Last update: 23 July 2009
- suspended from the rafters, direction SW-NE, ready for action -
A Slinky® is a toy made from a flexible 90-turn metal spring with a 2¾ inch diameter. Slinkies have been popular since the 1940s, and can still be bought today (e.g., at Wal-Mart for $2 a piece). Each Slinky contains about 67 feet (20 m) of flat steel wire, and weighs approximately ½ pound. Compressed, a Slinky coil is only 2¼ inch long, but it can be stretched into a helix as long as 15 feet (4.5 m) in length without deforming it permanently.
An antenna made from a Slinky is light, simple to extend, suspend, and stow: good for portable work and in limited space situations such as in an apartment. A standard Slinky coil resonates as a quarter wave between 7 and 8 MHz when it is stretched to a length between 5 and 15 feet. Dipoles resonant at frequencies above the 7-8 MHz range may be created by removing turns to shorten the helices, or by shorting out turns. However, the simplest way to obtain multi-band results with a pair of Slinky coils, is to stretch them as far as space permits and connect them to a feed line made of coax or twin lead. This creates a compact version of the good old "center fed Zepp" antenna.
Feed it through an antenna tuner. This simple antenna will work all bands 7 MHz and above. With a tuner, it may be usable on the 80 meter band, depending on metallic and other objects near the antenna - 80 meter operation is easier when expanding the antenna to 2+2 coils. Here are some insights as to how/why this antenna works (which is not because of the wire length).
1999 US
commemorative stamp
Note: the standard (brittle) steel coils are not corrosion protected. Corrosion will take place quickly due to weather exposure, and to some extent due to RF energy when transmitting. So the Slinky is best suited to indoor or temporary outdoor (portable) deployment. To mark the 40th anniversary of the Slinky product, brass coils were made. Their availability has become very limited. Powder-coated steel Slinky coils are still readily available in several colors for about $7; though not as weatherproof as brass coils, they hold up better than the plain coils. A clear-coat of acrylic lacquer from a spray can won't hurt any Slinky.
Slinky antennas are easy to make, see below. They are also cheap: about $10 worth of components (available commercially for about $30-$60 + shipping & handling charges; powder-coated, solid brass and gold-plated brass versions are also available).
Update 26 June 2008: have ordered a solid-brass 2-Slinky long-wire antenna for $40, and will transform that into a brass Slinky dipole.
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Articles:
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The
components of my Slinky Antenna
(support wire and coax
feedline are not shown )
List of
components:
2 Slinky coils
1 hard-PVC (Schedule 40 sprinkler pipe) T-piece, ¾ inch Ø, 2½ inch long
3 hard-PVC plugs that fit the T-piece
3 eyebolts
3 nuts for eyebolts
6 washers for eyebolts
3 locknuts (nuts with nylon insert) for eyebolt
1 BNC jack (through-hull, not screw-on)
1 washer + lock washer + nut for BNC jack
50 cm of insulated heavy gauge multi-strand copper wire
20 m of nylon or dacron cord (pre-stretched is best; slinky coils are not self supporting so you need to use a messenger line to support the weight of the
coils)
Tools required:
soldering iron + solder
large pliers
regular pliers
drill + drill bits
socket wrench + small sockets (for nut on BNC jack and eyebolt locknuts)
sand paper
Construction notes:
1. drill hole in center of each PVC plug (2 to fit an eyebolt, 1 for the BNC
jack).
2. install the eyebolts (nut + washer on outside, washer + nylon self-locking
nut on inside of plug and of the T-piece)
3. install the BNC jack (washer + washer with solder lip + lock washer + nut on
inside of PVC plug)
4. drill hole next to eyebolt and BNC jack, into the plug. Avoid hitting the
washer/nut
5. take 2 pieces of 25 cm of heavy gauge insulated multi-strand copper wire.
6. strip off about 5 mm of the insulation
7. solder wires to center conductor and ground (solder lip) of the BNC jack
8. insert the wires and plug with the BNC jack into the center hole of the
T-piece
9. press in tightly with large pliers (no glue required!!)
10. run each wire through the hole in one of the 2 remaining PVC plugs
11. insert the two plugs into the T-piece, and press fit.
12. pull out the wires
13. VERY carefully bend half of the final turn of each Slinky coil (both ends of
each coil), such that it is perpendicular to the cross section of the coil. See
pictures. Use a bending radius of about 1 cm. The steel wire is very brittle.
You can only bend it once, and only if you use a sufficient radius.
14.
were the bent part of the coil separates from the rest of the coil, sand the
surface of the coil and of the bent part over a length of about 1-2 cm. Solder.
15. open each eyebolt and hang the bent part of one end of each coil in it.
Close the eyebolt.
16. strip about 6-7 cm of insulation of each wire
17. sand the surface of the first turn of each coil over a length of about 1-2
cm.
18. solder 1-2 cm of the stripped wire that is closest to the insulation, to the
coil.
19. wrap the rest of the stripped wire around it, and solder. See photo below.
20. compress each coil, and run about 6 m of 3 mm Ø nylon cord through it, and
tie it off to the eyebolt.
21. suspend the T-piece from the eyebolt on top.
22. run the nylon messenger (support) wire through the coils, from both sides of
the T-piece to whatever (tree, wall, ...), and tie off. If desired,
end-insulators can be used. E.g., traditional egg-insulators or tie-wraps
(simple & cheap).
23. Leave enough support wire to run back to
the end of the stretched coil.
24. stretch the coils both coils to the same length (up to 4.5 m), and tie off
the bent turn of each coil. See photo below.
25. attach coax to the BNC jack.
26. ready for operation!
Performance notes:
1. As a receiving antenna, signals are about 30 -
40 dB stronger than with a thin 5 m multi-strand wire (no tuner).
2.
With 25½ m (84 ft) of RG58A/U coax and the slinky coils
stretched to 4.25 m (14 ft) each, I can easily tune to SWR
lower than 1.5 on all bands 10 m through 80 m + 6m (can't get it tuned on 160 m).
With 10 m (33 ft) of coax, I get significantly more output (per the power/SWR
meter of my tuner) on 40 m, but very sharp tuning peaks on 20 m.
3. I have added a
W2DU-style balun at the antenna base (this is
actually a
ferrite-loaded coaxial RF-choke that just happens to yield the same result as a
1:1 current balun).
It is made of 24 ferrite beads (Amidon FB-77-6301) on a 25 cm (10 inch) piece of
RG400 coax. The beads fit snugly on RG400, whereas RG58 is too thick (use 3
dozen FB-73-2401 beads instead). Without the
balun, SWR is 1:2 on 20 m, and 1:1.4 on 40 m. No change on the other
decade bands. I do not use a station ground.
4. I
still have to try out this antenna
with twinlead feed line or "ladder line" plus a 4:1 balun, as I have already
done with my "¼
size"
Cobra folded dipole.
Close-up
of the T-piece and coil
attachment
Close-up of the far end of the coil
Construction
details of commercially available Slinky Dipoles
Construction
details of commercially available Slinky Dipoles
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