I tested some common Slinky's, purchased at Toy-R-Us in a
shopping mall ($1.65 each). I'm not sure if they are the same as
the Slinky's everyone else is using or not.
This what I've found for a height of 6 feet over 4 mS/m soil. I'm
sure the results are soil critical. So they may not hold for other
locations. The antenna tested was about 200 feet long.
Anyone interested in measuring their antenna can ask, and I'll
describe how. An Autek or MFJ-259 through a good transformer should
work, although I used a Network Analyzer because of the high Z.
Terms used:
d= diameter
s=spacing
Vp = velocity of propagation
Zo= surge impedance at 2 MHz
For different s/d's (turn spacings divided by turn diameters):
s/d = 0.5 Zo 1280 ohms Vp= .42
s/d = 0.75 Zo 1060 ohms Vp= .55
s/d = 1.0 Zo 930 ohms Vp= .63
A Vp of .5 means the wave travels half the speed of
freespace, so the antenna looks twices as long as it PHYSICALLY is
from end to end. A Vp of .75 means a 100 ft LONG structure is 133 ft
long electrically...no matter HOW much wire length is in the
structure.
As you uses more turns per foot, impedance climbs and Vp slows, but
Vp's less than .5 were deleterious to performance according to my
ancient data from the 70's. From my experiments in the 70's with slow
wave receiving antennas, optimum Vp was about .5 to .6, so it looks
like a s/d of .75 is about optimum. The best length I found was
about 1/2 wl end to end, but that was cut and try.
At that s/d, the surge Zo is about 1000 ohms or so, so that would be
the optimum terminating and input resistance.
Power loss measured about 3 dB per hundred foot ANTENNA length for a
s/d of .75.
I hope someone with Eznec or some other program can model this
antenna. This can be done by inserting a series of what Eznec calls
"loads" (spaced maybe every 20 feet or so) and adjusting wire size
and the "load" inductance until the Vp and Zo matches, while tweaking
the resistance of the loads to bring the loss in line. The antenna
should show an even and smooth current taper that amounts to half
the current remaining in about 200 ft.
(If anyone goes through this exercise I'd appreciate the values
used.)
I would expect a model should give a good approximation of the
pattern, but I'm too lazy to hack away at the model.
73 Tom
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