"Cecil Moore" wrote in message
...
Reg Edwards wrote:

"Cecil Moore" wrote

A high-Q loading coil in
the center of a vertical is much more efficient than spreading
the loading out over the entire antenna.

Wrong! Spreading a multi-turn coil allows a MUCH thicker wire
diameter to be used with spaced turns. Also the coil diameter can
be
increased to minimise the number of turns.

Result : higher coil Q, lower coil loss, greater efficiency.

Got to disagree with you on that one, Reg. Mobile shootout
field strength measurements put all the helicals, no matter
what wire diameters were used, considerably down from the
well-designed bugcatchers and screwdrivers. That meter of
wire in each turn of the helical has more resistance than
the centimeter of radiating bottom section that it replaces
at the feedpoint.

It is well known and accepted that moving the mobile loading
coil from the center of the antenna to the base will reduce
the efficiency even though the inductance required for loading
is decreased. With a helical, part of the loading coil is
at the base and that's simply a bad idea when efficiency
is important.

One mobile, in particular, should have performed well. It
was made from 1/4 inch copper tubing with a large diameter
and proper spacing between turns but it was about equal to
a Hustler and considerably down from the top performer which
was top-loaded.

What wins the mobile shootouts is the longest possible straight
bottom section under the coil where the highest current occurs.
That maximum current occurs all up and down that straight bottom
section when a good top hat is added to the antenna. I once won
the shootout competition by putting all the loading (coil+top-hat)
at the top of the antenna using cheap stuff from my junk box.

If one wants to win a mobile shootout, one cannot afford to
install a helical coil at the maximum current section.
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==================================

Cec,

(1) A mobile antenna is NOT a 1/4-wave resonant, base-fed,
ground-mounted loaded vertical which behaves reasonably predictable.

(2) A mobile antenna is a relatively-isolated-from-ground, 1/2-wave
resonant, loaded, off-centre-fed vertical dipole which defies rational
analysis.

(3) It is impossible to separate the many different behaviour modes
and effects, differentiate between them and allocate relative
magnitudes. One has to be careful to control one's imagination when
describing effects.

I've never seen one near to, but I understand "screwdriver" type
mobile antennas are akin to long helicals specially at the lowest
operating frequency.
----
Reg.