While going through my low band antenna notes covering the last 40 years or
so, I came across a few contradicting pieces of data. Trying to resolve the
issues with computer modeling exercises only resulted in more questions.
Since it looks like I will have to construct a few antennas soon for
portable and mobile-in-motion use, I will put the questions to the
collective wisdom of the list.
In my experience, (near) top loading with an inductor has always been the
easiest way to build a short low band antenna. Although both center and
bottom loading show improved performance (less loss), they are more
difficult to use to construct a practical antenna. Mobile antennas being
the exception where having a coil way up there will soon result in no
antenna at all.
My own antenna projects as well as my limited computer modeling bear this
out. Conventional wisdom suggests that top loading with a "hat" will be
better than any of the coil based solutions. Computer modeling suggests the
same answer. Maybe. This is where I start having trouble.
If you assume a short vertical with a wire "hat" composed of two equal
length horizontal wires attached to the top and adjusted in length to
"resonate" the antenna, it will certainly show less loss over any ground
you want. In a practical installation, the two horizontal wires are seldom
horizontal. If the wires are drooped down to a point near ground level at
the ends, and the lengths adjusted to maintain resonance, the feed point
impedance drops a lot. So much so that it looks like matching and real
ground losses will more than make up any difference between the wires
solution and the center coil solution.
My notes from years ago bears this out. A short vertical I had over a fair
ground system used a coil near the top. It was easy to build and easy to
get working. In an effort to improve performance, the coil was removed and
two drooping top wires were installed. Before and after observations made
over a period of many years through a complete sun spot cycle showed no
overall improvement.
It looks to me like drooping wire "hats" create as many problems as they
solve. So, is this real? What have I missed? Comments?
I have another question to pose, but I'll save it for another post.
73,
Larry - W7IUV
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