On 02/24/2016 12:19 AM, Richard Fry wrote:
"Wayne" wrote

Dead on explanation. Some have insisted that inefficiencies
result from antenna size and not from matching challenges.
_______

Kindly note that the radiation efficiency of an antenna _system_ is
related to its radiation resistance compared to other resistive losses
present in that antenna system.

It is possible to perfectly match the impedance at the feedpoint of an
antenna system to the impedance of the transmission line connected
there. But that antenna system still can have very poor radiation
efficiency at that frequency.

An example of this is a Z-matched, but electrically short vertical
monopole driven against a poor r-f ground connection such as a few
buried ground rods. Most of the available transmitter power is
dissipated in the r-f ground resistance, rather than being usefully
radiated as e-m waves.
===============================
That all depends on the length of the antenna against the operating
frequency .
A half wave vertical has a high RF voltage at its "bottom" but a low RF
current at that point ,hence less ground loss ,meaning that not all that
many radials are required compared with a quarter wave vertical .
Of course an impedance transformer is required .
A well known transformer is an inductor of 14 windings between ground
and vertical antenna and over it one of 2 windings connected to the
feeder and hence transmitter ,resulting in a 1:7 voltage transformation
,hence a 1: 49 (say 50) impedance transformation . Across the 14
windings is a HV variable capacitor to adjust for minimal SWR.

A well known matching circuit (tuner if you wish) used for low power HF
ARDF transmitters is the L-circuit by G3ZOI
Here the low impedance of the transmitter's output is in series with a
variable capacitor the other side connected to the(high impedance) short
wire antenna and to a grounded inductor . With 2 diodes in series with a
LED in de output to the antenna the LED is adjusted for max RF current
(max LED brightness). For 1-5 W transmitters the variable capacitor can
be a polyvarcon type from a portable MW receiver ....works very well !

Frank GM0CSZ / KN6WH in IO87AT