MadSci Network: Astronomy Query:

### Re: How is the effective collecting area of a radio telescope calculated

Date: Fri Dec 5 10:01:31 2008
Posted By: Madhu Siddalingaiah, Physicist, author, consultant
Area of science: Astronomy
ID: 1225781094.As
Message:

Hi Noe,

That's a great question!

In the case of a dish antenna, the effective area is more or less the
physical area of the dish. In the case of wire antennas, the effective area
can be calculated given the directive gain. The directive gain is well
known for any number of antenna geometries and widely published by
manufacturers.

The effective area given the directive gain is:

A = L^2 * G / 4*pi

Where A is the effective area, L is the wavelength of incident radiation,
and G is the directive gain. For a half wave dipole antenna, the directive
gain is approximately 1.64, substituting that value for G simplifies the
equation to:

A = 0.131 * L^2

If you have 100 such antennas and their power is collected coherently (in
phase), the total effective area is simply 100 times the area of one
antenna. This is just like 100 parabolic mirrors concentrating their power
on one spot. The total area is the sum of all individual areas.

There are some issues to be concerned about. If the antenna power is not
added in phase, power will be re-radiated and lost. Also, the antennas must
be more than a wavelength apart. If not, each antenna will not operate
independently and you will have created a single, more complex antenna with
different directive gain and effective area. Your antennas spread a mile
apart should not cause any trouble unless you plan to measure wavelengths a
mile long!

Here are some links to dipole antennas and effective area:
http://en.wikipedia.org/wiki/Dipole_antenna
http://en.wikipedia.org/wiki/Antenna_effective_area

Good luck!

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