Topband: Antennas

[Tom Rauch]

Hi Larry,

The best thing with any of this is to model the antenna.

> Very true. That's why I always base my conclusions on observations made
> over a span of years. Log entries are a fair filter of "measurement noise"

Not at all. Log entries reflect propagation, equipment, noise , and 
many other things including the mood of the operator. There is so 
much data clutter that affects the results, the results are 
meaningless unless it is a blind A-B switch repeated at very short 
time intervals many times.

> and, after all, isn't the "best" antenna the one that make the operator
> the happiest?

Of course you are correct. The best antenna is always what we 
think works no matter what the truth is!

> It is a very big problem if it is not practical to put guys at the top of
> the structure. Remember that my original premise was for portable and
> mobile antennas.

A standard portable vertical for lake and river dredging operations 
before GPS came along was a push up mast with the upper guy 
lines used as partial top loading. The "bleepers" ran right below 160 
meters. It's a good system, and the hat wires can be folded in until 
they nearly parallel the vertical before the advantages go totally 
away. In that system, they had two adjustments on one 
inductor...one for a feedline tap and one for resonance. 

My mobile antenna uses a large flat hat around 5 feet in diameter. 
That hat lets me put the coil anywhere in the structure I like with 
almost no change in efficiency or radiation resistance, and greatly 
increases bandwidth and efficiency because it is there.

Remember what happens in the structure, folding the wires down 
may decrease height from a flat wire....but some top loading with 
"capacitance" is virtually always better than none at all if bandwidth 
is important and the ground is less than perfect. That can be true 
even with some pretty tight angles in the toploading wires.

Of course we all know having the ends of an antenna near lossy 
dirt isn't even a good idea with an inverted Vee dipole, or a regular 
dipole. So it certainly can't be better in a vertical loading system. 
One good rule would be to keep the loading wires as far above 
ground as possible.
73, Tom W8JI
W8JI@contesting.com