September 2021
A stacked array of three 5 element 6m Yagis was built to replace the YU7EF EF0606 Yagi which vibrated in strong winds due to the light weight materials used. The 5/5/5 array should provide 3dB more gain than the EF0606 (e.g. similar gain to an 11 element Yagi on a 50ft boom). The 5 element design has a good radiation pattern for its size and 'stacks well' (the F/B and minor lobes remain 'under control', unlike some short boom Yagi designs). Here is a screen shot of the 4NEC2 computer analysis for the 3-stack system ...
Material for two of the Yagis was recovered from the 'antenna graveyard' behind my shed, and material for the third Yagi came from the old EF0606 antenna. The element lengths from the Yagi's .nec file were increased by 4mm as the Boom Correction factor required for the particular element mounting system used (Nuxcom element mounts on a square aluminium boom). The design was also tweaked to provide low SWR at 50.3 MHz to suit 6m band FT8 operation.
A split dipole feed point with an impedance of 28 Ohms was used and matched to the 50 Ohm coax feedline using a "DK7ZB match".
The first two Yagis were tested individually on a 6m length of scaffold tubing to check their 'tuning' using a Sark-110 antenna analyser.
The analyser was 'OSL calibrated' including the 25m length of length of LMR400 feedline used, so the results should be a fairly accurate representation of the Yagi's feedpoint impedance. After a small adjustment to the driven element length to compensate for the 'coax tails', the SWR curve was almost identical to a 4NEC2 plot for one Yagi on it's own.
A three-way power divider was made from aluminium and copper tubes, and N-type connectors.
UPDATE late-August 2021: The tower was tilted down and the EF0606 Yagi removed (to be reconfigured as the third 5 element Yagi). The rotating mast was extended from 8m long to 9.7m long to accommodate the 3-stack array (stacking distance = 4.7m + 4.7m). Three LMR400 phasing lines and a new 'rotator loop' cable were cut to exactly one electrical wavelength long using the SARK-110 antenna analyser.
UPDATE 05 September 2021: Finally the winter weather cooperated and the 3 Yagis could be mounted to the extended rotating mast ...
The array's SWR curve is almost exactly as shown in the 4NEC2 diagram above ...
The F/B as seen on the local 6m beacon is about 25dB ...
The Auckland 6m beacon 200km to the SSE is stronger than ever before ...
This antenna system was used on 6m until May 2022 and then was dismantled as part of the 'revised tower project'.
The 3 x 5 element Yagis were reused in November 2023 as a second 6m antenna system on a shorter tilt-over mast as seen here.