[Fwd: LF: Multiple Tuned Vertical]

Andre' Kesteloot akestelo@bellatlantic.net
Wed, 25 Aug 1999 18:39:40 -0400

vernall wrote:

> 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 ....
> Regards,
> Bob ZL2CA