In article ,
Robert11 wrote:
Am interested in receiving only, around the Marine VHF bands of 156 MHz, or
so.
I understand that "Gain" is achieved in many antenna configurations by
making them very directional.
My understanding is that you in effect re-shape the lobes to be prominent
along one axis, and minimal at right angles.
That's correct. However, it's important to realize that the antenna
pattern is three-dimensional, not two-dimensional.
Antenna (directional) gain is achieved by (in effect) shaping or
compressing the lobes on both the vertical and horizontal axes.
So, how is Gain achieved in a
vertical whip (the specs usually mention 3 or even 6 db), or the
rubber-duckie, types of antennas used so often on handheld scanners (or
mounted to recreational boats) ? They are, I believe, truly omni
directional.
They are omnidirectional in the plane which is perpendicular to the
length of the antenna. That is to say, they have the same gain or
sensitivity "on the horizon", no matter which compass direction you're
looking in.
They are *not* omnidirectional in elevation.
The gain pattern of a vertical whip looks something like a donut. The
pattern shows the greatest sensitivity in the direction of the
horizon. At angles above or below the horizon, the sensitivity
becomes less and less. A theoretically-perfect vertical antenna, in
free space, has an extremely deep "null" in the direction along its
axis... it's very insensitive to signals arriving from overhead or
below.
The way that a vertical antenna achives gain, is to compress the
"donut" pattern vertically. Compared to a standard reference dipole,
such a "gain vertical" is more sensitive in the direction of the
horizon, and for some number of degrees above and below the horizon.
Once you reach a certain elevation, the gain antenna's sensitivity drops
down to the point at which it's equal to a reference dipole... and at
higher elevation angles, it's _less_ sensitive than a reference dipole.
The word "omnidirectional" can be a bit confusing, as to a beginner
this can convey the idea that the antenna radiates equal power in all
directions of the sphere (and has equal receive sensitivity in all
directions). That isn't the actual meaning of the term, when
discussing antennas.
An antenna which does have equal sensitivy in all directions of the
sphere is an "isotropic" antenna. The sensitivity / gain of an
isotropic antenna is often used as a reference point for measuring or
describing other antennas... if you see antenna sensitivity referred
to in "dBi", you know that this is the type of reference being given.
Isotropic antennas don't (and can't) actually exist in practice...
the EM field equations don't work out. It's just a convenient
theoretical reference.
The other antenna gain figure you'll see is "gain, as compared to a
reference half-wave dipole". This is denoted by "dBd".
For any given antenna, its gain dBi will be 2.15 dB higher than its
gain in dBd (in other words, a reference half-wave dipole has a gain
of 2.15 dBi or 0 dBd).
When you look at antenna descriptions or ads, make sure you know which
reference is being used. Antenna manufacturers sometimes like to
simply say "dB" without saying which reference... and in this case
it's likely to be dBi, because it makes the gain number larger and
makes the ad read better :-)
Also, are there vertical whip antennas that are directional, with "gain"
perhaps ?
A single vertical antenna, with no other active or parasitic element,
is going to have the same pattern in every direction towards the
horizon. It's rotationally symmetrical, and therefore its sensitivity
pattern is also symmetrical around the vertical axis.
If you want greater sensitivity in one direction, you either need a
second element, or you need to bend the antenna somehow so that it is
no longer symmetrical around the vertical axis.
--
Dave Platt AE6EO
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