TopBand: Slinky test results
km1h @ juno.com
km1h@juno.com
Tue, 19 Aug 1997 17:02:17 EDT
On Tue, 19 Aug 1997 15:09:13 +0000 w8jitom@postoffice.worldnet.att.net
writes:
>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.
What was the part # on the box?? There are 3 sizes available.
>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.
How many did you use in series?
>Anyone interested in measuring their antenna can ask, and I'll
>describe how.
So Im asking...but why not just tell us all??
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
I assume that s = turns per foot and d = diameter in inches ????
Each Slinky is 85 turns and the large ones are 2.875" diameter and the
smallest ones are 1.5" in diameter. The turns are the same in all
versions.
Using the 2.875" diameter version and 5 in series over 175' I wind up
with 2.43 turns per foot. Using s/d 2.43/2.875 I get a ratio of .845.
This would then seem to translate to a Vp of about .58 which makes some
sense here at my QTH. Altho in reality the A:B comparison with a full
wave conventional Beverage keeps giving the Slinky the edge in S/N .
The Zo is also what was predicted by others on this reflector last year.
In very basic tests several years ago, trying to null a BC station on
1590KHz directly to the rear of one Slinky Beverage, I found 1100 Ohms to
be the magic number. Since I had no way of telling if this was a
reactive or resistive termination I just got lazy and used the standard
~600 Ohms for a resistor to cover 160-30M .
>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.
In what way? Loss, F/B, pattern??
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.
In 1970 or 1997?
>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.
That comes as no suprise since the DC resistance is a healthy 2.56 Ohms
per Slinky. These are, after all, cadmium plated steel.
I have related on this reflector that my own setup ( 5 over 175') is good
on 160M and really kicks butt on 80M. Would the DC resistance play a big
factor in the aperiodic performance over say 160 thru 30M; which is how I
use them here. I can definitely feel the loss on 40 and 30M where a
preamp is used....but they blow the yagis away in snow/rain static condx.
>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.)
We ALL would appreciate it. This concept is begging for modeling and more
experimentation.
73...Carl KM1H Slinky Beverages (tm) since 1985
>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
Thanks for actually making the effort Tom. And just recently you were
condeming the concept.
BTW, for your friends in Missisippi, the term "Slinky Beverage" has
already been trademarked.
I also wonder if the DC resistance is not, in some way, contributing to
the performance...particularly the F/B and apparent S/N. Someone a few
weeks ago mentioned similar findings with galvanized fence wire
Beverages.
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