LF: LF-antenna with top load]

Andre' Kesteloot akestelo@bellatlantic.net
Fri, 17 Sep 1999 12:17:43 -0400

Rik Strobbe wrote:

> At 14:04 17/09/99 +0100, G3XDV wrote:
> >PA0SE 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 :
> gain   tophat
>  1dB    1.4m
>  2dB    3.5m
>  3dB    7.0m
>  4dB   14.1m
>  5dB   35.1m
>  6dB   endless
> 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