Hint: a click on an image will show it full size in a new tab.
How to start ...
In all operations on the Compute & Display
page, we first give
and the Dish diameter
of the antenna. We may
specify the diameter in wavelengths or in meters. Hit the enter key after
you entered the number in the chosen field, to make this field green to
confirm your choice.
Then you will find below the ideal HPBW, which is for an uniformly illuminated
circular dish. The true HPBW and the effective diameter are computed and displayed
only when you had chosen to display the antenna pattern
Then you give the parameters of the Feed:
Specify its diameter (we assume that it is of circular shape, too), and
choose among a list the type of feed. This list contains:
- several theoretical patterns, from a uniform illumination to various
more focussed patterns. The screenshot shows that of a simple dipole
whose radiation is shielded towards the outside
- measured or simulated feeds from W1GHZ: waveguides, horn antennas,
helical feeds, and others
- read the data from Input page: here you enter the feed pattern
in a text area at Input: Feed Pattern, following the example
given there. It is the same syntax as in W1GHZ files.
- read the data from User computer: When this option is chosen, a button
Browse... appears at the top of the page. Clicking this will
open a file selector menu on your computer, with which you select your
file. Note that your file must be a text file, conforming to the
format given in the example on Input: Feed Pattern!
Furthermore, you may modify any chosen pattern by cutting feed pattern
beyond a specified angle. This lets you explore what would happen if the
feed pattern could be made narrower...
The HPBWs in the E-plane and the H-plane are displayed. Note that some patterns
are only given in the E-plane, including the data from the Input page or from
the User computer!
The sketch left of the plot shows the true geometry of the antenna and feed,
with the feed pattern (red curve) and the opening angle of the dish as seen
by the feed (blue lines).
Common to all plots is the display of the current coordinates of the computer
mouse, which are displayed at the lower left corner.
Next, chose the Display
... view the Feed Pattern
The plot shows the pattern of the feed antenna: if the pattern is available
in both E- and H-plane, the red curve displays the E-plane pattern. Otherwise,
a single magenta curve shows the feed pattern. The dot on each curve indicate
the feed HPBW in the corresponding plane. The vertical blue line marks the
angle to the dish's rim; thus, the power in the part of the pattern that extents
beyond this angle is lost by spill over.
... view the Dish illumination
The plot shows how the illumination varies over the dish: if the pattern is available
in both E- and H-plane, the red curve displays the E-plane pattern. Otherwise,
a single magenta curve shows the feed pattern. The vertical blue line marks the
outer rim of the dish; thus, the power in the part of the pattern that extents
beyond this radius is lost by spill over.
... view the Efficiency Plot
This plot shows the same quantities as the plots in Paul's book, as functions of
dish f/D ratio.
The red curve is the feed efficiency, i.e. the fraction of the power fed to the antenna
which is actually radiated into the antenna beam and thus in the desired direction.
Like in Paul's program, this and the other quantities are computed by taking
the mean of the E- and H-plane amplitudes at each angle as the amplitude which
illuminates the dish.
The green curve shows the spill-over loss, i.e. the fraction of feed power that is
radiated beyond the dish's outer rim, and thus is not available for being focussed
into the antenna beam. The grey curve is the illumination loss which is a measure
of how much the dish differs from the ideal of fully illuminated antenna.
The dots in the curves mark the currently chosen f/D ratio, as shown on the
left hand panel. Blue marks indicate the half opening angle of the dishes as a function
of f/D ratio.
Please note that this feed efficiency
is neither the aperture efficiency
which is the fraction of the radiated power of what could be radiated by a uniformly
illuminated dish nor what may be called the overall efficiency
which would include
the beam efficiency
. Any other effects, such as scattering due to surface
irregularities and ohmic losses, would further need to be taken into account.
Sometimes, several green curves are present, as in this example of an isotropic feed.
They show the spill-over loss in more detail:
- A curve with long dashes is the spill-over to angles higher than 90° from the
main beam, and thus towards the rear of the dish. This is important because of the
pickup of thermal noise from the ground.
- Shorter dashes are the spill-over to the sides, to angles between 30 and 90°
off the main beam. Usually this is not too important if there are no high trees
or buildings block the sight.
- Very short dashes is the spill-over to the front, within 30° of the main beam.
Obviously this is of little importance, as one would pick up noise from the sky.
- The full line is the total spill-over, i.e from all directions.
The curves are marked by the letters B, S, and F or a dot at the currently chosen
f/D ratio, as shown on the left hand panel.
One may also investigate the effects of a conductive
collar at the dish edge. This cuts down the spill-over from the rear of the
antenna, and therefore reduces the noise.
Also, a magenta curve gives the illumination at the edge of the dish. This is in
linear units, so that the usually recommended value of -10 dB corresponds to 0.1.
... view the Gain Fraction
This plot shows how much each circular ring of the dish contributes to the antenna
gain. One notes that the central 10 percent of the dish provide only a small amount
to the gain, much less than a ring with a width of 10 percent of the dish radius.
The reason is the difference in surface area of each part. The small horizontal
black bar near the left top indicates the geometric obscuration by the size of the
... view the Antenna Pattern
The far-field radiation patterns (in the E-plane or H-plane; the "overall"
view assumes as feed amplitude the mean of E and H plane values, as used for
the calculation of efficiencies) are computed for the chosen configuration of the
antenna. This may take a few seconds, before the curve is displayed. The red
dot marks the true HPBW whose value is also displayed on the left hand panel,
along with the effective diameter of the dish. This is the diameter of a uniformly
illuminated circular dish which gives the same HPBW as the chosen antenna.
The aperture efficiency is the ratio of the effective and the geometrical area
of the dish (i.e. the ratio of the squares of effective and geometrical diameters).
In this example it is 72%.
Several parameters are displayed: the antenna solid angle ΩA
the beam efficiency ηB
spillover, illumination loss, edge and feed taper.
... view the Antenna Pattern including Spill-Over
A second plot shows the far-field radiation pattern for the entire hemisphere,
together with the spill-over contribution as the green curve. The vertical black
line at 90° indicates the border between the forward beam (left) and the
backward side of the antenna. It also displays
the beam efficiency factor which is the fraction of the forwardly radiated power
that is in the main beam, thus a measure of the (un)importance of the sidelobes.
... play with parameters
When you change a parameter, such as the diameter of the feed, hit the enter key
and the plot will be updated with the new data. In this way you can explore how
the parameters influence the results.
... compare different models
Normally, a change of parameters will clear the plot to show the current version.
Click the button overplot
. As long as it has a green colour (and is active),
the curves from all following models are superposed. This allows you to compare
directly how different values for a parameter change the results. If you take
a screenshot, you may want to edit that image file (with "Paint") to mark the
different curves with their associated parameters, as shown in the above picture.
Once the plot gets too cluttered up with curves, use the button clear
to wipe all curves.