One way of having off-centre fed antennas is to use 3 wires, e.g. 10m, 0.1m, 20m and feed the antenna at the centre of the very short segment, but there is an easier way that uses just a single wire.
You can specify the number of segments and the specify the feedpoint as a number, rather the just ‘w1b’, ‘w1c’ or ‘w1e’ (where b/c/e are beginning, centre or end).
It’s easiest to explaining using an example of a wire with a small number of segments:
#e.g. a 6m wire long, 1m up, 0.8mm radius, 4 segments
w1: 0,-3,1, 0,3, 1, 0.8, 4
feedpoint (sources/Pulse) = w1b2
A wire above with 4 segments has 5 nodes (possible feedpoints). They’re numbered from ‘-1′ to ‘n-1′ where ‘n’ is the number of segments.
e.g.
w1b-1 # beginning – the first coordinate (-1 really does work)
w1b0 # (that’s b zero), 25%
w1b1 # the centre
w1b2 # 75%
w1b3 # far end
Therefore if the number after the “b” is called the “feedpointNo”, we can get two useful equations:
fraction-along-wire = (feedpointNo+1)/no-of-segments
OR
feedPointNo = (fraction-along-wire * no-of-segments) – 1
A wire with 100 segments should be precise enough for most uses and is easy to work with.
If want to feed it 20% of the way along:
feedPointNo = (0.20 * 100) – 1 # as 20% is 0.20
so w1b19 is the feedpoint.
I found the reference to digits after the wire/pulse in the documentation somewhere. The above writeup sounds like I had it all clear in my mind, but your question prompted me to go and play with MMANA and define a wire with 4 segments and play to sort out the details. Apparently the number of segments must be even, you can have 4 or 6 segments, but not 5.
IMPORTANT: make sure that the feedpoint number is between -1 and n-2
- if it’s n-1, the START button doesn’t seem to do anything, but there’s no warning
- if it’s the same as the number of segments (or greater) the feedpoint vanishes, but the “Start” button does give an error message.
Do try the example above with 4 segments. As there are so few it’s very clear in the geometry view where the feedpoint is.
Giovanni – ZL2GX
