[Fwd: LF: Multiple Tuned Vertical]
Wed, 25 Aug 1999 18:39:40 -0400
> Here is my two cents worth ... done on the fly ...
> The multiple feedpoints in a multiple tuned antenna give an apparent
> increase in the real component (resistance) by transformer action
> (currents divide by the number of separate vertical feeds). However,
> that does not alter the basic radiation characteristic of THE SYSTEM AS
> A WHOLE. The radiation resistance of the whole system is still related
> to overall physical dimensions. Multiple tuning is just a "trick" to
> get an apparent increase in radiation resistance, but it does not trick
> the laws of physics!
> Where gains could be had is with reduced current density at the earth
> mat around each feedpoint. For a given resistivity earth mat, the power
> loss is I squared R. As the I values are divided according to vertical
> feeds, then the current densities and I squared R losses are all much
> lower than having one feedpoint for the given antenna structure, taking
> full current.
> Reducing the ground loss component of an electrically small antenna is
> an effective way of improving gain (as the radiation resistance is
> swamped by loss resistance).
> In an electrically large system (like commercial systems) running high
> power, the multiple feeds, each with earth mats, disperse the ground
> current much better than for a single feedpoint system. I have read
> somewhere that there have been ground systems that have literally "burnt
> out" trying to work with applied high power.
> In summary, for a typical amateur system, size limited, I do not see any
> advantages in trying multiple tunes systems. Neither it seems do the
> commercial NDB users, where "Marconi T" LF antennas are standard
> installations. They do however invest in rather good ground radial
> systems ....
> Bob ZL2CA