Topband: T loaded vertical

[Tom Rauch]

It's not how our antennas work, it's how we *think* they work that makes us
feel good.

> Jeez, I am just reacting to the thought of 260 feet of top loading on a
> vertical. It almost seems as though one is using a single wire feed line
to
> feed an horizontal antenna. I may have to get out my ON4UN book and see
what
> the equations tell me the feed point resistance would be -- I have never
> thought of doing something like this and maybe I have missed something
that
> would work better than what I am using.

Maximum radiation resistance occurs when current integrated over the
vertical section is as high as possible, which means maximum current
mid-height in the vertical section.

Feedpoint resistance is not the same as radition resistance, of course. With
a poor ground system maximum gain would occur with a larger flat top and
current further up from the center. With a reasonable ground maximum gain
would appear when current maximum is near  mid-height. With a good ground,
at mid-height or lower. But all of these are small changes anyway. The real
cure is to use the best ground system possible.

> Maybe this is just an oddly proportioned DDDR antenna.

The flat top of the DDRR being close to the groundplane only "loads" the
short vertical antenna, and does so somewhat inefficiently. Only the very
short vertical section radiates useful RF. The DDRR isn't a good antenna,
typically in the few percent efficiency range, because of the large flattop
and short vertical radiating section.

With a T or L antenna, the optimum length would depend on the ground system
and vertical height.  I don't think there is any combination of modest
height vertical and reasonable ground loss that would have maximum
performance with current maxima exactly at the top of the vertical. As the
ground system gets better, where you place the current maximum generally
means less and less (fractions of a dB).

Not using any ground or a poor ground wouldn't be a good idea, unless the
antenna is 100 feet above ground at the lowest point! Electric fields induce
large amounts of loss in soil also. We often forget about that and only
consider direct current paths.

I'd use whatever made me happy, no one else would ever notice the signal
change. I wanted a real signal change, I'd work on the ground system.

73 Tom