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How is the radiation pattern of a parabolic reflector antenna plotted?

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2008-03-21 08:18:00

The paraolic antenna has a high degree of "directivity" compared

to many other antennas. That gives these puppies big gain. But they

need to be pointed in the "right" direction to work well. This

antenna design is used in many radar (and other microwave)

applications, as well as in satellite communication. And is has a

home with radio astronomers, too, but they're usually listening

instead of transmitting. It has a parabolic reflector, and some

kind of support for the feedhorn, sub-reflector or whatever is at

the focus. We're talking about a transmission antenna here, so

there will be some kind of feed assembly to put the signal onto the

parabolic reflector to "send out" or transmit that signal. How do

we test it? It's so simple that you're not gonna believe it. The

reflector can be modified a bit to "broaden" the primary lobe of

the radiated signal either horizontally or vertically. But let's

work with a simple parabolic reflector. Imagine a parabolic antenna

that is fixed so it's stationary. Let's look at which way the

signal goes. There are two variables to assess when plotting the

radiated pattern, and they are usually referenced to the

"direction" or "directivity" of the antenna, or the direction of

what might be termed the primary lobe or beam of the radiation. Put

another way, there is one direction that is the "center of the

beam" for this antenna, and once we establish this line, we

reference to it. Something is either left or right of the line by

"x" number of degrees, or something is above or below the line by

"y" degrees. You gonna put this up on a pole and walk around

measuring radiated power at different points left or right of, or

above or below the beam? Remember I said this was ease? Put the

antenna on a stand and make it point horizontally. Make that stand

like a heavy duty lazy susan so the whole thing rotates. Hook up a

signal generator to it, and put in the desired operating frequency.

(The signal generator won't be generating high power, and that's

okay. A milliwatt isn't even necessary for the test.) We're now

ready to transmit, and that's our test antenna setup. Step off a

hundred meters (or whatever) and set up a receiving antenna

(pointed at the antenna under test and at the same level). Hook up

a receiver to the receiving antenna so that signal strength can be

measured. Turn on your equipment and rotate the turntable slowly.

As the turntable rotates, it causes the antenna being tested to

"sweep" the horizon with its little output signal. As it moves

around, the receiver will be getting more and more and more signal,

or less and less and less signal, depending on whether the test

antenns is sweeping toward or away from the receiver. With a

computer hooked up to the receiver (via a handy IEEE bus) and doing

some recording, a relative signal strength can be plotted. Presto!

You've got a 360o plot of the relative output signal strength. All

you have to do is raise you receiving antenna a touch, and then

point it down a tiny bit so it's aimed directly at the test

antenna. Then turn the test antenna and record for another 360o

view at a bit high of beam center. Keep moving up the receiving

antenna in steps, realigning it, and testing a circle. Do this for

a bunch of vertical levels above beam center. Then come back and do

it again for a bunch of levels below beam center. You're done! The

trick is to set up the test antenna on a turntable and point it

flat out and level with the horizon, and then to begin with a

receiving antenna level with and pointed directly at the test

antenna. The turntable does most of the work, and it makes it easy.

The work is in raising or lowering the receiving antenna in

calculated steps and realigning it at each step to point it

directly at the test antenna. By the time the test crew get

finished, the computer can plot a nice 3D chart (in the form of a

thick cylinder with the test antenna at the center that will

demonstrate the performance of that test antenna. Piece of cake.

The paraolic antenna has a high degree of "direcitivity" compared

to many other antennas. That gives these puppies big gain. This

antenna design is used in many radar and in satellite communication

applications, as well as having a home with radio astronomers. It

has a parabolic reflector, and some kind of support for the

feedhorn, sub-reflector or whatever is at the focus. We're talking

about a transmission antenna here, there will be some kind of feed

assembly to put the signal onto the parabolic reflector to "send

out" or transmit the signal. How do we test it? It's so simple that

you're not gonna believe it. The reflector can be modified a bit to

"broaden" the primary lobe of the radiated signal either

horizontally or vertically. But let's work with a simple parabolic

reflector. Imagine a parabolic antenna that is fixed so it's

stationary. Let's look at which way the signal goes. There are two

variables to assess when plotting the radiated pattern, and they

are usually referenced to the "direction" or "directivity" of the

antenna, or the direction of what might be termed the primary lobe

of the radiation. Put another way, there is one direction that is

the "center of the beam" for this antenna, and once we establish

this line, we reference to it. Something is either left or right of

the line by "x" number of degrees, or something is above or below

the line by "y" degrees. You gonna put this up on a pole and walk

around measuring radiated power at different points left or right

of, or above or below the beam? Remember I said this was ease? Put

the antenna on a stand and make it point horizontally. Make that

stand like a heavy duty lazy susan so the whole thing rotates. Hook

up a signal generator to it, and put in the desired operating

frequency. (The signal generator won't be generating high power,

and that's okay. A milliwatt isn't even necessary for the test.)

We're now ready to transmit, and that's our test antenna setup.

Step off a hundred meters (or whatever) and set up a receiving

antenna (pointed at the antenna under test and at the same level).

Hook up a receiver to the receiving antenna so that signal strength

can be measured. Turn on your equipment and rotate the turntable

slowly. As the turntable rotates, it causes the antenna being

tested to "sweep" the horizon with its little output signal. As it

moves around, the receiver will be getting more and more and more

signal, or less and less and less signal, depending on whether the

test antenns is sweeping toward or away from the receiver. With a

computer hooked up to the receiver (via a handy IEEE bus) and doing

some recording, a relative signal strength can be plotted. Presto!

You've got a 360o plot of the relative output signal strength. All

you have to do is raise you receiving antenna a touch, and then

point it down a tiny bit so it's aimed directly at the test

antenna. Then turn the test antenna and record for another 360o

view at a bit high of beam center. Keep moving up the receiving

antenna in steps, realigning it, and testing a circle. Do this for

a bunch of vertical levels above beam center. Then come back and do

it again for a bunch of levels below beam center. You're done! The

trick is to set up the test antenna on a turntable and point it

flat out and level with the horizon, and then to begin with a

receiving antenna level with and pointed directly at the test

antenna. The turntable does most of the work, and it makes it easy.

The work is in raising or lowering the receiving antenna in

calculated steps and realigning it at each step to point it

directly at the test antenna. By the time the test crew get

finished, the computer can plot a nice 3D chart (in the form of a

thick cylinder with the test antenna at the center that will

demonstrate the performance of that test antenna. The output

pattern should look like a long, skinny teardrop. Piece of

cake.


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