|Subject:||[TowerTalk] Re: 20M Yagi|
|From:||David J Rodman MD <email@example.com>|
|Date:||Sun, 12 Dec 2004 07:28:29 -0500|
Mike, I have designed, matched and built many HF yagis, including a
20/15m interlaced yagi over 15 years ago. Computers were not that
sophisticated then and their use was rather new and programs were not
too sophisticated. Fortunately, now many sources are available and
there is considerable information on the physical design. Experienced
yagi builders know what taper works and how to build a boom but first
timers will not be sure. Even if you are not used to working with
aluminum stress numbers you can and should purchase and or download some
software to assist you in developing your project. This goes for the
electrical and physical characteristics. First, you will learn
something and secondly, you will be able to build a product which is
more likely to be repaired in a remote location of the world with the
hardware on hand rather than shipping parts from some manufacturer.|
From my experience, you should use 3" OD boom material (if it is available) for this project. Yes, 2" is used for some antennas but without seeing what you are doing or running a stress calculation, I would not guess how practical your project is. In my experience, you want to take your time in the design phase, get it right, then move on to constuction.
As far as matching this antenna, I have used gamma matching and hairpin matching. You should spend some time reading on the types of matches and construction before you decide. For example, you must know if the match is happier with either a capacitive (-j) on inductive (+j) reactance and set this accordingly. A great idea is to use the coaxial transformer method as noted in other posts too. What ever you do, the match is roughly set by selecting the driven element (DE) length for resonance then setting the match for best VSWR at that frequency. They interact a bit so double check. The project you propose is relatively simple with two elements. My antenna had 8 elements and there was considerable interaction before I found the best match.
Use sufficient conductive grease on the elements. I recommend stainless screws. You can rivet or screw elements together. Coat electrical connections with liquid electrical tape or some paint that will be long lasting and cover the material against corrosion from the elements.
Good luck. _______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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