[TowerTalk] Re: 20M Yagi

[David J Rodman MD]

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.