> Is this real or a myth? I wondered and so asked Al K3LC.
He studied the
> question and had a good article about the topic. For 160m
I use a 90 foot
> irrigiation pipe vertical with four
> wires in part of the top guys, each 42 feet long. The
antenna looks
> electrically like a quarter wave. So there is some
improvement in the base
> impedance, which helps make a matching network and antenna
more efficient.
If you use a very poor loading coil or have a poor ground
system, changing radiation resistance will have some effect.
If you have a good ground system and use a good inductor,
changes caused by top loading have a minimal effect.
> question however was about the shielding or signal
cancellation effect of
> using such top loading. If I interpret his article
correctly, the antenna
> radiates equally well with or without such top loading.
Probably so. You can feed it where you want and do almost
anything to self-resonance or loading placement and not
greatly affect efficiency.
However, Chip had a very SHORT vertical and had top wires
sloping down. He also had a modest ground system. In his
case the capacitance at the top and how it is arranged could
oppose radiation, resulting in an effective reduction of
height.
Contrary to popular myth, loading coil losses are an
insignificant part of this equation if we use a reasonably
good loading coil. Loading coil location is also not
important when a large hat is used.
In Chip's case he could make the hat wires too long and hurt
efficiency, he could slope the wires down and hurt
efficiency. The optimal solution depends on just how poor
his ground system is, but as a general rule Chip would be
able to do anything he wanted and not affect efficiency a
noticable amount so long as the hat is horizontal and
capacitance of the hat greatly exceeds capacitance of the
vertical section.
Your vertical is so tall very little you do with a small hat
would change system efficiency.
There is however a big
> benefit of using such top loading - much better bandwidth
when that is used.
> My results compare well on bandwidth with this data in
his article.
....and the interesting thing is bandwidth increases
independently of efficiency changes. We often hear
statements claiming wide BW means low efficiency. That's
another myth. Just like the myths moving the feedpoint
(elevated feedpoint) or changing the feedpoint style (folded
monopole style) changes efficiency.
73 Tom
_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband
|