Richard Fry wrote:
"Cecil Moore":
Jimmie D wrote:
One such mistake is calling a 4 ft long antenna that has a coil to
make it resonate on the 10M band a "10M loaded 1/4 wavelength antenna".

They should be more careful and specify that they are
talking about electrical lengths, not physical lengths.

They also should point out that, although a radiator
physically/electrically shorter than needed for first self-resonance can
be "loaded" to resonance, this does not mean that loaded and
self-resonant radiators perform equally well in an installed system. In
some applications there can be as much as a 100:1 difference in their
radiated powers, for a given power at the transmitter output connector.

Good point, Richard. An antenna's ability to "load"
is proportional to its electrical length. An antenna's
ability to radiate seems to be proportional to the
physical length of the antenna that is carrying the
highest current. In 75m shootouts, the mobile antennas
with the loading coil furtherest away from the feedpoint
(closest to top-loaded) generally won the shootout.
--
73, Cecil http://www.w5dxp.com

"Cecil Moore" :
An antenna's abilty to "load"
is proportional to its electrical length. An antenna's
ability to radiate seems to be proportional to the
physical length of the antenna that is carrying the
highest current.
__________

Perhaps unexpectedly, the intrinsic pattern and directivity of a
physically/electrically short, unloaded monopole radiator are not greatly
different than those of a self-resonant 1/4-wave monopole. The big problem
with an unloaded, short radiator is the reactance at its feedpoint, which
means that very little current will flow into the short radiator from any
practical r-f source. But for the current that DOES flow in it, its
radiation performance will not be much different than that of a
self-resonant 1/4-wave monopole, at that same current flow (as NEC will
show).

Using a "loading" reactance to resonate the radiator allows maximum power
transfer from the r-f source into the feedpoint. But the remaining issue is
the low radiation resistance of the short radiator even when it is resonant,
which is a small fraction of the other series resistances in the antenna
system (ground and coil loss, mostly). As a result, much of the available
transmitter power produces heat rather than EM radiation.

RF

"Richard Fry" wrote in message
...
"Cecil Moore" :
An antenna's abilty to "load"
is proportional to its electrical length. An antenna's
ability to radiate seems to be proportional to the
physical length of the antenna that is carrying the
highest current.
__________

Perhaps unexpectedly, the intrinsic pattern and directivity of a
physically/electrically short, unloaded monopole radiator are not greatly
different than those of a self-resonant 1/4-wave monopole. The big
problem with an unloaded, short radiator is the reactance at its
feedpoint, which means that very little current will flow into the short
radiator from any practical r-f source. But for the current that DOES
flow in it, its radiation performance will not be much different than that
of a self-resonant 1/4-wave monopole, at that same current flow (as NEC
will show).

Using a "loading" reactance to resonate the radiator allows maximum power
transfer from the r-f source into the feedpoint. But the remaining issue
is the low radiation resistance of the short radiator even when it is
resonant, which is a small fraction of the other series resistances in the
antenna system (ground and coil loss, mostly). As a result, much of the
available transmitter power produces heat rather than EM radiation.

RF I had an electronics instructor (not sure what he knew about antennas)
say thet there was a 10/90 rule about antennas. That an antenna 10 % as
long as a 1/4 wl will radiate 90%as well, wile he didnt say it I assume
this means with all other sources of loss minimized. Is this anywhere near
true?
/Jimmie

"Jimmie D":
RF I had an electronics instructor (not sure what he knew about antennas)
say thet there was a 10/90 rule about antennas. That an antenna 10 % as
long as a 1/4 wl will radiate 90%as well, wile he didnt say it I assume
this means with all other sources of loss minimized. Is this anywhere near
true?
_____________

Theoretically yes, but not so much in practice.

Without losses, an "infinitesimally short" linear dipole has 91% of the
peak directivity of a self-resonant 1/2-wave dipole (1.5 vs 1.64). And
for a given applied power both of them would radiate the same total amount
of power, just with marginally different pattern shapes.

The problem is that system losses in a real, "loaded" short antenna can be
much higher than the radiation resistance, so a loaded short antenna may not
radiate much of the available power -- in some applications not even 1% of
it.

RF