutely useless in chalk.   .

As for other antennas, Every so often thoughts return to big loops.   I
still feel sure that at a certain dimension there will come a point
where a loop is as efficient as a vertical wire for a given TOTAL amount
of copper - it will be quite big but G2AJV was very pro-loop in the
early 73kHz days and he had a one of  100s of m^2   Certainly for a back
garden sized antenna we can't do better than a vertical even when ground
conductivity is poor, but for people who have access to orchards and
paddocks.........

As the efficiency of a loop goes up at a rate somewhere between the
third and fourth power of its linear dimensions (Loop radiation
resistance varies as area ^2 = dimension ^4  against R loss proportional
to dimensions), and wire antennas roughly as length^2, there ought to be
a point where the graphs cross and loops become more efficient.  But I
don't know how to model both in a way that equalises the amount of
copper which is the parameter we probably should keep constant.

And then there are other effects such as ground conductivity (poor
ground favours loops), proximity of trees (local trees again favour
loops) to be taken into account as well.
Of course, a superconducting loop could have very high efficiency, but
at a bandwidth that makes Visual CW at days per dot mandatory :-)

Andy  G4JNT

> Depressing as it may be, Marconi got it right 100 years ago and we 
> have failed to improve on his findings. There must be another magic 
> antenna out there somewhere but that isn't it.


-- 
The Information contained in this E-Mail and any subsequent correspondence
is private and is intended solely for the intended recipient(s).
For those other than the recipient any disclosure, copying, distribution, 
or any action taken or omitted to be taken in reliance on such information is
prohibited and may be unlawful.