Tom w8ji makes some interesting but uninformed comments about the Tennadyne
feed system. The booms are in fact the feedpoint for the antenna and the
collins balun belongs at the transition from the unbalanced coax line, to
the balanced line necessary in any log periodic construction. The parallel
booms substitute for the balanced line and make the antenna much more
efficient. The potential of the lower (or upper) boom is the same as that
of the feed line. The booms are simply aluminum feedline, they are not
radiators, just balanced line. The idea that the balun at the feedpoint
forms a dead short is simply untrue when you study the construction if that
were true every antenna forms a short at some point because the shield
grounds the antenna. Whether it is a log, a vertical, or a yagi.
By the way you do not have to strap the coax to the lower boom if you don't
want to, it can drape down or use separators if you like, but they are
unnecessary. It is true that you have to be careful not to have any barrel
connector touching the boom but a single layer of tape is sufficient
insulation. You can also add a RF choke if you like, again it isn't
necessary but it would prevent spurious emissions from the unbalanced
coaxial feedline from the choke to the antenna.
I own 2 models of Tennadyne LPDA's (1 HF, 1 VHF) and both work
wonderfully. No RF on the coax either, hi hi! I have also owned The
Sommer hybrid log, as well as the other generic brand, as well as F12 C4E,
and other yagis but none performed with the pattern and forward gain of my
T-10. I have never owned any other VHF log so I will not comment on others
in that field.
Chuck designs and sells antenna systems to military and civilian entities
all over the world, I am sure they would note any problems that I have
not. My advice is to examine the theory of Log Periodic Dipole Antenna
construction, I found it fascinating. Once you have, I believe that you
will find that no one exceeds the quality or efficiency of Tennadyne.
I respect Toms idea but it simply isn't true if you study the construction
of the Tennadyne LPDA. You can have my pair of Tennadynes when you pry
them from my cold, dead, fingers as the saying goes. (No, I am not related
to Chuck, I don't get paid, or a discount etc......)
Thanks for your time. Please no flames, this is meant in discussion not
attack of any other brands.
Excerpt of comments to save bandwidth:
W8JI wrote: " That means the shield leaves the antenna with voltage on it, and
also that the balun at the feedpoint is electrically shorted and does
nothing.
Assuming the shield connects to the lower boom and the coax
runs along that boom, the correct location for the balun is at the
exit point where the cable leaves the boom and goes to the tower.
Most certainly not at the feedpoint! "
73,
Frank, kb8xu
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