Re: [amsat-bb] Homebrew 70cms Crossed Yagi

[K5OE@xxxxxxx]

howard@howardlong.com writes:
> The designer is Kent Britain WA5VJB. All I did was use a hacksaw.
> 
> But, suffice to say, the J pole DE configuration allows (a) the use of an
> unbalanced feed and (b) raises the impedance to 50 ohms.

WA5VJB's design is easy to verify with most any antenna modeling software.  The J-feed is an impedance transformer with a ratio of 3:1.  Thus, if you model the design with a dipole (simplest method), you will see a feedpoint impedance at resonance of about 17 Ohms.  This is a good compromise between a dipole at a nominal 25 Ohms (requires a matching transformer) and a folded dipole at 12.5 Ohms (does not require a transformer due to the 4:1 ratio, but is very "touchy" to tune).  

Modeling this antenna, as designed by WA5VJB, will show a good compromise also between F/B ratio and gain.  He appears to have given up a little gain, maybe 1 dBi, to produce an easily reproduceable design with excellent F/B ratio and good bandwidth (translates to:  "easy to tune").  I highly recommend it for a weekend project.  I have one up with 10x2 elements made from 3/4" PVC pipe and #10 AWG bare wire.
73,
Jerry, K5OE
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