Capture Area Ae effective aperture

Capture Area Ae effective aperture

Capture area, or
effective aperture,
is one of the most
misunderstood
antenna system
terms. In amateur
radio or layman
antenna theory,
capture area or
effective aperture
is often used to
justify
unjustifiable
fantasy antennas.
Looking carefully we
find capture area or
effective aperture
used to justify
totally absurd
claims in systems
that promise great
advantage where none
exists.

Capture area or
effective aperture
has a use in antenna
theory. Let’s look
at what capture area
or effective
aperture means and
what it can be used
for, and where and
how it is misused.

What is Capture
Area?

Capture area, or
more correctly
effective aperture (Ae), 
is a direct function
of antenna gain and
operating
wavelength. Ae is
determined by the
voltage available
across a load
matching the antenna
feed impedance for a
given electromagnetic
field strength
density. In simple
terms if the antenna
is placed in a
electromagnetic
field of a certain
intensity, a certain
amount of power will
appear in the load
at the antenna
terminals. The area
of space around the
antenna that
provided this amount
of power is the
effective aperture.

Many people
confuse physical
area, or Ap, with
effective aperture.
They are not the
same. Physical
size only determines
effective aperture
as physical size
might affect gain of
an antenna. Gain and
wavelength
determines capture
area, but capture
area itself has
nothing to do with
actual physical size or
physical area of the antenna.

For example a 1/2 wave long dipole in freespace has a capture area of about
.13λ².
This means a
lossless freespace
dipole has an Ae of
approximately .13
square wavelengths.
This effective
aperture is about
100 times larger
than the actual
physical area of a
thin wire dipole
antenna. Energy is
extracted from an
elliptically shaped
area slightly longer
than the dipole and
about 1/4 wave
diameter at the
center. This is why
increasing conductor diameter
or using a cage of wires
will not increase
electrical aperture
or capture area. 
As a matter of fact
if we built a
lossless or very low
loss small dipole,
perhaps λ/20 (1/20th
of a wavelength)
long, capture area
or Ae would be within a
few percent of a
full size dipole!

A change in
antenna element diameter does not
affect gain, except
as it might very
slightly reduce
power losses in
conductor
resistance. Length
itself has very
little effect unless
the change in length
significantly
affects antenna
gain. We must have a
change in gain to
change Ae (effective
electrical
aperture). Physical
aperture (Ap)
changes do not
affect Ae unless the
gain changes.

From
the Antenna
Engineering handbook
by Jasik we have:

We see from the
beginning of
text above that
gain, absent
significant losses
in the antenna,  is
equal to directivity
of the antenna. This
is another way of
saying something we
all should know and
remember. It is
impossible to have
antenna gain without
having a more
directional pattern.
In order to increase
gain, an antenna
must focus more
and more in certain
directions and
radiate less signal in
other directions.
Always. The only
exception is when a
lossy reference
antenna is compared
to a much lower loss
antenna. Virtually
all dipoles are in
the upper 90%
efficiency range.
Properly installed
dipoles make an
excellent reference
antenna.

From the text in
2.7 above we learn effective aperture
is directly related to
gain and
operating wavelength
of the system,
nothing else.
Physical size does
not enter the
equation, nor does
conductor surface area.
While certain very
large antennas with very low loss may
have a rough
relationship between
physical area and
effective aperture,
that relationship is
more coincidental
than a rule with typical antennas we use. Only
certain structures,
like parabolic
reflector antennas,
horn antennas, 
and mattress arrays,
approach a 1:1
effective aperture
to physical aperture
relationship.

Using equations
derived from or
embedded in the
above engineering
text , we find the
following effective
apertures or capture
areas:

Or using Ae =

λ²G/4π where G is the “multiplication” (not in dB) we have .13λ² for a
dipole.

We often hear a
quad antenna
“receives weak
signals better” than
a Yagi or dipole
because the quad
“has more capture
area”. Yet the basic quad
element, despite
being touted as
a huge capture area
antenna,
only has about 28%
more capture area
than a short dipole.
This is because the quad element in freespace under ideal conditions has about
28% more gain than a dipole! This ~28% increase is
only under ideal
case of a single
quad element in free
space compared to a
dipole in free space. When multiple
elements are added
(to make a cubical
quad antenna or a
Yagi antenna),
or even when the
dipole and quad
antenna elements are placed over
earth, the 28% difference in
capture area
decreases
significantly! In
many cases the
dipole or Yagi winds
up with more capture
area than a
similarly located
quad antenna, but
for most cases of
well-built antennas
with similar boom
lengths the Quad and
Yagi are equal in
gain and capture
area.

The
increase in capture
area of a single
quad element over a
dipole has nothing
to do with the 600
fold increase of
space enclosed
inside the antenna’s
area. The increased
capture area ties
directly into the change
in gain, the loop
having about 1.1 dB
gain over the dipole
under ideal
conditions.  The
loop’s 2 dBd gain
figure bantered
about is actually wrong. The
2 dB figure came
from flawed
measurements made
on UHF way back in the 1950’s.
Back then UHF equipment and
UHF measurement techniques were
poor. This resulted
in a measurement
error that was
published in several
places and initially
accepted as fact.
Once published (even
if later corrected),
the bad information
keeps reappearing.
Despite being
corrected we are
likely to keep
seeing the 2 dB
figure for the rest
of our lives, even
though the correct
figure is 1.1 dBd
for a free space
single element quad
antenna and even
less for a quad over
earth or a
multi-element
cubical quad.