Dave wanted a small 6m Yagi for portable operation from his Camper Van. It had to be light-weight and compact to be supported on a Buddipole telescopic mast, and break down to less than 1.5m lengths for easy stowing inside the van while travelling.
A 3 element design was selected that provided good gain and radiation pattern on a 2 meter long boom. It has a split dipole feed point impedance of 28 Ohms which could be matched to a 50 Ohm feedline with a 'DK7ZB match' (essentially an integrated balun/transformer) made from two 1/4 WL pieces of 75 Ohm coax in parallel ('electrical' 1/4 WL including the 75 Ohm coax's velocity factor).
The boom was made from two pieces of 25mm square aluminium tubing joined by a short length of 21mm square tubing recovered from the 'antenna graveyard' behind my shed. The elements were made from 12.7mm diameter and 9.6mm diameter round tubing from another aluminium scrap pile. The original antenna .nec file was modified to suit the tapered elements, and tweaked to provide low SWR at 50.3 MHz for 6m band FT8 operation. Here is the 4NEC2 computer analysis plot of the final tapered element design ...
The Yagi was built in an afternoon and tested at 6m high using my Sark-110 antenna analyser which was calibrated including the 25m of feedline to the Yagi thus accurately representing the Yagi's feedpoint impedance after the balun/transformer. A small adjustment to the driven element length was required to compensate for the 'coax tail ends' and lugs on the 'DK7ZB Match', then we obtained this result ...
We checked the Yagi's F/B using the local 6m beacon as an on-air signal source, and measured 28dB which is in line with the 4NEC2 prediction for F/B at the bottom end of the band.
The travelling package is just 1250mm long ...
The split dipole feed point mounted on a piece of white HDPE (kitchen breadboard) with Stauff pipe clamps. Stauff clamps are also used to fix the reflector and director to the boom ...
Here is the assembled Yagi installed on a portable mast in Dave's back yard ...
