At 14:04 17/09/99 +0100, G3XDV wrote:
Refering to the last part of my second report (16.9.99 14.17) it is clear
that top loads are hardly a substitute for height. Even raising the
antenna from 10 to 15 m increases radiation resistance more than even the
largest top load can do. This is easily understood as radiation resistance
is proportional to height squared whereas the most a top load can do is
doubling the radiation resistance.
Ah, but that's EFFECTIVE height squared, and one effect of the top
load is to increase effective height by anything from 50% of actual
height (no top load) to 100% (quarter-wave top load). Are you really
saying that a 15m vertical will outperform a 10m inverted-L with a
1000m top load?
Why does a top load only double the radiation resistance? Isn't it a
function of total length, whether vertical or horizontal?
As far as I understand short verticals the radiation resistance depends on :
1. the height of the vertical section
2. the average current through the vertical section
Radiation resistance is proportional to the square of the height and to the
square of the average current.
Adding a tophat to a vertical antenna increases the average current through
the vertical section and an 'endless' tophat will double the average current.
So a 10m high vertical with an 'endless' tophat will have the same
radiation resistance as a 20m high vertical without tophat.
eg. in a prviuos mail PA0SE calculates a 10m vertical without topload as
7.8 milli-Ohm (example 1) and a this same 10m vertical with a 4 times 20m
tophat as 29 milli-Ohm (example 16). This is close to the 31.2 milli-Ohm (4
x 7.8) that you would get with an 'endless' tophat.
As a 'bonus' the tophat will increase the antenna-capacitance, resulting in
a smaller loading coil and lower antenna-voltage.
The increase of the radiation resistance is large for the first few meters
of tophat but the 'gain' decreases fast and the absolute maximum you can
get is 6dB.
Following table gives the dimension of the tophat (for a 10m vertical)
compared to the gain :
But all the above is only valid as long as both the vertical and tophat
section are short compared to the wavelength. In most publications with
'short' is meant less than 5% of the wavelengt (= 110m). This should be
always the case with the vertical section (unless you are kiting or
ballooning) but some of us might have tophats that are not 'short' and will
contribute to the radiation.
73, Rik ON7YD