I am planning on building an 80m yagi to cover the CW portion and hopefully
retain good performance up into the phone band up to 3800. I currently have a
4 el OWA design on a 76' boom that covers approx 3650-3975. It has been up for
6 years and performs well, but not into the CW portion. The new 80m yagi will
not replace the one I have, but will be installed on a similar 150 foot tower.
I am planning to use a 100 foot tower that I have as a boom (40" triangular
lattice structure) with perhaps 5 full size elements as an OWA design if it
has enough bandwidth. I am not interested in loaded designs with relay
switching. The elements will be of the tapered pipe design, as is the current
one.
Denis VE6AQ designed and we both built a 40m yagi 5 element OWA design on a 59'
boom that has wide bandwidth - about 400 khz - that if scaled would be
adequate for 80m coverage. That would require a 118 foot boom, but I am hoping
I can squeeze it down to use the existing 100 boom that I have. It isn't
impossible to add another 20 feet but I don't have the matching tower section
to do that so would like to avoid that if possible.
As it correlates to current towertalk threads, the tower will be guyed with
multiple screw in anchors. The guy wires (5/8 EHS) will be broken up with at
least one insulator to reduce potential corrosion issues. The rotor will be at
the bottom of the tower and I will be using 4" IPS sked 40 black (4.5" OD water
pipe with welded flanges) for the mast. I have used this on the current 80m
yagi without any problems. The mast near the top will be something
significantly better.
Some design innovations that I have thinking about include:
having the elements winched into place to allow for modification and
repair by lowering them to the ground.
having a heavy steel 4 sided "pyramid" shaped structure on the top of the
mast. A matching structure will be built into the boom, so that the antenna
can easily be lowered (by crane) over the self guiding structure and simply set
in place without any critical placement issues. It will be bolted down later
for security.
Anyway, the main issue at hand is the electrical design not the physical. I
will have lots of time to ponder the mechanical aspects once it has been
decided how much boom and how many elements are needed.
73 Don
VE6JY
A note from Denis Ve6aq
I have modeled the 100 ft Yagi on EZNec and it can be downloaded from
http://alfaradio.ca/ve6jy/downloads/ The spacing between the DE and D1 is
0.025 wavelength and I noticed that some of the modeling programs give
inconsistent results at these close spacings. I would like comments on the
design and perhaps someone could run it on a NEC4 engine to see if the results
are similar to what I have obtained. Comments would be appreciated.
Denis with Ve6Fi
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