Re: Tesla Coil RF Transmitter

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Ed Phillips <evp@xxxxxxxxxxx>

"If we keep making a transmitting antenna smaller, but we also keep
stepping the voltage up which drives that antenna, then the antenna
doesn't act smaller.  As long as the strength of the EM field at
1/4-wavelength distance is not decreasing, then we can keep making the
antenna physically smaller.  We only pay for this in wasted power, since
step-up transformers use coils which get hot.

Or in other words, it's hard to judge how well a small transmitting
antenna works.  Even the shortest antenna must always radiate SOME
electromagnetic waves, and if an impedance-matching network is involved,
then far more energy gets out than one might expect.  To make your small
antenna act larger, just step up the drive voltage while stepping down
the
current.  Don't forget: if a superconducting Tesla coil was used, with
superconductor primary and superconductor capacitor plates, hooked to a
superconducting ground plane, then the Tesla coil would be just as good
as
a humongous quarter-wave antenna.  Of course the voltage would get a bit
high, and you might have to bury the thing inside a block of solid
teflon
a mile across."

	The radiated power from the antenna is the RADIATION RESISTANCE times
the input CURRENT squared.  The radiation resistance is proportional to
the square of the ratio of the physical height to the wavelength.  The
efficiency is the ratio of the radiation resistance to the ground
resistance plus the total loss resistance of the matching network which
will increase with the lowered radiation resistance and increased
antenna reactance so things go to pot in a hurry as the height gets
smaller. In most small antennas the ground resistance is the limiting
factor.  If not then the effective bandwidth gets very small as the
height decreases.  The effective circuit Q is the ratio of the antenna
reactance to radiation resistance plus loss resistance so if the latter
is made small in an attempt to raise the efficiency then the bandwidth
goes way down.

	It's customary in the design of low-frequency broadcast stations (there
are still quite a few super power LF stations left around the world) to
attempt to reduce the antenna circuit Q and get the bandwidth large
enough to handle the audio modulation frequencies by making it of a big
cage of vertical radiators all in parallel.  This reduces the reactance
but doesn't increase the radiation resistance.

Ed