Slinky-Coil Dipole (2021)

• [Introduction]
• [My steel Slinky dipole]
• [My brass Slinky dipole]
• [More on tuning a Slinky dipole]
• [Slinky dipoles built by others]
• [References]

©2001-2021 F. Dörenberg, unless stated otherwise. All rights reserved worldwide. No part of this publication may be used without permission from the author.

Latest page update: 25 May 2021 (added ref. 22)

Previous page update: 6 July 2019

INTRODUCTION

A standard Slinky® is a toy made of a flexible 90-turn metal spring. It has diameter of 2¾ inch (≈7 cm). Slinkies have been popular since the 1940s and can still be bought today. Each Slinky contains about 67 feet (≈20.4 m) of flat steel wire (1/10 inch wide), and weighs approximately ½ pound (225 gram). 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. Note: some Slinky coils only have about 58 ft of wire (≈17.7 m).

Figure 1: 1999 US commemorative stamp and 1946 advertising

Antennas can be made from any piece of wire, so why not a Slinky?

A Slinky is made of 67 feet of steel wire. However, a 15 ft long antenna made with a Slinky coil does not have an electrical length of 67 ft! The Slinky-wire is "helically" or "spirally" wound - basically a cylindrical coil. The diameter of the coil is small ( < 0.5%) compared to the wavelength that the antenna is used for. Likewise, when the coil is stretched, the pitch of the turns ( = axial distance between successive turns) is similarly small. In this case, the antenna is called a "normal-mode" helix. In this mode, the coiled wire has maximum radiation in directions perpendicular to the axis of the coil. The coiling of the wire adds inductance that is distributed all along the antenna. I.e., the antenna has built-in continuous inductive loading. As the loading is obtained by the helical shape of the antenna wire, it is also referred to as "helically loaded". Note that the inductance also adds resonance frequencies that a same-length straight wire antenna does not have. As with all antennas, operation at a resonance frequency is not required - that only makes matching to a feedline easier.

See the references for more background information and examples of Slinky and other helical antennas built by others. And no: no one claims that this is a science-defying "miracle" antenna. It is a compromise antenna - with some advantage: it is light-weight, simple to extend and suspend, and very compact to stow: good for portable work and in limited space situations such as in an apartment.

Figure 2: Concept of the Slinky dipole

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, by shorting-out turns (incl. by pushing a number of turns up against each other). 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 an antenna tuner/coupler via a feed line made of twin-lead or ladder line. This creates a compact version of the good old "center fed Zepp" antenna.

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.

Note: the standard 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 40^th anniversary of the Slinky product, brass coils were made. Their availability has become very limited ( = expensive). Un-coated and powder-coated steel Slinky coils are still readily available; 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.

MY STEEL SLINKY DIPOLE

In 2004, I put together a steel Slinky dipole. Here are the components of this dipole:

• Two Slinky coils
• For the dipole center-insulator / support:
• 1x hard-PVC (Schedule 40 sprinkler pipe) T-piece, ¾ inch Ø, 2½ inch long
• 3x hard-PVC plug that fit into the T-piece
• 3x eyebolt
• 6x washer for the eyebolts
• 3x locknut ( = nut with nylon insert) for the eyebolts
• 1x BNC jack (through-hull, not flanged screw-on)
• 1s washer + lock washer + nut for the BNC jack
• 2x 25 cm (10") of heavy gauge insulated multi-strand copper wire
• 2x 10 m (2x 30 ft) of 3 mm dacron cord (pre-stretched is best; nylon typically stretches too much). Note that Slinky coils are not self supporting! So you need to use a "messenger line" (guy wire) to support the weight of the coils)!

Figure 3: Components of my first Slinky dipole

My center-insulator is easy to make:

• Drill an appropriately size hole in each of the PVC plus (2x for an eyebolt, 1x for the BNC jack).
• Install the eyebolts (nut + washer on outside, washer + self-locking nut on inside of plug and of the T-piece)
• Install the BNC jack (washer + washer with solder lip + lock washer + nut on inside of PVC plug)
• Drill a small hole (big enough to pass the insulated wire with some effort) in the PVC plugs with the eyebolt. Avoid hitting the washer/nut on the inside of the PVC plugs!
• Strip about 5 mm (1/4") of insulation of one end of each piece of insulated wire. Solder them to the center conductor and the solder-lip of the BNC connector. Insert the wires and the PVC plug with the BNC jack into the appropriate hole of the T-piece. Press the PVC plug in tightly (I used large adjustable pliers - no glue required).
• Pull each wire out through the other holes in the T-piece. Pass them through the small hole drilled into the PVC plugs with the eyebolt. Firmly press in those PVC plugs.

The photos below show the finished center-insulator, attachment of the guy-wire and final turn of the Slinky to the eyebolt, and connection of the insulated wire to the Slinky:

Figure 4: Some construction details of my first Slinky dipole

The photos above show that the final two turns of both end of each Slinky are held together with small clips. The final half-turn of the Slinky (both ends) is very carefully bent over by 90 degrees. at the clips. The cheap steel wire is very brittle - use a bending radius of about 1 cm (1/2"). You can only bend it once, and only if you use sufficient radius! Lightly sand about 2 cm (1") of the coil surface near the clips, and solder the wires from the center-insulator to the two Slinkies.

Now all that is left to do, is to tie each dacron cord to an eyebolt, pull it through the compressed Slinky coil. Suspend the center-insulator from the top eyebolt. Tie off the ends of the dacron cords to ... whatever (walls, trees,...). If desired, end-insulators can be used. E.g., traditional egg-insulators or tie-wraps (simple & cheap).Leave enough support wire to run back to the end of the stretched coil. Stretch the coils both coils to the same length (up to 4.5 m / 15 ft), and tie off the bent final turn of each coil. We are now ready for action!

Figure 5: Suspended from the rafters in the loft of my previous apartment - ready for action

As the photo above shows, I could only install the antenna inside my apartment in the south of France. Still managed to make QSOs throughout Europe, and even a QSO with a station in Argentina on 20m, in Hellschreiber mode and 50 watts. This doesn't mean that it is a good antenna, nor that it is a bad one... I compared it with a simple wire dipole with the same overall length (2x 4.5 m / 15 ft) installed at the same spot: with the Slinky, received signals were noticeably stronger.

With a simple antenna tuner (MFJ-945E) at my transceiver and 25 m (80 ft) of RG58A/U coax to the Slinky dipole, I could easily load & match the antenna system from 80 to 6 m. Yes, I know: 1) this says absolutely nothing about the antenna performance, and 2) this setup is rather bad practice for non-resonant and multi-band operation! Should have used some 300 or 450 Ω twin-lead, but did not have any on hand...

MY BRASS SLINKY DIPOLE

The standard Slinkies are made of very cheap, low-grade steel. I'm not sure if they are galvanized (zinc plated). If plain steel, it is also a relatively poor conductor compared to copper. I have no idea if the steel wire generates any strange magnetic antenna effects. So, mid-2008 I splurged and ordered a "double Slinky" solid-brass long-wire antenna for $40.

Figure 6: My "longwire" antenna, made of two brass Slinkies

Did northing with them until August of 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 much more than the Slinkies made of cheap steel. In 2007, I moved to a penthouse apartment with a nice size terrace, so I can install (small) antennas outside. My automatic antenna tuner/coupler (ATU) is mounted on the outside wall of the corner of my living room. A short section (1.5 m, 5 ft) of 300 Ω twinlead cable connects the dipole to the ATU. In this experiment, the antenna was only installed about 7 ft (2m) off the ground.

Figure 7: Installation situation on the terrace of my apartment

Figure 8: My brass Slinky dipole, installed on my terrace

Figure 9: Looking down the guy-wire of the stretched brass Slinky

Figure 10: Side view of the suspended stretched brass Slinky

I took some quick measurements with my miniVNA antenna analyzer at the end of the short twin-lead feedline. With the Slinky-dipole stretched to 2x6 meters (2x 20 ft), I obtained the following SWR plot:

Figure 11: A 1-30 MHz SWR sweep of my brass Slinky dipole, stretched to 2x6 m

The SWR plot shows two deep dips in the 1-30 MHz range: around 7.4 and 20.2 MHz. By adjusting the length of the stretched coils, these can be moved to the 20 and 15 m bands. A medium dip is located around 17.1 MHz. Two shallow dips are at 8.7 and 26.9 MHz. None of the five dips seem harmonically related. The fact the Slinkies provide inductive loading may cause additional resonances with stray capacitances. I also cannot exclude coupling with the long zinc rain gutter and the steel railing of my terrace.

Note that a simple wire-dipole with the same span, would have a resonance frequency that is around 9.4 MHz (in free-space). This suggests that this Slinky dipole appears to be 20% longer than a same-size wire dipole. My tuner/coupler loads this antenna without any problems from 80-10m, though tuning took a long time in the 10 m band.

MORE ON TUNING A SLINKY DIPOLE

Clearly, there are (at least) two ways to tune a Slinky dipole:

• by symmetrically varying the the spacing between the coil turns (amount of "stretch" and overall tip-to-tip span of the dipole), and/or
• by symmetrically varying the effective number of coil turns, by shorting out a number of adjacent turns (that are then preferably compressed against each other).

Of course, this is like adjusting the inductance of any coil by deforming it! This way of tuning a Slinky dipole has actually been patented in the USA in 1974 (ref. 18)!

Figure 12: US patent Nr. 3,858,220: Tunable Spiral Dipole Antenna (coil diameter 4 inch (10 cm) , 2 x 75 turns)

(source: ref. 19)

The 80-40-20 dipole was sold "patents pending" by Teletron Data Corp., at least as early as 1973 (ref. 4). The kit included 2 Slinky coils, a center insulator, nylon cord, hooks, 50 ft of coax cable with a PL259 connector (six turns of the coax to be installed at the feed point, as a common-mode choke), and an instruction sheet:

Fig. 13. Installation and tuning instruction for the Slinky dipole that was marketed by Teletron data Corp.

(source: ref. 4)

According to the instructions, SWR better than 1.4 could be achieved by careful tuning. At some point, the patent rights were apparently sold to Jon Titus, KZ1G, who sold the antenna for a couple of years; the patent has since expired.

Figure 14: Slinky Dipole advertising - 1973-1981

(note the price variations from $25 to $50)

As the Slinky dipole is just a dipole, there is at least one more standard way to tune it: asymmetrically. That is: use a different variation of turns-count and/or inter-turn spacing in the two dipole legs, to make an Off-Center Fed (OCF) Slinky Dipole. This way, the resonance frequency and feed-point impedance can also be adjusted. Maybe I should apply for a patent! I have also used a single Slinky coil as a variable "counterpoise" for a short 20m vertical antenna.

SLINKY DIPOLES BUILT BY OTHERS

I am definitely not the first one to build a Slinky antenna, see references 1, 4-7, 9-14, 16, 17, 20 and 21 below. Rich Morgan (KF9F), was inspired and also decided to build one himself. He tested an initial version with two small Slinky coils (1.75 inch diameter, 58 ft wire) in his backyard. He two support "arms" of PVC tubing (10 ft long, 3/4 inch diam.), a 4:1 balun and coax. He constructed a second version, made with "full size" Slinky coils (67 tf of wire). It also had a span of 20 ft (6m), but was fed via a 450 ohm window line. It was put to the test at the 2017 Field Day of the Lewis & Clark Radio Club (K9HAM) on a manually rotable mast:

Figure 18: 80-10 Slinky dipole prepared for raising at the 2017 Field Day of the LCRC (K9HAM) in Godfrey/Illinois

(source: Rich, KF9F)

Figure 19: The 80-10 Slinky dipole being erected

(source: Rich, KF9F)

Figure 20: The 80-10 Slinky dipole erected - final adjustment of the guy wires

(source: Rich, KF9F)

Much to the surprise of the radio club members, it worked - as demonstrated in this YouTube clip!

REFERENCES

• Ref. 1: "The NJQRP "Slink-ette" - A SlinkyTM-based Doublet or Loaded Vertical QRP Antenna", by Joe Everhart (N2CX), and Tom Arey (N2EI), in "NJQRP Meeting Recap", 29 September 2001
• Ref. 2: "Some Of Our Technical and Non-Technical Notes On Our Slinky^® Antenna", from SlinkyAntennas.com [appears to have gone out of business]
• Ref. 3: "Slinky and Loaded Beverages", by Tom Rauch (W8JI)
• Ref. 4: "Slinky - A lot of antenna in a little space" [80-20 mtr "Boinger"], by Teletron Data Corp. [business inactive since 1982], 1973, 7 pp.
• Ref. 5: "Platzsparende Vertikalantenne (Wendelantenne)", by Gerd Schreiber (DL1MO), in "QRV", year?, pp. 341-342
• Ref. 6: "Holi-D-Box: avec pour bagage... une Slinky", by Luc Smeesters (ON4ZI), in "NMRevue", nr. 42, July 2007, pp. 7-11
• Ref. 7: "A Multiband Slinky Coil for the PAC-12 Antenna", by James Bennett (KA5DVS), in "QHB Extra!" (extra feature of "QRP Homebrewer" of the New Jersey QRP Club, April 2003
• Ref. 8: "A Helical Loop Antenna for the 20-meters Band", by Vladimir Kuzmin (UA9JKW), in "Antentop", Nr. 5, 1-2004, pp. 60-62
• Ref. 9: "A Compact Spiral T/R HF Antenna", by Richard Marris (G2BZQ), in "Elektor Electronics USA", November 1992, pp. 14-15
• Ref. 10: "The Slinky Antenna - A lot of signal for not many $$$", by John Wehren (N6XN), in monthly newsletter of the Napa Valley Silverado Amateur Radio Society, June 2007
• Ref. 11 "Suitcase antenna" [helix trap dipole] by Brian Warman (VK5BI), in "Ham radio Magazine", May 1973, pp. 61-63
• Ref. 12 "A nine band helically wound doublet antenna", by Constantine Feruglio (IV3VS), in SPRAT, 1993, Issue 75, p. 19
• Ref. 13: "Some helical antenna experiments - 15 M helical", by Ben Johanson (CT4CH), in SPRAT, 1983, Issue 36, p. 10
• Ref. 14: "The Clandestine Multi-band HF Slinky Antenna", by Bill Hayden (N1FRE), in "Spectrum" (wewsletter of The Whitman Amateur Radio Club, Inc.), December 2006, pp. 8-9
• Ref. 15: "Slinky Loop aerial", by Tom Haylock (M0ZSA), in "RadCom", November 2010, p. 49
• Ref. 16: "Die Wendelantenne", by Kurt Trapp (DK2VJ), Georg Neis (DL4VAN), in "CQ-DL", 9-2011, pp. 651-652
• Ref. 17: "Directional helical antennas" [slinky 2-element beam], I. Kapustin (UA0RW), in "Antentop", Nr. 12, 1-2010; reprinted from "Radio", Nr. 7, 1958, pp. 34-35
• Ref. 18: "Helical Antennas", Howard King, Jimmy Wong, Chapter 13, 24 pp. in "Antenna Engineering Handbook", Richard C. Johnson, 3^rd ed., McGraw-Hill, 1993.
• Ref. 19: "Tunable Spiral Dipole Antenna", Sidney Arrow, US patent Nr. 3,858,220, December 31, 1974.
• Ref. 20: "The Super Slinky - Inverted-L Antenna for Top Band", Dave Ahlgren (K1BUK), in "QST", May 2018, pp. 43-45; retrieved 6 July 2019. [pdf] See note at bottom of this page.
• Ref. 21: "Endgespeiste Halbwellen Monoband-Antenne" [Half wave End-Fed], Jürgen Schäfer (DL7PE), in "CQ-DL", 3-2019, pp. 24-26. [pdf] See note at bottom of this page.
• Ref. 22: "Slinky Coil Dipole for 40 to 10 metres" [This web page in magazine article form], by me Frank Dörenberg, N4SPP / F4WCN,  in "Radio ZS" (The Journal of the South African Radio League / Die Tydskrif van die Suid-Afrikaanse Radioliga), Vol. 73, No. 5, May/Mei 2020, pp. 27-37.

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External links last checked: October 2015 unless noted otherwise.