Finding the focal length, adjusting the focal length, measuring the effects of changing the focal length of the SRT.
Once the dish and the feed supports of the SRT have been assembled the focal point of the dish must be determined and adjusted to maximize antenna efficiency. The focal point is the common point that all of the collected radio waves will be reflected back to. The distance from the center of the dish to this point in front of the dish is the focal length of the antenna. SRT antennas have changed in size, depth and location of feed supports. This in turn has changed the focal length of the collecting dish. The diameter and the depth of the dish, determine the focal point of the dish itself. The feed supports determine the placement of the receiver, which must be at the focal point of the telescope.
Step
one: Determining dish measurements:
With
the dish in the stow position measure the diameter of the dish and the depth of
the dish. Using a length of string or thin wire determine the diameter of the
dish.
1.
Tie
the string or wire to one edge of the dish and stretch the wire across the
dish. When you have the measuring device at it’s maximum possible length, you
have the diameter. Anything less would be a chord of the circle. Tie the second
end to the dish at its maximum length.
2.
Measure
the length of the string with a tape measure.
3.
From the center of the string measure down to the center of the dish. This is
the depth of the dish.
4.
From the center of the dish measure up to the bottom of the plastic circle
(phase center) on the bottom of the receiver. This is the distance of the
receiver from the dish.
A)
Dish
Diameter: ______________
B)
Depth of the dish: _____________
Step Two:
Finding the focal length of the SRT:
The
formula for finding the focal length of the dish is: f=Dsquared/16x
D=
diameter of the dish (inches)
X =
depth of the dish (inches)
Using
your data, what is the focal length of the SRT?
C)
Focal
length of SRT: _________________
Step Three: Current
placement of the receiver & adjustment:
What
did you find for the current distance receiver to center of the dish in step
one?
_________________
What
is the focal length of the SRT that you found mathematically?
________________
What
is the difference between the two measurements? (= focal length adjustment)
________________
There
have been at least two different feed support systems supplied with the dishes.
The most recent and easiest to adjust features the feed arms located about ¼ of
the way inside of the dish. The length of the feed arms can be changed by
inserting lengths of ½” ID pvc water pipe as stand offs.
Cut
four pieces of pvc to the length of the focal length adjustment. Cut two other
sets of stand offs, one set one half inch longer, a second set one half inch
shorter than the calculated focal length adjustment. These pvc sections will
serve as sleeves around the bolts. You will also need to obtain four 3/8” bolts
that are about 3” long to replace the existing feed support bolts.
Procedure:
Using
the AzEl command, direct the SRT to an empty point in the sky. Set the
frequency of the SRT to 1415. MHz and 4 bins (1415 4). Using the “vane” or
“calibrate” command (whichever is available on your SRT), calibrate the SRT
without any focal length adjustments. Record in your data table: date, time,
frequency, and length of stand off if any, tysys, calcon, trec, counts, and
temp (k).
Aim
the SRT at the sun and record in your data table: date, time, frequency, tysys,
calcon, trec, counts, and temp (k).
With
the SRT antenna in the 88 degrees of elevation position, loosen the nuts on the
bolts of two adjoining feed arms. Remove the nuts off of the remaining two feed
arms. Lift the one of the feed arms that has had the nut removed. Remove the
old bolt; insert the new, longer bolt. Place the smallest pvc stand off over the bolt between the feed arm and the
antenna. Loosely attach the nut to the bolt. Move to one of the feed arms that
still has a nut on it. Follow the same procedure to replace the bolt &
insert the stand off. Repeat until all four feed arms have been extended. Go
around and tighten all of the nuts.
Aim
the SRT at the sun and record in your data table: length of stand off, date,
time, frequency, tysys, calcon, trec, counts, and temp (k).
Return
the SRT antenna in the 88 degrees of elevation position; loosen the nuts on the
bolts of two adjoining feed arms. Remove the nuts off of the remaining two feed
arms. Lift the one of the feed arms that has had the nut removed. Remove the
old bolt; insert the new, longer bolt. Place the largest pvc stand off over the bolt between the feed arm and the
antenna. Loosely attach the nut to the bolt. Move to one of the feed arms that
still has a nut on it. Follow the same procedure to replace the bolt &
insert the stand off. Repeat until all four feed arms have been extended. Go
around and tighten all of the nuts.
Aim the SRT at the sun and record in your data table: length of stand off, date, time, frequency, tysys, calcon, trec, counts, and temp (k).
Return
the SRT antenna in the 88 degrees of elevation position; loosen the nuts on the
bolts of two adjoining feed arms. Remove the nuts off of the remaining two feed
arms. Lift the one of the feed arms that has had the nut removed. Remove the
old bolt; insert the new, longer bolt. Place the pvc stand off that equaled your focal length adjustment
over the bolt between the feed arm and the antenna. Loosely attach the nut to
the bolt. Move to one of the feed arms that still has a nut on it. Follow the
same procedure to replace the bolt & insert the stand off. Repeat until all
four feed arms have been extended. Go around and tighten all of the nuts.
Aim
the SRT at the sun and record in your data table: length of stand off, date,
time, frequency, tysys, calcon, trec, counts, and temp (k).
Questions:
Which
of the four receiver settings (no stand off, etc.) produced the highest
temperature readings?
It
would be expected that the highest temperature readings would be with the stand
offs that were designed to correct the focal length. Was this the case? If not, why would this have happened?
What
effect does a ½” change in focal length have on receiver efficiency?
Do
you think time of day (morning versus noon, afternoon) would have an effect on
your temperature readings?
Make
a graph of temperature reading versus length of stand off.
Does
the graph yield a straight line? Why or
why not?
Data table: