What are you not satisfied with - the SWR curve? I have two 20DX-6 antennas
with 2:1 SWR at the lower band edge but they play very well. If I ever take
them down, I'll try to improve it, but it's not really a problem and there's
really very loss of power having a 2;1 SWR. Gain, F/B and good SWR occur at
different points so antenna design is a compromise. I also have two 15DX-6
15m antennas that had a very bad SWR curve. I had these antennas down last
year for other changes. I tried modeling and adjusting and could not
improve it. Then I noticed that the hairpin was the same size as my 10m
antenna. I made a larger hairpin and the SWR curve greatly improved so I
believe they gave me the wrong hairpin. I recently modeled the 20DX-4
which appears to have a poor SWR curve, but reasonable gain and F/B.
Instead of using that design I just built a 4el 20m on a 36' boom and I
sacrificed some F/B for better SWR - 1.9:1 max. It seems unlikely that
there would be a strong lobe up, unless something else on the tower is
causing the weird pattern. You may want to look at your hairpin or you can
maybe decrease your first director for a better SWR curve and worse F/B.
John
----- Original Message -----
From: "W0YG Charlie Summers" <w0yg@plusten.net>
To: "TowerTalk" <towertalk@contesting.com>
Sent: Monday, August 04, 2003 22:13
Subject: [TowerTalk] DX Engineering 20DX-5 modeling
Has anyone modeled the old DX Engineering 20DX-5 antenna? This was a 5
element 20M yagi on a 43.75 foot boom. The reason I asked is I just spent a
whole morning trying to tune one by yanking it in the air after every
adjustment and it just never came into what I considered acceptable
parameters. So, I modeled it in EZNEC and find that it has some problems
the way it is factory set. For one thing, the forward gain and f/b are not
what I would expect. For another, there is a strong lobe straight up, a
cloud buster. For another it is hard to move the frequency down.
Let me explain. If you set it the way DX Engineering said, then it
resonates at 14.280. By playing with the driven element, the first director
and the reflector, one can get it to move down to about 14.200 where it
plays but still I am not quite satisfied.
I asked in the first paragraph above if anyone has ever modeled this
antenna. Perhaps I should have stated has anyone ever run YO or some other
yagi optimizing program on it. That is what I would like to see the results
of.
73,
Charlie, W0YG..>>
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
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