In 2008, when I was between jobs for a while, I decided to build an HF antenna with more directivity than a simple dipole. Installation at my QTH basically had to be on my terrace, so space is rather limited. Such a directional antenna requires at least two elements, a full-size antenna for wavelength over 10 mtr will not fit, and 10 mtr is (usually) not my favorite band. So I narrowed it down to a simple half-size 2-element "Yagi".
This famous type of antenna was invented in Japan in the 1920s, by Dr. Hidetsugu Yagi and Dr. Shintaro Uda. So it should actually be referred to as a Yagi-Uda antenna, but unlike the Japanese patent, only Mr. Yagi's name appeared on the 1926 US patent application. Substantial additional research was subsequently done by Yasuto Mushiake. The 2-element version is the simplest form: a dipole as the driven element (feed element), and a passive "parasitic" element at some distance, in parallel with the driven element. This second element is either a "reflector" or a "director". A reflector is slightly longer than the dipole, and placed behind it. A director is slightly shorter than the dipole, and placed in front of it. The coupling (interaction) between the two elements causes the transmitted energy to be concentrated in the forward direction ( = looking from the reflector to the dipole, or from the dipole to the director). I.e., a "beam" is formed. Hence this type of antenna is sometimes called a "beam antenna". In a multi-element Yagi antenna, there is (typically) one reflector, and one or more directors. In accordance with the universal Law of Diminishing Returns, each additional director N+1 increases the gain relatively less than the adding the preceding director N.
Two-element Yagi and general Yagi geometry
As with all "beam" antennas, performance characteristics of a Yagi antenna are front-to-back ratio (F(B, a.k.a. front-to-rear, F/R), gain (in the forward direction), feedpoint impedance (hence, SWR bandwidth), and side-lobes These characteristics depend in a complex manner on design parameters such as spacing between the elements and element tuning (electrical length, element diameter and tapering, matching system,...), installation height, and the environment. Ref. 1, 2. Yagi antennas are "mono-band". However, it is possible to combine several 2-element Yagis on the same boom, e.g., for the 20-17-15-12-10 meter band (ref. 3).
A full-size ( = ½λ) dipole antenna for the 20 mtr band spans about 10 meters (33 ft). Searching for a half-size half-size 2-element Yagi, I found the design by Gary Hanson (KJ5VW), ref. 4. See ref. 5 and 6 for similar designs.
Nominal dimensions of the KJ5VW 2-element yagi
The G3YCC "shorty" dipole is made of aluminium tubing, whereas the KJ5VW model is a wire-dipole. Both are "half size" dipoles. The G3YCC "shorty" dipole is made of aluminium tubing, whereas the KJ5VW model is a wire-dipole. Both are "half size" dipoles. Scaling-down an full-size antenna by more than 50% causes serious performance reductions. However, I have actually built very small loaded dipoles for 80/40/20 that are scaled-down by 95% (!) and work interestingly well for their size (but, sorry, no miracles...).
To make a shortened dipole resonant in the 20 mtr band, some form of loading is required. One standard solution is "inductive loading": adding a loading coil to each "leg" of the dipole. The loading coils is placed somewhere between the feedpoint and the tip of each "leg". The current-distribution along the main elements is such that the current is highest at the feedpoint. Placing a loading coil here, requires the smallest inductance. The current-distribution tapers off, from maximum at the feedpoint to zero at the tip of the radiator element. So, as the loading coil is placed farther away from the feedpoint, a larger inductance is required. At the tip of the main element, the current is zero. This would require an infinitely large inductance. See the diagram below. The placement of the coils does affect the shape of the current distribution, but does not change the fact that it is maximum at the feedpoint and zero at the tip.
Required loading-coil inductance as function of coil placement
Center-loading (also called "base loading" when talking about vertical monopole antennas) requires the smallest inductance, and is often easier to construct than off-center loading. So why not always use center-loading? This is primarily driven by coil losses, hence, efficiency of the antenna. The coil losses basically depend on the current, coil dimensions, material, construction, and core. Depending on the diameter of the radiator element (tubing, wire) and installation height, compared to the wavelength, the most efficient placement of the loading coil is somewhere between 30 and 60% away from the feedpoint (ref. 8, 9). I.e., around the mid-point. The G3YCC and KJ5VW dipoles are "mid-element loaded". Note that the efficiency-vs-placement curves are fairly flat over a relatively large range around the mid-point.
I decided to build a lightweight version of the KJ5VW design. Instead of attaching the dipole wires to wooden dowels, each leg of my dipole and reflector consists of a 10 ft ( = 3 m) lightweight telescopic fiberglass fishing poles. Got them at the end-of-summer sale at a local supermarket. The poles have 3 collapsible sections of 4 ft. Each pole only weighs 4½ ounces (130 grams)! Note: some such poles are made of carbon fiber or with graphite. I do not know if they are conductive for RF and cause undesirable loading of the dipole. The total weight of my design is only 2.4 kg (5.3 lbs) - about one third of the KJ5VW design (6.8 kg, 15 lbs). Obviously, the fishing poles are more flexible than dowel rods, so they flop around when its windy!
Nominal dimensions of the N4SPP yagi
The first step in making the 2-element antenna, is building the dipole. So, first I built & tested the dipole with loading coils by itself, see this page. This is how I made my loading coils:
- Each coil core consists of a 7 cm (2¾") long section of ¾" Schedule-40 PVC pipe from the building supply store (21.4 mm outer diameter; 15.6 mm inner). This is not the lightweight light gray pipe, but heavy duty, thick-walled "rigid non-metallic conduit - above ground underground" pipe. However, this is not critical for this application!
- I have drilled out a small groove from the edge of the coil core to each of the holes. (see photo below)
- For each dipole leg (incl. the coil), I used 5m7 (12+ ft) of #22 AWG (0.64 mm Ø, 0.33 mm2) multi-strand insulated hook-up wire:
- 2.2 m (7+ ft) of wire for the coil itself.
- 1.5 m (5 feet) sticking out beyond each coil-end. This includes margin for "tuning-and-pruning" the antenna to the desired center frequency.
- note that the wire is not cut - I simply pulled 2.2 + 1.5 m through the starter hole in the coil core, wound the coil, and pulled the remaining 1.5 m wire through the exit hole of the coil core.
- With this type of wire, the required 32 turns produce a coil that is just over 5 cm long (2"). Drill feed-through holes with a distance of 5.1 cm (2") between the holes (not center-to-center) to nicely fit the coil. See photo below. I used a 5/64" (2 mm) drill bit.
- The wire is fixed in place with small dabs of waterproof glue on the outside of the first and the last turn. I have used both the universal 2-component epoxy glues "UHU Endfest 300" and "Eccobond 286 Blue" (which even holds up in the dishwasher and the microwave oven). Gorilla Glue® also works.
As always, "Harry's Law of Coils" applies. As Harry (SMØVPO) says
- You cannot wind coils like I, and I cannot wind coils like you.
- Coil-winding data is a constant that varies from person to person.
Two blank PVC coil cores and a finished 32-turn coil
(notch of groove is visible in top blank core)
Loaded-dipole calculators (e.g., ref. 10A/B/C) suggest that - for the given dimensions of the antenna - the loading coils should have an inductance of about 10 μH. Coil calculators (e.g., ref. 11A/B/C) suggest that the KJ5VW coils have an inductance of about 9 μH. That's in the ballpark - and, as over 30 years of professional engineering experience has taught me: anything within ±20% is within engineering accuracy, hi! Out of curiosity, I checked the actual inductance with my miniVNA antenna analyzer (series & parallel resonance) and with my dip-meter (ref. 12), and found 8.9 μH. Of course, accuracy of the measurement is affected by the tolerance of the capacitor value. When new, ceramic disk caps typically have +80/-20% tolerance, milar polyester typ. ±5 or ±20%, tantalum typ. ±10 or ±20%, metalized polypropylene typ. ±5, ±10, or ±20%, electrolytic typ. ±20%, etc. Note that cheap LC-meters typically measure inductance and capacitance well below 100 kHz, and the indicated values are not valid at RF frequencies.
Test set-up for parallel-resonance inductance measurement
My coils are cylindrical and wound on a PVC core. The same inductance can be obtained with small toroidal iron powder cores: e.g., about 30 turns on a T-130-2 core (1.3" diameter), and about 45 turns on a T-50-2 core (0.5" diameter). Ref. 13. You may have to stack 2 or 3 such small cores to get 4x or 9x power handling.
The components of my antenna (2 m PVC mast included, fishing poles partially collapsed)
I used 2 meters of PVC tube of 4 cm diameter as a temporary mast, stuck into a heavy umbrella stand. The antenna boom is made of the same material: 2 sections of 135 cm (4 ft 5") in length (cut from a standard 3 m section). They are mounted on top of the mast with a T-piece. I did not glue the sections into the T-piece. This way, it is easy to disassemble and experiment with the boom length. Once installed into the T-piece, two long nails prevent the boom-sections from turning and slipping out, and the T-piece from turning about the mast. Obviously this won't hold up under windy conditions, but it is fine for experimental trials and fair weather mobile/portable use. The fishing poles fit tightly onto 18 mm diameter dowel rods that run through the ends of the PVC boom sections. The dowels were cut to length such that the fishing poles are almost up against the boom when they are fully "impaled" on the rods. Shrink tube was used at the tip of each pole segment, to keep the wire taught. For the same purpose, tie-wraps were used at the base of the poles (didn't have shrink tube of sufficient diameter). It takes about 10 minutes to assemble or disassemble the antenna.
Close-ups: quick-disconnect of the reflector and T-joint of the PVC boom and mast
My Yagi, fully assembled and installed
As indicated above, I first built the dipole by itself. By trimming the wire on the outside of the loading coils, I tuned the dipole to about 14150 kHz, so as to be able to cover both 14230 kHz (SSTV) and digi-modes around 14070 kHz. The reflector has to be tuned to a resonance frequency that is about 5% lower than that of the dipole. In my case: 0.95 x 14150 = 13445 kHz. This requires that the wires on the inside and/or the outside of the loading coils be longer than those of the dipole. Alternatively, the loading coils can be made larger.
An SWR plot of the antenna is shown below. The original KJ5VW design indicates a 2:1 SWR bandwidth of about 250 kHz. My Yagi has about 220 kHz bandwidth. I did not play with the antenna geometry to get to SWR = 1. My transmitter has no problem with SWR = 1.34. I also do not have an acceptably reliable way to determine the radiation pattern.
SWR curves from original KJ5VW design vs. mine
Idea: crimp-on ring terminals with a rubber grommet for the tip of the fishing poles
OTHER 2-ELEMENT WIRE-BEAMS
The Yagi described above is a straight-forward 2-element beam antenna. The driven element and the reflector are straight and parallel. There are many variations that have a bent driver and reflector. These shapes allow a significant reduction in size (and turn radius) compared to a full-size 2-element Yagi - without having to resort to loading coils and associated losses. On top of that, they can have larger forward gain and significantly larger front-to-back (F/B) ratio than the standard full-size configuration. Ref. 14.
Some reduced-size 2-element wire-beam configurations - size and turn radius referenced to full-size
(source: ref. 14)
- Ref. 1: "Yagi Antenna Design", Peter Viezbicke, NBS Technical Note 688, US Dept. of Commerce, Nat'l Bureau of Standards, December 1976, 30 pp. Source: NIST
- Ref. 2: "Simple gain antennas fir the beginner", Doug DeMaw (W1FB), QST, August 1981, pp. 32-35
- Ref. 3: "5-Band Yagi (20M/17M715M/12M/10M)", product brochure of JK Antennas. Note: I cannot endorse this antenna, as I have no experience with it. The manufacturer is welcome to contact me, to donate such an antenna for trials.
- Ref. 4: "The KJ5VW 20 Meter Mini Yagi" by Gary Hanson, KJ5VW
- Ref. 5: "Build a high-performance low-profile 20-meter beam", Cornell Drentea WB3JZO), in "Communications Quarterly", Spring 1993, pp. 85...
- Ref. 6: "Monoband Yagi for 20 meters - more dBs for the buck" by Ken Kemski, AB4GX (aluminum tubing instead of wires; still center-loaded but the director and reflector are about 1m2 / 4 ft longer).
- Ref. 7: "The G3YCC 'Shorty' Dipole for 14 MHz" by Frank Lee (G3YCC, SK)
- Ref. 8: "Shortened Dipole Study for Conditions On BVARC’s Rag Chew Net", Larry Jacobson (K5LJ), Rick Hiller (W5RH), expanded from same-title article in "Newsletter of the Brazos Valley Radio Club", September 2009
- Ref. 9: "Element Loading to Achieve Dipole Resonance", part 3 of "Half-Length Dipoles (for 40 Meters", L. B. Cebik (W4RNL, SK)
- Ref. 10: Dipole loading calculators
- Ref. 10A: "On-line short Off-Center-Loaded dipole calculator by Martin Meserve (K7MEM)
- Ref. 10B: "Shortened dipole calculator (.exe)" by Alexander Stirling (VE3KSK)
- Ref. 10C: "Loaded dipole calculator (.exe)" by Al Legary (VE3SQB). Note: the GUI of this calculator shows inductance as "mH" instead of "μH"!
- Ref. 11: Coil calculators
- Ref. 11A: "Helical coil calculator" on pages of the Tesla Coil web-ring.
- Ref. 11B: "K1QW Inductor Calculators"
- Ref. 11C: "ON4AA Single-layer Helical Round Wire Coil Inductor Calculator"
- Ref. 12: "Measurements of an antenna loading coil" by Frank Dörenberg (N4SPP), 13 August 200
- Ref. 13: "Turns-length calculator for ferrite and iron powder core toroids"
- Ref. 13: "A family of wire beams" by Steve Hunt (G3TXQ), May 2008 [pdf]
External links last checked: October 2015
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