Hi Dave,
The goal of top loading a vertical antenna is to raise its radiation
resistance without increasing antenna height. For a fixed amount of ground
loss this translates into more radiated power. Increasing the height of a
vertical antenna will also raise its radiation resistance until it reaches
1/2 wavelength in height.
It helps to think of the top loading structure as one plate of a physically
large capacitor with ground being the other plate. Since capacitance
increases with surface area it's clear that increasing surface area will
increase capacitive loading; therefore, a simple horizontal wire at the top
of the vertical element is not necessarily the best and certainly not the
only top loading scheme. The best top loading structure for any given case
will hinge on practical considerations ( a government project being the
exception).
The radiation angle of a vertical antenna depends on the height of the
vertical element up to about a half wavelength, so the higher you can make
the antenna the lower its angle of radiation will be. Unfortunately,
raising the top loading structure also decreases the amount of capacitive
loading it provides because capacitance depends on plate spacing as well as
surface area.
The amount of radiation from the top loading structure vs. the vertical
element will be small as a general rule as long as the loading structure is
reasonably symmetrical about the vertical element. Keep in mind that there
are no magic numbers for an antenna of this type so don't agonize over
dimensions - build what is practical for your situation. Unless you are
unusually lucky, what you finally decide on won't match your feedline. A
simple LC matching network will cure that problem and it won't degrade
antenna performance if it's constructed with good quality components.
One lesson I've learned in fifty years of hamming is that no matter how good
an antenna may be from a performance standpoint, it's useless once it comes
down in a storm, so mechanical considerations often override what would be
ideal in theory. I hope this helps resolve your problem.
73, Al W6DZ
----- Original Message -----
From: "Dave NØRQ" <n0rq-1@dfwair.net>
To: <topband@contesting.com>
Sent: Tuesday, December 02, 2003 4:05 PM
Subject: Topband: 160m T-wire vert
| I'm hoping to build a good 160m xmit antenna, hopefully a vertical with a
| T-wire at the top.
|
| According to the ON4UN book, 3rd edition (which I love, by the way, but am
| having problems digesting all of it, as well as having difficulties
turning
| great theory into aluminum and copper), pages 9-45 and 9-46 talk about a
| "short" vertical with T-wire loading. Figure 9-52 even gives a nice
| drawing, but for 80m.
|
| Realistically, I could put up about 60 feet of vertical tubing (possibly a
| Rohn H-50 mast, with 10' of aluminum sticking out the top... unless that
is
| a bad idea for some reason), supported by good rope, and have reasonable
| supports for the T wires, which would have to be (ballpark) 75-85 feet
each
| (modest slope down). My question -- is that too much "T-wire" and not
| enough true vertical for it to work well on 160m? Obviously, at least
| *some* of the thing has to really be vertical, but I couldn't figure out
if
| 60' is enough to expect at least decent xmit performance. Going higher
| would be nice, but it starts getting difficult and expensive to build...
|
| Comments appreciated. (And thanks for putting up with rudimentary
| questions from someone new to 160m!)
|
| 73, Dave NØRQ
| n0rq-1@dfwair.net
|
|
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