Slinky
and Loaded Beverages
When space is
limited and a
Beverage antenna is
too short for normal
operation,
increasing
inductance (or
capacitance) along
the length of a
“Beverage”
will increase
directivity.
Inductance must be
adding in series
with the antenna,
and doing so will
increase the
antenna’s surge
impedance.
Capacitance must
be added in shunt
with the antenna,
and the extra
capacitance reduces
surge
impedance. Additional
shunt capacitance is
very difficult to
implement because
each capacitor would
need a ground, or
you would need to
surround the antenna
with a slow velocity
factor very thick
dielectric. (Most of
the air between the
antenna and ground
would have to be
replaced with the
dielectric.) There
aren’t any practical
or easily
implemented methods
of slowing Vp to
anything near the
amount required
through increased
capacitance (at
least none I could
think of), so I will
set that method
aside.
There are three
practical ways to
slow velocity of
propagation through
inductance:
- Use a spiral
or zig-zag
element
- Use lumped
inductor
loading
- A string of
beads
How They Work
Contrary to what
we might intuitively
think, loaded
antennas DO
NOT work
because we pack
800-ft of wire in a
200-ft area! They
exhibit improved
directivity because
wave velocity is
slowed, altering
phase shift along
the length of the
antenna. Slinky
(helical) and loaded
Beverages show
increased
directivity because
velocity of
propagation along
the antenna is decreased.
As Vp is slowed,
the antenna shows
increased end-fire
response and a
gradually
narrowing pattern.
End-fire arrays fire
in the direction of
lagging phase, and
the slower Vp causes
more optimum phasing
but only within
certain limits. If
phase is retarded
too much, it
actually starts to
bring the signal
back out-of-phase.
With too much delay,
the array tries to
fire in the reverse
direction.
Because of the
reverse firing
effect, there is a
definite limit in
phase delay (or Vp
slowing) a system
will tolerate. If
the design goes
beyond the optimum
value, the antenna
pattern tends to
reverse direction,
causing directivity
to decrease. I’ve
found optimum Vp for
a 1/2 wl wire is
generally around 0.5
times freespace
velocity.
Slinky users
should be
particularly
cautious to extend
the coils an optimum
amount. With 1/2 wl
of distance, you
would want somewhat
less than 1wl of
total conductor
length. Too many
“turns-per-foot”
and the slinky, like
any loaded Beverage,
will try to fire
backwards (towards
the feedpoint).
Optimum
termination
impedance will
always be somewhat
higher than a
conventional
Beverage antenna.
Surge impedance of
the antenna is
increased by the
additional series
inductance.
Helices vs.
Lumped
For all practical
purposes helices and
lumped antennas are
electrically
identical. It
doesn’t matter at
all if the antenna
uses a string of
soft-iron beads, a
series of lumped
inductors, zig-zags,
stubs, or helices (a
Slinky). The only
criteria is the
spacing between
lumped loads must be
a small fraction of
a wavelength. For
all practical
purposes, 1/8th wl
or less load spacing
distance will make a
lumped system
perform identical to
continuously loaded
systems (slinkies).
Lumped
reactances, whether
in the form of a
bead, stub, or
conventional coil
should have at the
minimum a somewhat
modest Q. This is
just another way of
saying the
inductances should
not add excessive
series resistance. Total
reactance of ALL
loads should be
between two and
fives times the
surge impedance of
the antenna. With
nine inductors along
a 1/2 wl wire, I’ve
found optimum
reactance to range
between 150 and 250
ohms. Surge
impedance was about
700-1100 ohms in
systems I tested. A
Q of 20 or more
would be acceptable
in each
inductor.
Attached
Model
I’ve added an
Eznec file of a
loaded Beverage. A
few things to note
are I used radials
to avoid a
connection to
high-accuracy ground
at the termination.
The feedpoint isn’t
critical, since it
does not greatly
affect directivity
in this antenna.
Unlike many antenna
models, it appears a
Mininec ground can
be used without
harming results.
Using a variety
of ground-types I’ve
found RDF remains at
about 6dB. This
places this antenna
slightly below most
elongated terminated
loops, but well
ahead of
conventional short
beverages.
loaded_bev
