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
====================
|
