Advertisement
tres
distances.
Unfortunately, there are catches
here: UHF waves
travel only by line
of sight
path and
get absorbed or
reflected by
obstacles.
The
curvature
of
our
planet is the biggest of
all.
On per-
fectly
flat
ground,
with the
antenna at
your head
height,
the
range is
a
mere 10 km. To fully
exploit our 200
km limit, one station has to
be
3000
m
high,
or
both
of them at 800
m
above
the terrain. Also,
our signal
is not the
only
in the
air and
the receiver tends to pick up
the
strongest one,
which,
especially
further
up,
doesnt have to be
the
one we want.
For
this reason,
we
often change
our
simple
quar-
ter wave antenna (which
has
a
radiating pattern
close
to the ideal
omnidirectional one) for a
different
gadget, antenna with a gain.
What is a gain?
Like many
other technical
terms, it is
an incorrect expression.
Antenna,
being
a
passive
element,
doesnt
gain anything.
In
fact,
it
always
has losses
and
ironically,
the higher
gain of the antenna,
the
more losses it usually has! If
you
find this hard to
follow,
do not
panic, keep
reading
...
If
we
talk
about gain,
we
always
talk direc-
tionality.
Our
antenna is
not omnidirectional any longer. Instead, we
concentrate
THE SAME AMOUNT OF ENERGY
(or even a
bit
less) in
certain
direction(s).
As
a
result,
we
have
only a
part
of our imaginary globe
to
cover,
ie
a stronger signal
in the desired heading, but weaker
or
no
signal in the
other
directions.
This might
explain
why you, with
your superb,
high
gain,
heli-
cal antenna, cant
contact
somebody
a
couple of
hundred meters
away
but
substantially
lower or higher than
you are, especially over
a
flat terrain
(no ground
reflections)
and
if the receiving
station
is
fitted
with the
same
type of radiator
(receiving
pattern is
always
the
same as the
transmitting
one).
I had
some
inexplicable problems
during towing myself. As
soon as
I reached
some
100 m of height,
my driver
stopped,
lost contact with me ..
.
Helical
antenna (do
not
confuse
with helical beam
antenna)
doesn
't
come from
hell,
as
we might
easily conclude:
the name
originates
in
Greek
heli, meaning circle. This
antennas
radiating ...
pattern is
a
fairly flat
disc,
only slowly
thickening with diameter.
Anything below
or
under it is
simply
out.
However,
the
signal
with-
in the circle can
reach
a
gain of
some 8
Decibels. This unit is
slight-
ly
confusing.
Gain is
expressed as a
numeric
comparison on a
loga-
rithmic
scale,
with the
signal of either
the ideal
omnidirectional
antenna or
half wave diPole. Nobody usually bothers to
say
which
standard
is being used
,
despite there being
a considerable
difference
here. The term GAIN is meaningless by itself. It makes
sense only
if we know in which
direction.
The best way to
show
it is a dia-
gram,
which
should
be
available
for
any
high
gain antenna.
To get
at least
some
idea,
3
dB means a double
fold, 6
dB
four
fold
and
15
dB
a
thirty fold
of
the field
strength of
the respective
simple anten-
na,
in
the direction
of
the maximum radiation. Anyway, in the
stick
like, vertical antennas we use, GAIN is usually meant
as an
enhancement of a
horizontal radiation in
360
dg
angle at
the
expense of vertical emission.
A UHF 15 Db
antenna
would be most likely Yagi
(common ly
used for TV
reception),
about 2
m
long,
with
some
12
elements.
A
bit impractical for
our
handheld baby. Also its
forward
radiating
pattern,
being
roughly conical
with
about 30
dg
angle
between 1/2
power points, would
cause
inconvenience
in
the
air.
1m using it
on
my base
station
to
access a
busy
channel
3
Melbourne repeater. If
needs be, I do not have to beg for
a
break. I
simply
press the PPT
button
and
talk,
over
virtually
anybody else,
from the distance
of
50
kilometres. Not too
considerate,
you might
say ...
If you listen to this
repeater,
you will
realise that it has to be
covering at least one
men-
tal institution, where the
venerable schizophrenics and
manic
depressives have
a
free
access
to UHF radios. You
cant seriously
expect somebody
who is burping
and f#$%ing
in the mike,
or
worse,
re-transmitting Heavy Metal from his
car
radio,
to
give
you
a
break when you politely
ask
for it!
What I'm
getting
to is
,
there is not
such a
thing
as
the best
antenna.
There
are only suitable or
unsuitable devices for
each
par-
ticular job. High
gain can
turn quite bad in
our application
because
April
1996
ANNOUNCING
NEW
INSURANCE
OPTIONS
TAILORED FOR
HGFA
MEMBERS
...
In
consultation with the HGFA we are delighted to
announce
the development of new HGFA
approved
insurance options
for
all members.
We can offer:
• Affordable
"Flight
Risks" Personal
Accident/lnjurV/lllness
Cover
• Options for
insurance
of Trikes and Hang
Gliders:
*
limited transit risks plus fire & theft or
...
*
Full cover Ground & Transport or
...
*
Full Aircraft policV
-
Flight, TaxVing, Ground
& Transport
As Registered General insurance brokers, with spe-
cial expertise in aviation insurances, we have the
experience and qualifications to assist all members
with a broad range of insurance advice and cover
placement. We currentlv handle the HGFA liabilitv
Scheme as well as specialist services for a number
of other aviation Associations.
Please contact us for expert aviation insurance
advice.
H G FA I nsurance Scheme Adm i n istrator,
Kevin Chamberlain
Chamberlain Knights PtV Ltd
Insurance Brokers ACN
002 493 521
Telephone
(02) 868 4044
Facsimile
(02) 868 5262
PO Box
329,
EPPI NO NSW
2121
Approved and initiated by the HGFA for the Members
17
Advertisement
