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Re: Cheap Yagi Antennas for VHF/UHF: experiences?



John,
The arrangement you describe is exactly the design of the "AO-40 Yagi" I have 
on my web page.  Kent uses a feedpoint design of 17 Ohms and accomplishes a 
"transformation" of 3X using the J-feed element to get 50 Ohms.  I used a 25 
Ohm design with a 4X transformation from a folded-dipole feed to accomplish 
100 Ohms:
http://members.aol.com/k5oe/index.htm

For whatever reason, I found the folded dipole easy to tune on 2 m, but very 
touchy on 70 cm version (and never got a perfect SWR like I could get on 2m). 
 Kent's J-feed is quite easy to tune if built per his specs.  Of course you 
could play with the design to accomplish any feedpoint impedance, but 
generally gain and feedpoint impedance are inversely proportional.
73,
Jerry, K5OE


tosca005@tc.umn.edu writes:

> Kent's originally designed this series of "cheap yagis" first to get a
>  good 50 ohm feedpoint impedance, and then he worked out the rest of
>  the elements for the other properties like gain, F/B, etc.  I wonder
>  if the design is flexible enough that he (or you, or someone else)
>  could re-do it so that the feedpoint of each part was 100 ohms.  That
>  way, you could combine the horizontal and vertical feeds (or more
>  likely, the / and \ diagonal feeds) without any additional impedance
>  transformation -- no 75 ohm quarter-wave transformers or 2-port power
>  divider, just relay selectable phasing lines for the different
>  polarities, right into a T connector (two 100 ohm antennas in parallel
>  = 50 ohms).  I'd love to see such a design and attempt to build it, if
>  it worked.  Seems like it would be in keeping with the KISS and keep
>  it cheap philosophy.  (Ok, once you add 4 coaxial relays to get 4
>  different polarities, the cost and complexity go up.  But hey, if you
>  were willing to settle for just a single, non-switchable circular
>  polarity, it really DOES stay cheap and simple!)
>  
>  Just thinking out loud.  (I have a great desire to get started in
>  antenna modeling and design, but no time to begin the process now.
>  I've got so many pending projects right now that I'm sure I'll never
>  be bored for the rest of my life!)
>  
>  73 de KB0ZEV
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