# [amsat-bb] Antennas

Howie DeFelice howied231 at hotmail.com
Wed Aug 5 19:38:52 UTC 2015

```>From the w8ji.com:

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
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
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
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
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.

> Date: Wed, 5 Aug 2015 09:49:59 -0500
> From: tomdoyle1948 at gmail.com
> To: amsat-bb at amsat.org
> Subject: [amsat-bb] Antennas
>
> Reading the posts on the 23 cm antennas brought back some nightmares and a
> lingering question I have about antennas. As an undergrad in Electrical
> Engineering we had  to take a course in Maxwell's equations. I really
> suffered through that course. I remember one thing that I have wondered
> about since then. Almost everything was based on comparisons to an
> isotropic radiator (dBi). Since I was a ham I had little interest in things
> that could not be built. When measuring the real gain of a real antenna it
> has to be compared to something like a dipole. Sometimes a gain is
> converted to dBi by adjusting for the fact that the reference was a dipole.
>
> Consider measuring the gain of an antenna for 70cm with the reference a
> dipole for 70cm. Suppose the gain is 10 dB.
> Now consider making exactly the same measurements on a 23cm antenna with
> the reference a dipole for 23 cm and finding the same 10 dB gain.
>
> At first glance it would seem that the two antennas would be equally
> effective. Another view is energy. If you are trying to receive a signal
> there is only a certain amount of energy that is distributed through space.
> The goal of the antenna is to pick up as much of this energy as
> possible and deliver it to the radio. There is a term 'effective aperture'
> Ae or more commonly  'capture area'. The antenna picks up the energy in
> its Ae. Ae is not just the physical size of the antenna but bigger antennas
> tend to have a bigger Ae. Ae also depends on gain - the higher the gain the
> bigger the Ae (big surprise).
>
> Here is my question. All other things being equal (ignore things like path
> loss) I believe that the 70 cm antenna with 10dB gain will deliver more
> energy to the radio than the 23 cm antenna with 10 dB gain. Bigger is
> better ?
>
> If this is wrong please do not fill a chalk board with Maxwell equations to
> convince me. If it is correct, is the difference simply related in a linear
> fashion to the difference in wavelength or is there more to it ?
>
> 73 W9KE Tom Doyle
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