[TowerTalk] 100ft 80meter Yagi
coolican at telus.net
Fri Mar 31 14:21:57 EST 2006
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.
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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