My next antenna project is a compact 2-element yagi wire-beam for the 20 meter band. It is based on designs from KJ5VW:

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The KJ5VW 20 Meter Mini Yagi which is an expansion of The G3YCC 'Shorty' Dipole for 14 MHz

This famous type of antenna was invented in Japan in the 1930s, by Dr. Hidetsugu Yagi and Dr. Shintaro Uda (so it should actually be referred to as a Yagi-Uda antenna). The 2-element version is the simplest form: a dipole as driven element, and a reflector in parallel. Multiple director elements can be added in parallel to the driven and reflector elements:

I will use the same construction techniques that I used for my 20 meter mini-dipole. Basically, I will have to construct a second mini-dipole, tune it to a resonance frequency that is 5% lower than the primary mini-dipole (i.e., make it 5% longer), connect the two dipole halves, and mount this reflector dipole parallel to the driven dipole at about 2.6 meters (8½ feet) distance (⅛ l).

Update 11 July 2008: I have purchased two more fiberglass telescoping fishing poles of 3 meters length. I.e., same as the ones I used for my 20 meter mini-dipole, but a different brand and different color (neon-orange on silver vs. black). Price had come down slightly, from €7 to €6.55 (about $10 mid-2008). Already had two additional coil cores cut when I constructed the mini-dipole. Wound the two coils in about 10 minutes each, leaving  plenty of wire (about 1m5, 5 feet) sticking out beyond each coil-end. The KJ5VW design is different in this respect: separate pieces of wire are used  for the coils and the dipole wires. I prefer my own solution (of course, hi!). Installed the coils on the poles, and the poles on the interconnecting dowel rod of the mini-dipole. Trimmed the wire on the inside of the coils to 1m20, such that the ends would easily meet at the dowel and be connected with crimp-on quick-disconnects. On the outside of the coils, I started out with 1m6 wire. With 15 meter coax, my miniVNA antenna analyzer found a resonance frequency of 12900 kHz. Trimmed 5 cm at a time (2"), down to 1m25 and a resonance frequency of 14472 kHz. I.e., roughly 50 kHz per cm or 125 kHz per inch. My mini-dipole has a resonance frequency of 14152 kHz (at the same location): I typically use frequencies around 14074 for Hellschreiber and 14230 for SSTV. So I needed to trim this reflector-dipole for about 14860 kHz. Trimmed twice more, this time 4 cm at a time, and arrived at 14890 kHz. Close enough! Note that I now have 120 cm on both sides of each coil. This is actually shorter than my mini-dipole!? Must be the color of the poles, hi!

Update 12 July 2008: have decided to use 2 meters of PVC tube of 4 cm diameter as a temporary mast, stuck into a heavy umbrella stand. The boom is made of the same material: 2 sections of 1m35 in length (cut from a standard 3 m section). They are mounted on top of the mast with a T-piece. I'm not gluing the sections into the T-piece. Once installed into the T-piece, two long nails prevent the 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; started with a 5 mm drill, then used several drills to get up to 13 mm, and finished with a conical grinding bit to get to just under 18 mm. 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).


The elements prior to assembly (2 m mast included, fishing poles partially collapsed)

  
 Close-up of quick-disconnect of the reflector                                          Close-up of the T-joint  


The beam, fully installed

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Antenna analyzer results: see plots below. The original KJ5VW design indicates a 2:1 SWR bandwidth of about 250 kHz . My yagi has about 220 kHz bandwidth between the frequencies with SWR 2:1.

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Front-to-back (F/B) ratio and directivity is still to be determined. Somehow need to do far-field field-strength measurements, e.g., with a beacon station or some other fixed-strength carrier at sufficient distance. KJ5VW did not indicate F/B ratio or radiation pattern in his write-up.

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Assembly time: less than 10 minutes

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Total weight of the antenna is 2.4 kg (5.3 lbs), excluding the mast. Note that KJ5VW claims almost 3 times this weight (just under 6.8 kg or 15 lbs) for his version of this antenna !

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Total cost: about €45  ($70 against mid-2008 exchange rate). It would have been a lot less if all components had been bought in the US, and obviously using fishing poles make it more expensive. KJ5VW claimed $30 for his version several years ago...

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2 oak (!) dowels @ €2.75 ea.

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4 fiberglass fishing poles @ €7.00 ea.

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3 m PVC boom, 40 mm diam. @ €5.00

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PVC T-piece, 40 mm diam. @ €1.75

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20 ft of 16 AWG wire @ €2.00

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4 PVC coil cores @ €0.25 ea.

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BNC jack @ €1.50

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3 large nails @ €0.10 ea.


Plot from the antenna analyzer (antenna + 12 m coax)


SWR results from antenna analyzer: original KJ5VW design vs. mine

Now I need to decide whether I want to to reduce the turn radius of the beam, by clipping the 40-50 cm of the fishing rods that stick out beyond the wire. If I do, I may crimp a ring-spade terminator lug onto each clipped tip (18 AWG size should do, with or without a rubber or silicone grommet), as an attachment point for the antenna wire.

And I need to buy a small antenna rotor. Maybe this one... Or I'l make one myself.

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If for some strange reason you need to run (much) more power, then you may want to beef things up a bit. You can glean some ideas from "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).



Last update: 13-July-2008
©2007-2008 F. Dörenberg N4SPP

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