Hello all,

I built in the years several Yagis (and other antennas). Until now all
Yagis I built were calculated with the great program from DL6WU and
fed with folded dipole and 4:1 coaxial balun with good results. A
couple of them has been fed with gamma match and are working good but
needed a lot of tweaking to have low SWR.
Two weeks ago I built a 6 elements yagi for the 70 MHz band, the
design this time comes from YU7EF (http://www.yu1arc.com/yu7ef/
EF0406.htm), it is made with 10mm elements supported over a 26mm boom
(both aluminium). Element to boom spacing is 25mm (end to end) but
there is a steel plate (35x55x2 mm) between each element and the boom.
The plate is located ad 10mm from each element end (for end I mean the
5mm radius from the center of the element). Each element is supported
in a plastic shell bolted to the steel plate with two steel bolts (not
touching the element).
I didn't apply any correction factor to the elements, is this right or
it's my first error with this antenna?
For the feeder I decided to try with a T match as described in the
DL6WU program and the program itself gave me a 504mm as length for
(each?) arm of the T match. The "dipole" isn't split in the middle
obviously. Then there's the 4:1 coaxial balun and then about 12m of 50
ohm cable. I hadn't enough aluminium tubing for a folded dipole.
The antenna is mounted about 2m from the tip of the roof (V-inverted
roof) and about 2m down a 2-lambda 2m yagi (no other place for it,
really).
I expected to have a poor match and infacts with 1W forward I first
got 100mW reflected (with the T shorts about at the full length of the
matching arms). Also the actual reflected power depends on the
direction of the antenna, beeing the roof quite polluted with metal of
other antennas and poles, but it never reflects less than 80mW with 1W
forward.
I then tried moving the shorts toward the boom to find a better match,
but to my surprise I discovered almost no effect on reflected power.
Now the shorts are about 220mm from the end of the matching arms and
reflected power now peaks at about 85mW instead of 100mW, seems like
too independent from the short position to be true.
The antenna indeed shows a directional pattern as it should. I tested
with local noises and during an ES opening (heard a beacon 2000 km
away, peaking at S7 when correctly turned and almost no audible with
the side of the antenna), I also worked a station at about 2000 km
with power output of less than 4W (but yes, during ES openings you can
make QSO even with 1W in a dummy load).
Now I really wanted to understand why this T-match is so "broad", what
errors I might be doing? Can the poor placement of the antenna justify
this behaviour?
Thanks in advance

Francesco IZ8DWF