[Introduction]    [My steel Slinky dipole]     [My brass Slinky dipole]      [References]

 

INTRODUCTION

A Slinky® is a toy made from a flexible 90-turn metal spring with a 2¾ inch diameter (≈7 cm). Slinkies have been popular since the 1940s, and can still be bought today (e.g., at Wal-Mart for $2 a piece, 2014). Each Slinky contains about 67 feet (20 m) of flat steel wire (1/10 inch wide), 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.

1999 US commemorative stamp

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 overall wire length).

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 steel 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 available commercially, for about $30-$60 + shipping & handling charges; powder-coated, solid brass and gold-plated brass versions are also available. However, Slinky dipole antennas are easy to make, see below. Home-built is also cheap: about $10 worth of components.


Construction details of commercially available Slinky Dipoles

Construction details of some commercially available Slinky Dipoles

 

MY STEEL SLINKY DIPOLE

 

In 2004 I put together a steel Slinky dipole. I could only install the antenna inside my apartment. Made QSOs throughout Europe from my QTH in the south of France.


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.
4I
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             


- suspended from the rafters, direction SW-NE, ready for action -

MY BRASS SLINKY DIPOLE

           My brass "longwire" antenna, made of 2 Slinkies

 

Mid-2008 I ordered a solid-brass 2-Slinky long-wire antenna for $40. August-2014 I dusted them off, turned them into a quick-and-dirty dipole, and took some measurements. I stretched each Slinky to a coil of 6 meters in length (≈20 ft). The brass Slinkies can be stretched more than the  Slinkies made of very cheap steel. In 2007, I moved to a penthouse apartment with a nice terrace. So I can install (small) antennas outside. My automatic antenna tuner/coupler (ATU) is mounted on the outside wall of the souther corenr of my living room. A short section (1.5 m, 5 ft) of 300 Ohm twinlead cable connects the dipole to the ATU.

 

Note that, even though the antenna is installed about 21 meters (≈70 ft) above street-level, it is only about 2 meters (≈7 ft) above ground (the steel-reinforced concrete floor of my apartment and terrace). Especially on 80 meters, this means that DX is basically excluded - not because of the antenna, but because of the installation height.

 

           Installation situation on the terrace of my apartment

 

Update of Aug-2014: I have dusted off the 2-Slinky long wire, and converted it into a 2-Slinky dipole! The brass Slinkies can be stretched much more than the cheap steel version - and still return to their original shape.

 

           One leg of the brass Slinky dipole

 

 

          The other leg of my brass Slinky dipole

 

Looking down the guy wire of the stretched brass Slinky
 

Side view of the stretched brass Slinky
 

The SWR plot below (taken at the end of the short section of twin-lead) shows three deep dips in the 1-30 MHz range: around 6.4, 9.5, and 11.2 MHz. A simple wire dipole of the same overall dimensions would have a resonance frequency that is much higher (around 9.4 MHz in free-space)... My tuner/coupler loads this antenna without any problems from 80-10m. I could trim the extension wires (or rebuild a new, longer Cobra) such that the lowest dip is around 7050 kHz, to be able to use it at that QRG without a tuner/coupler.

 

I took some quick measurements with my miniVNA antenna analyzer at the end of the short twinlead feedline. With each Slinkies stretched to 6 meters, I obtained the following SWR plots:

 

           1-30 MHz SWR plot of my 2x6m brass Slinky dipole - without balun

 

The above plot show two very usable SWR dips: near 7.4 and 20 MHz. No antenna tuner/coupler needed! With minor adjustments to the stretched length of the Slinky coils, the 7.4 MHz dip could be moved to the 40 mtr band, and the 20 MHz dip to 17 and 15 mtr bands. In that case, the antenna could be fed with a coax (for those bands). My ATU had no problem loading the antenna on the 80 - 10 meter bands. "Tuning" in the 30 MHz band (10 meters) took the longest, by far.

 

Purely out of curiosity, I repeated the measurement with a 4:1 balun installed at the end of the section of twinleed feedline. The SWR plot below shows that this is not a good idea.

 

           1-30 MHz SWR plot of my 2x6m brass Slinky dipole - with 4:1 balun

 

 

REFERENCES

 

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The NJQRP "Slink-ette" - A SlinkyTM-based Doublet or Loaded Vertical QRP Antenna by Joe, N2CX, and Tom, N2EI

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Some Of Our Technical and Non-Technical Notes On Our Slinky® Antenna from SlinkyAntennas.com

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Slinky and Loaded Beverages by Charles, W8JI

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Slinky - A lot of antenna in a little space by Teletron Corp.

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Platzsparende Vertikalantenne (Wendelantenne) by Gerd, DL1MO

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Holi-D-Box: avec pour bagage... une Slinky by Luc, ON4ZI

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A Multiband Slinky Coil for the PAC-12 Antenna by James, KA5DVS

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A Helical Loop Antenna for the 20-meters Band by Vladimir, UA9JKW

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A Compact Spiral T/R HF Antenna by Richard, G2BZQ

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The Slinky Antenna - A lot of signal for not many $$$ by John, N6XN

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Suitcase antenna by Brian, VK5BI (helix trap dipole)

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A nine band helically wound doublet antenna by Constantine ,IV3VS

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Some helical antenna experiments - 15 M helical by Ben, CT4CH

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The Clandestine Multi-band HF Slinky Antenna by Bill, N1FRE

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Slinky Loop aerial by Tom Haylock (M0ZSA)

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Die Wendelantenne by Kurt Trapp (DK2VJ), Georg Neis (DL4VAN)

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Directional helical antennas [slinky 2 el. beam], I. Kapustin (UA0RW), Antentop, Nr. 12, 1-2020; reprinted from "Radio", Nr. 7, 1958, pp.34-35



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