A little yagi and a field test.

This summer I built a little 2-element yagi. The goal was an easy assembly and to have it portable into a bag so each single part must be no longer than one meter. Btw this forced me to split elements into four different tubing section. The yagi was feed via gamma match, as usual. Nothing new, you're right.

Later on October, during the AT World Contest, I raised up an hybrid antenna. This needs more explanation ... I was used to run an old Galaxy 4-element at contest, that's a clone of the Moonraker-4. Because we were *only* able to classify 9th at Italia Contest 1999, Sandro 1948 saw me: "this antenna is dead, it's deaf". Wasn't the antenna the problem of course, we moved this year to a new contest site and that turned out to be a bad choice, but we had to try to know it so I'm happy anyway. But I had decided to convert the gamma-feed Galaxy into a direct feed one. I used the quad reflector, the old directors without any change but put up a new dipole as radiator.

Why this looong story, you say ... I had used the computer to project a new spacing and the new radiator, to obtain a 50 Ohm direct feed, and when I put up the antenna I was enough happy to find a fair match between the expected tune and the real antenna.

After the World Contest I was thinking ... I built tens aerials but a lot were yagis with gammas, or folded dipole and 4/1 balun at vhf, a lot were wire antennas where swr is a compromise. I newer ran an extensive test on software ability to forecast the antenna impedance and swr.

So now we moved back to that little yagi we started from. I built a new radiator for the 50 Ohm direct feed and I used it to run more tests. Here half antenna's dimensions and the Eznec card.

    dia. 20 mm. dia. 18mm dia. 12 mm dia. 10 mm
reflector at 0.0 m. 50 cm 97 cm 56 cm 83 cm
driver at 1.7 m. 46.5 cm 97 cm 55.5 cm 59 cm

Tests were run with 50w power, 20m of H100 cable and the Revex W560 as swr-meter. Yes I know I'd better use an HP network analyzer :-)

Anyway the below tables summarize expected and measured swr. I think the experiment was successful.


Expected and measured SWR at different height over real, average quality, ground

3 m. 4 m. 5 m. 6 m. 7 m.

 

Now with a 4 cm. longer radiator, to have a better fit at 27.600

    dia. 20 mm. dia. 18mm dia. 12 mm dia. 10 mm
reflector at 0.0 m. 50 cm 97 cm 56 cm 83 cm
driver at 1.7 m. 46.5 cm 97 cm 55.5 cm 61 cm

 

4 m. 6 m.

 

And some photo: click for full size.

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