[TowerTalk] Mystery Hy-Gain tribander

[Joe Subich, K4IK]

> Alan C. Zack wrote: 
> 
> You will have to argue with the Mosley engineers, not me.  
> That was a direct quote from:
http://www.mosley-electronics.com/faq.htm
> Works fine for me plus a whole bunch of LORAN station ham stations.

Quite simply Mosley's "engineers" are dead wrong.  You need only 
refer to the ARRL Antenna Book, the NBS work on Yagi antennas, or 
any of dozens (if not hundreds) of academic papers in the field. 
If you still do not believe the literature, buy a copy of YO (or 
other good optimization program) and build some experimental antennas 
in the computer.  

The trends are very clear, unless one is using some special techniques 
for antenna design ... the OWA or "open sleeve" feed systems are two 
examples ... antennas with 50 Ohm driving point impedance are 
inferior to those with driving point impedances in the 15 to 30 Ohm 
range.  For example, for a 14 foot boom three element 20 meter Yagi 
(typical three element tribander) free space gain is as much as 1.5 dB 
higher and F/R is as much as 10 dB better when the feed impedance is 20 
Ohms vs. 50 Ohms. 

In order to achieve a "native" 50 Ohm feed impedance, one must reduce 
the coupling (and thus the effectiveness) of the parasitic elements.  
This can be one by either detuning the elements (making the reflector 
"too long" and the director "too short") or by making them farther 
from the driven element.  Making the boom longer will assist in 
improving gain (e.g., a 20 foot boom antenna has higher potential 
gain than an antenna with a 14 foot boom) but even with a 20 foot 
boom a native 50 Ohm three element Yagi is inferior to a 20 Ohm 
Yagi by more than 1 dB in gain and 10 dB F/R.  

73, 

   ... Joe, K4IK