2 Meter 7 Element Yagi for the QTH
The source for the dimensions and layout of this antenna project can be found here: https://archive.org/details/73-magazine-1993-09 The article is on pages 34 & 36 and entitled - recycled TV beam to 2 meters for 2$.
Although it was intended that an old TV beam be used to build this antenna, I had already bought some aluminium with the building of a 2 meter Yagi in mind, though not yet having decided on any design.
This particular design appealed to me because although having seven elements, was shorter than other designs I had seen. I wanted to keep it as small as possible so as to not draw to much attention. Also the antenna was not mounted as high as would be ideal, this too was a compromise in order to make the antenna as unobtrusive as possible.
The boom section was not quite long enough and so was extended with a short length of tubular section inserted in the end as can be seen in the picture above.
Through the boom and electrically bonded mounting of the elements was chosen for this beam for a sturdy construction.
Because of this I made the elements 30% of the boom diameter longer than the specified dimensions shown in the picture above.
This was going by a general rule of thumb correction of about 25…45 % of boom diameter.
For the gamma match adjusting clamp, I cut a piece of thick aluminium plate 25mm x 70mm and drilled to holes for the driven element and gamma match.
Then placing the plate in a drill clamp, a hole was drilled through the middle of the side with a pillar drill.
The drill size chosen to match the 6BA thread Tap I intended to use. I had a large quantity of hex head bolts of that size which I would use.
Next step was to cut the plate in half down its length through the centre of the element holes while firmly held in a vice.
One half was taped with a tread for the fixing bolt, The other half hole was opened up with a reamer to give free clearance for the bolt to pass through.
Also a recess was filed into the side at the hole to counter sink the bolt head.
The completed clamp assembled and clamping the gamma match to the driven element.
This clamp proved to give a good strong grip and was easy to adjust.
A right angled bracket was made and drilled out to take the PL259 socket.
I taped 6BA threads into the socket fixing holes and using the same hex head bolts secured to the bracket. Excess tread was then cut off.
This assemble was then fixed to the boom with self-taping screws at the appropriate position.
The Gamma Match construction
My gamma match is a deviation from the original construction where the inner of the coax was slid inside the gamma tube. My Tube was of a larger diameter and because of this, the coax inner was far too loose and likely move side by side in windy weather. I felt this movement would produce fluctuations in the capacitance of the gamma match and may cause problems.
The construction shown here was far more ridged and the antenna was easily tuned to the SSB portion of the band with a low SWR.
After tuning up was complete, heat shrink tube was placed over where the gamma slid in and out and a plastic stopper inserted at the open end.
Five clip on ferrite chokes were placed on the coax just behind the PL259 plug and sealed and bound to the boom with self seal tape.