Today I did a little modeling with 4NEC2 on 80 meters. First a regular dipole
showed a bandwidth of 200 KHz at the SWR=2:1 points.
Then I decided to see how close to a real cage a pair of dipoles has to be to
get the same benefit. So I added another dipole, fan style, to it and got 320
KHz bandwidth. I varied the length of the second dipole all over the place. I
was modeling on 80 meters, and made the 2nd dipole small (like the length of a
40 m dipole) and could see SWR dips in the 80m and 40m bands when I did a
frequency sweep. Then I lengthend the small dipole and watched the 2nd dip
drop in frequency till it got close to the 80 m band and the bandwidth began
increasing. But I was never able to get a "double hump" inside the 80m band as
I had hoped.
But 320 KHz bandwidth was easily obtained.
Now the question is, how different is a pair of dipoles (with 10-20 degrees
angle between them)from a real cage dipole ? So to do this I modeled three
dipoles, parallel to each other and separated a few feet apart. The bandwidth
was the same as the fan, indicating to me that very likely a simple two dipole
fan antenna (both elements can be the same length) will give good swr
performance compared to a dipole and might be worth the effort. Sure would be
simple to build compared to a cage
Rick K2XT
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