tomdoyle1948 at gmail.com
Wed Aug 5 14:49:59 UTC 2015
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
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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