Re: SSTC As a transmitter.

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

>
> > a vertical conducting rod extending about 1800 feet above the
> > earth's surface and excited at a frequency of 136 kHz would be
> > a fairly efficient radiator.
>
> Certainly would.
>
> > a much shorter Tesla-type transmitting structure, say, about 50'
> > overall height... would not be as efficient a radiator.
>
> Indeed.
>
Actually, as far as radiating power uniformly in all directions (i.e. an
isotropic radiator) goes, they're equally good. What DOES change is the
various losses. The physically small radiator will have a low radiation
resistance, implying high currents, which makes the power loss due to the
inevitable resistance in physical structures greater (quickly... I^2*R adds
up fast). If you could build your antenna out of superconductors, it would
make no difference (for transmitting... receiving is another story).

The other practical side effect is that the impedance of a full size antenna
varies less as a function of frequency than a small radiator, making the
small radiator tougher to drive with a good impedance match (with mismatch
also resulting in loss)... The large reactive component also contributes to
loss from the circulating currents in the resistive components.


> > It it reasonable to assume the capture area could be increased
> > by some technique that would increase RF current flow in the
> > receiver coil, such as regeneration?
>
> This implies a power source local to the receiver, enabling some
> sort of positive feedback to assist with signal recovery.


"Effective" Capture area can also be increased by locally perturbing the
magnetic field shape, as with a ferrite core.  A technique used to good
effect in a small transistor radio with the "loopstick antenna".  (There are
also other reasons for the ferrite core, by the way).  As always though,
bandwidth gets narrower, and resistive losses increase.  But, for low
frequencies, atmospheric noise dominates, so giving up some capture area is
probably not a big deal, for communications, but of course, complete fouls
up the power transmission aspect.