TopBand: Slinky test results

Tue, 19 Aug 1997 15:09:13 +0000

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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