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Message-ID: <5.1.0.14.0.20020918141307.02fe93a0@pb623250.kuleuven.be>
Date: Wed, 18 Sep 2002 14:32:11 +0200
To: rsgb_lf_group@blacksheep.org
From: "Rik Strobbe" <rik.strobbe@fys.kuleuven.ac.be>
MIME-Version: 1.0
Subject: Re: LF: Capacity hat puzzle
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Hello Mike,

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)

73, Rik  ON7YD


At 10:19 18/09/2002 +0100, you wrote:
>My antenna failed recently because one of the leads to the elevated loading
>coil had broken. To fix it I had to lower one of my 14m masts - a task
>requiring several people so I don't do it very often.
>
>Whilst the mast was down I modified the capacity hat. There are three 17m
>long parallel wires, about 0.5 metres apart (total span 1m). The wires are
>joined at both ends. (See http://www.lf.thersgb.net/antenna.htm for a
>picture).
>
>I changed this so that the wires were only joined at the far end. Now the
>vertical wire and the elevated coil are connected to the centre wire only.
>The RF travels along this centre wire to the end, then back using the outer
>two wires in parallel..
>
>I expected this to increase the inductance of the antenna and so lower its
>resonant frequency. The actual effect was the opposite - the frequency
>increased and I had to add inductance at the base of the vertical.
>
>Any thoughts?
>
>The next time I lower the mast, I will restore the connections.
>
>Mike, G3XDV
>http://www.lf.thersgb.net
>====================