Message-ID: <002001c281a9$d29918a0$266468d5@oemcomputer>
From: "mike.dennison" <mike.dennison@ntlworld.com>
To: rsgb_lf_group@blacksheep.org
Date: Fri, 1 Nov 2002 13:23:09 -0000
MIME-Version: 1.0
Subject: LF: Antenna puzzle
Content-Type: text/plain; charset=iso-8859-1; format=flowed
Content-Transfer-Encoding: 8bit
Sender: <owner-rsgb_lf_group@blacksheep.org>
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org

ON7YD wrote:
>>>>>>>>>
During lunchbreak I simulated your antenna (using MMAMA), making some
variations on the topload (always with 3 parallel wires). I guessed your
elevated inductance as 1mH.
Regardless of the topload configuration the radiation resistance is 0.047
Ohm.
The reactive part was :
for 3 wires joined at the near end (at the elevated coil) = -j3945 Ohm
(297pF)
for 3 wires joined at the far end = -j3924 Ohm (298pF)
for 3 wires joined at both ends = -j3900 Ohm (300pF)
for 3 wires joined in a zig-zag configuration = -j3921 Ohm (298.5pF)
According to MMAMA the way you join the wires shouldn't make much change
(less than 1%), but I do not know how well MMAMA behaves for very short
antennas (the figures I get for simple configurations such as a straight
vertical or inverted-L seem to be OK).
The most efficient way to increase antenna capacity at a limited space seems
to be spiral toploading as described by Alan at :
http://www.alan.melia.btinternet.co.uk/spiraltop.htm
MMAMA gives a for a 14m high antenna with a 10 by 10m spiral topload (5
turns)  and a 1mH elevated coil:
radiation resistance = 0.052 Ohm
reactance = -j2871 Ohm (408pF)
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

Rik,

Thanks for those observations (McAfee warned me about the unwelcome
attachment).

The elevated coil is more like 3mH. The one I am using at present is Litz
and has a DC resistance of 5ohms.

The frequency increase was caused by removing the bridging wire from the
coil end of the three parallel top wires. I concluded that this was not
because I reconfigured the top section but because the bridging wire was
acting as a capacity hat that affected the coil. This is why I did the
experiment this week that led to the fire at the top of the mast. I was
trying to increase the capacitance close to the coil.

I am sure a 10mm spiral top loading would work as per your simulation, but I
have also simulated the reaction of my neighbours and the town council. The
computer told me I had been lynched and had no more lives left!

Back to the drawing board.

73

Mike, G3XDV
http://www.lf.thersgb.net
====================