[amsat-bb] Antennas

[Howie DeFelice]

>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 
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.


> 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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