unconventional link dipole for 20/30/40m
last updated 30 November 2024.
This project came about because I had recently acquired a small QRP transceiver that covers 20, 30, and 40m. While my usual backpack dipole kit will work, I decided to make an unconventional link dipole to cover the 3 bands, because it is always fun to try something new. It uses the minimum amount of wire, and only 2 links rather than 4 as a conventional link dipole. (This approach works best when the bands are relatively closely spaced.) It can, of course, be adapted to other band combinations.
My standard configuration for designing portable dipoles is an inverted vee with the center at 6m (20 feet) and the ends at 3m (10 feet), which is broadly typical of many of my portable installations. At that height, the 20m center fed antenna is very close to 75 ohms by itself, so I used that as a starting point. One leg has a link with an extension to lower the resonant frequency to 30m (fed rather off-center), then the other leg has a link and enough wire to make the whole antenna resonate on 40m (fed a little bit off center) when the 30m link is also closed. The off-center feed puts more stress on the balun than for center feed, but many 1 : 1 current baluns should still work well enough.
As it turned out, both 20m and 30m were very close to 75 ohms on my models, and 40m was about 60 ohms. While the SWR is still acceptable using 50 ohm coax, this is a likely candidate for 75 ohm cable. (Many HF rigs work quite well into a 75 ohm load.) Decreasing the angle between the wires (by raising the center and/or lowering the ends) will lower the feedpoint impedance somewhat, and improve the SWR on 50 ohm coax.
details
My initial model called for the center 20m dipole of 5.1m (16.75 feet) on each side of the feedpoint. The 30m extension wire was 4m (13.25 feet), and the 40m extension was 6.2m (20.25 feet). Those dimensions are for bare wire: in practice, insulation will decrease the required wire length, while knots to secure the wire to ropes or insulators will tend to require more wire. So expect to do some adjustment.
construction
I used my usual construction methods. Wires are 0.4 to 0.7mm (AWG #22 – 26) insulated, stranded hookup wire. My initial thought was to color code the wires using the resistor color code: red (2) for 20m, orange (3) for 30m, and yellow (4) for 40m. But the only orange wire I had was rather thicker than the others, so I used a big spool of green/white (59? 95?) instead. Insulators at the links are short pieces of nylon twine.
For testing the prototype, the wires were just twisted together at the links. The final version will likely use alligator clips on the ends of the 20m wires. While I generally prefer using Anderson connectors, the clips also permit field tuning of the antenna if needed by clipping the wire ends back on the standing part of the wire, as explained here.
The antenna was attached to the insulator and coax from one of my portable dipole kits that includes a balun (which is more important with this design than a normal dipole), then hoisted up in my backyard antenna test range at roughly the target heights.
I never measure the actual length of my antennas. What is more important is how much wire I need to cut off the spool – that accounts for knots, etc. As usual, I started with the wire a bit long, and trimmed it to resonance. In this case, after tuning I ended up with exactly the modeled length of 5.1m (16.75 feet) for the 20m dipole. Minimum SWR is 1.6 : 1, and it covers the whole 20m band under 2 : 1 SWR.
The added wire for 30m was 4 cm (1.5 inches) shorter than my model. Minimum SWR was 1.7 : 1.
The added wire for 40m was 10cm (4 inches) longer than the model. Minimum SWR was 1.2 : 1, and the 2 : 1 SWR bandwidth was about 300 kHz.
SWR was lower on the two higher bands when the antenna was at a lower height (because I forgot to raise it back up after making adjustments). They will also improve somewhat if the antenna is mounted higher and the wires run down at a sharper angle, closer to a 90 degree apex angle.
I haven’t tried it yet using 75 ohm coax. But the antenna is certainly usable as it is.
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