# Antenna question

1. Nov 6, 2007

### seang

the radiation intensity of a dish antenna is supposedly given by K(theta) = blah blah something.

I don't understand why the radiation doesn't also vary in rho. Does this mean that the radiation is the same for all rho? I'm having trouble picturing that in my head I guess.

Thanks;
Sean

2. Nov 6, 2007

### NoTime

A dish antenna is similar to a parabolic mirror.
It produces a beam much like a flashlight.
Does that help any?

3. Nov 7, 2007

### seang

when i think parabolic mirror i think that as the reflected light moves away from the mirror, it converges. when i think flashlight, i think of a diverging beam of light. is this wrong, too? haha.

in either case, i still don't understand how a three dimensional lobe can be represented by a function of one variable. what am i missing?

4. Nov 7, 2007

### rbj

no, it moves away from the mirror in parallel lines.

with a semester of calculus (and the trigonometry you normally get by then), you can actually show that this is the case for a quadratic function (which is the shape of a parabolic mirror).

5. Nov 7, 2007

### Staff: Mentor

If you use spherical coordinates with the Z axis pointing directly out of the dish in the direction of the peak of the lobe, then the radiation intensity is dependent on the angle theta out from the Z axis and dependent on the distance R away from the antenna, but independent of the rotational angle Theta about the Z axis. Does that make sense?

6. Nov 7, 2007

### mdelisio

Many of the analysis procedures assume radial symmetry, which leads to a symmetric antenna pattern. In practice, symmetry is often, but not always the case. For example, the antenna pattern will be dependent on the geometry and polarization of the antenna feed. In the case of linearly polarized antennas, you sometimes will see differences in the so-called E-plane and H-plane patterns. Also, you may see offset parabolic antennas which are more elliptical in shape, which breaks the symmetry of the pattern. (Homes with satellite TV or the VSAT antennas often on top of gas stations are good places to see these.)

As a general rule of thumb for large, round parabolic dishes, however, these differences are usually small and only really affect the sidelobes far away from the main beam.

7. Nov 7, 2007

### seang

wow yeah that makes perfect sense. i don't know what i was thinking. thank you.