A 12.5-ohm four-element by Martin Steyer, DK7ZB, for 2 m SSB/CW. Aluminum tube, insulated above the boom. This page scales his 144.3 MHz table with wavelength. It is his design. The 22 dB front-to-back listing is on his portable-Yagi notes.
Piece
Length
Reflector
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Spacing · reflector to driven
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Driven · tip to tip, 10 mm tube
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Spacing · driven to director 1
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Director 1
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Spacing · director 1 to 2
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Director 2
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Element span · reflector to director 2
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Boom · his listed 1.00 m
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Published gain and front-to-back
On the 1.00 m boom his summary table lists 7.5 dBd and 22 dB front-to-back. The dimension page states F/B > 20 dB, gain 7.5 dBd, half-power widths 54.4° horizontal and 72.8° vertical, and a pattern marked 7.54 dBd at 144.3 MHz. 7.5 dBd is about 9.7 dBi, higher than the +8.15 dBi printed for the foil Yagi, with a deeper rear null than the 8–15 dB of gain-optimized short booms. His 1.25 m four-element is listed at 8.2 dBd and 21 dB front-to-back, so this shorter boom is the one with slightly less gain. He describes the 12.5-ohm set as high gain for the boom length, with a narrow band: 144.0–144.8 MHz.
Build notes
Aluminum tube in free space. Parasitic elements are 6 mm or 8 mm on 2 m; the radiator is 10×1 mm tube on both rows of his table. Lengths here are that table scaled with wavelength, and the tube diameter scales the same way. There is no substrate K.
Mount the elements insulated above a 15×15 mm square aluminum boom, with polyamide clamps or 3–4 mm of clearance. A screw in the electrical center of an element does not change the length. He says contact with the boom detunes these Yagis.
The driven-element length is tip to tip and includes the insulated gap, 10–12 mm on 2 m. The radiator is two halves on an 8 mm insulating rod. Ground the coax socket to the boom.
The feed is 12.5 Ω. The match is two 50 Ω quarter-wave cables in parallel. Coax lengths for 2 m are on his construction page.
He gives good SWR from 144.0 to 144.8 MHz, and a trim of about 1–2 mm off each radiator tip for SWR 1.0 at 144.3 MHz.
Enter 144.3 MHz and the cuts match his millimeters. With 6 mm parasitics: reflector 1068 mm, driven 985 mm, directors 961 mm and 930 mm, positions 200 mm, 505 mm, and 985 mm. The 8 mm parasitics are 1066, 985, 954, and 921 mm. He lists the boom as 1.00 m.