University of Hong Kong
Department of Electrical and Electronic Engineering
Lab ANT
EXPERIMENT: Introduction to antennas
OBJECTIVE : To investigate the radiation pattern of dipole antennas.
APPARATUS
The Feedback AMS506 Antenna Modeling System, an IBM PC compatible computer.
INTRODUCTION
As shown in Fig. 1, the system comprises an RF generator (transmitter) unit, a
receiver unit, a set of antenna elements and the Feedback MIC926 interface. The computer
has control of the generator/receiver frequency, antenna position, rotation speed and direction
as well as the received signal level. The software provided is a complete operating system so
that all measurements are controlled by selecting operations from a menu and the results are
displayed in graphical form on the computer screen. The following parameters can be
measured:
* Directivity and gain, displayed as polar diagrams
* Bandwidth, displayed as graphs of amplitude against frequency
* Matching, displayed as graphs of return loss against frequency
Log periodic array Test antenna
(Receiving antenna) (Transmitting antenna)
RF-in RF-out
Receiver unit socket Transmitter unit socket
(Generator unit)
Fig. 1. Antenna modeling system
Lab ANT
Radiation pattern of an antenna
The directive gain is defined as the ratio of the power flow at a given direction to the
overall average among all directions. The maximum of the directive gain is the directivity of
the antenna. The directional characteristic can be displayed by a polar plot. Corresponding
to each point on the plot with polar coordinates (r, ), the distance of the point from the origin
(i.e., |r| ) represents the intensity (usually in dB) of the radiation at direction .
Return loss measurement
It is desirable to match an antenna to the transmission line. One way to determine the
degree of match is to perform return loss measurements. Return loss is defined as:
Return loss = -10 log10||2 dB
where is the reflection coefficient.
The return loss represents the fraction of the forward power reflected back from (hence
unabsorbed by) the antenna. If the return loss is measured, the reflection coefficient can be
determined, and the VSWR can be calculated as follows:
1 | |
VSWR
1 | |
Return loss measurement can be carried out by a directional coupler. Please refer to Fig. 2.
As a signal flows from port A to port B, part of the signal is coupled to port F and port R. By
suitably designing the path lengths of the directional coupler, the interference at F will be
constructive while that at R will be destructive. The net result is that the signal flowing from
A to B will leak through port F but not through port R. On the other hand, the signal
reflected back from the load (the test antenna) will leak through port R but not port F. Hence
by measuring the ratio of the power leaked to port R and port F, the return loss can be
determined.
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Lab ANT
To the test (transmitting)
antenna (using a 15 cm
long cable)
B To a power measurement
A: main input port
B: main output port R apparatus (using a 1.8 m
F: forward port (reverse port) cable). See notes below.
R: reverse port
F (foward port)
To a power measurement
apparatus (using a 1.8 m A
cable). See notes below.
From the RF-out socket on the
generator tower (using the cable which
originally connects the test antenna to
the generator).
Fig. 2. Directional coupler for return loss measurement. The power measurement apparatus
in this experiment is the microwave receiver. To connect to the receiver, first
disconnect the receiving (log periodic) antenna from the receiver inlet (the RF-in
socket on the receiver tower), then connect the signal to be measured to the inlet.
Director Dipole Reflector
Cable
Fig. 3. Yagi-Uda antenna.
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Lab ANT
PROCEDURE
Part I -- Initialization and Operation of the Antenna system
Perform the following initialization steps
1. Boot up the PC in DOS mode. Execute the batch file ANT1.bat. The file activates
two programs: a screen capture program and the ANT17.exe antenna operation
program.
2. Make sure that the 'motor enable' switch on the generator base is switched OFF before
switching on the hardware. Switch on the AMS506 'power' switch, located on the
AMS506G base.
3. Input the appropriate response to the messages displayed.
Screen mode Select VGA mode
Computer speed This concerns matching the software speed to that of the
AMS506 hardware. The software makes an attempt to measure
the calculating speed of the computer and displays the result as
an index number. This index can then be used to set the
software speed by pressing ENTER or entering a different
number followed by ENTER, but some number larger than 100
is enough for general purpose.
Index Search Here the antenna drive shaft is rotated to its reference or index
position. Switch the 'motor enable' on at this stage. The switch
should be left in the 'on' position.
Data path The AMS506 can record data and allows you to set which drive
and which directory is used. You may choose the default path.
4. Select Signal Level (menu item 4). Choose a frequency of 1500 MHz. Align the
transmitting (test) antenna with the receiving (log periodic) antenna. The two
antennas should be separated by at least 3 metres. Try to maximize the level of the
received signal by aligning the antennas. The signal level should be within the range
of 40 to 60 dB.
This completes the initialization steps.
Useful keys during normal operation
SPACE Returns to the main menu, losing the data that has just been plotted.
ENTER Repeats the plot at the same frequency, this might be used after an
antenna adjustment has been made. The original data is lost.
C Re-plots the same diagram with the scale changed from a 35dB range
to a 70dB range. The raw data was taken over a 70dB range, and then
scaled to 35dB with the maximum placed at 0dB. When scaled to
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Lab ANT
70dB the data is simply the raw level information and therefore the
maximum is not set to 0dB.
S This facility enables several plots to be superimposed on the same
background, with the original plot as a reference. Press S and you will
be asked for a new frequency. Maximum five plots can be displayed
on the same background at the same time.
R This causes the plot to be recorded using the path specified during the
software start-up.
The filenames are generated by the system from the data and time read
from the computer's clock. After the recording has been made the
filename appears at the bottom of the screen. The format is
DDHHMM.xPD, DD is the date, HH the hours, MM the minutes and x
may be P, F or R.
All files containing recorded data are in a standard ASCII format.
Screen capture The screen image can be captured by pressing .
The image (in .GIF format) will be captured and stored under directory
C:\graph.
Playback recorded data
This option enables you to display data previously recorded by one of
the measurement functions.
Exit to DOS This function terminates the AMS506 operating system and returns to
the DOS environment. Any data not recorded on the disk is lost.
Warning
Before exit from the system, you MUST
switch the motor enable switch OFF or
spurious commands may cause the motor to
rotate continuously and cause cable demage.
Colour convention
The AMS506 operating system uses a 16-colour screen. The following colour convention is
used:
* Blue background Option menu or status information
* Green background Measured data
* White text Status messages not requiring a user response
* Yellow text Requests for user instructions
* Gray text Unavailable options
* Red text Error messages
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Lab ANT
Part II -- Radiation Patterns of Dipole Antennas
1. Adopt a frequency of 1500 MHz. Set the length of the dipole antenna to /2 (i.e., 10
cm).
2. Obtain a polar plot of the radiation pattern by selecting menu item 3. Save the screen
image into a file (.GIF format) by pressing .
3. Lengthen the antenna to 1.5 . Obtain the polar plot. Save the screen image.
4. Lengthen the antenna to 2.0 . Obtain the polar plot. Save the screen image.
5. Set the length of the dipole back to 0.5. Add a reflector with a length slightly longer
than 0.5 at the back of the dipole, as shown in Fig. 3 for a Yagi-Uda antenna.
6. Vary the distance between the reflector and the dipole and obtain polar plots of the
radiation patterns by selecting menu item 3. Keep the reflector at the position where
the directive gain is the maximum. Save the screen image.
7. Add a director with a length slightly shorter than 0.5 at the front of the dipole, as
shown in Fig. 3. Vary the position of the director and obtain the corresponding polar
plot. Find the position for maximum directive gain. Save the screen image.
Part III -- Return Loss Measurements
1. Adjust the length of the dipole antenna to /2.
2. Connect the system as shown in Fig. 2. Measure the power output from port F by
connecting port F to the receiver inlet (the RF-in socket on the receiver tower).
3. Select 'Return Loss Plot' (menu item 2). Follow the instructions on the screen. Then
measure the power output from port R by disconnecting the cable from port F and
connecting it to port R instead. The return loss plot (return loss vs. frequency) will be
displayed on the screen. Save the screen image.
4. Use a dipole antenna of length 1.5. Measure the return loss by repeating steps 2 and
3. Save the screen image.
5. Use a dipole antenna of length 2. Measure the return loss by repeating steps 2 and 3.
Save the screen image.
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REPORT
1. Compare the polar plots of the 0.5-, 1.5- and 2.0-dipoles obtained in the
experiment with theoretical values. Comment on the results.
2. From the polar plot of the 3-element Yagi-Uda antenna, determine the front-to-back
ratio and the beam width.
3. Calculate the theoretical values of the radiation resistance of the 0.5-, 1.5- and
2.0-dipoles. Explain the assumptions made.
4. Assume that the characteristic impedance of the cable feeding the antenna is 70 ,
and assume that the antenna load is equal to the radiation resistance obtained in Step 3
above, calculate the theoretical values of the reflection coefficient || and the return
loss. Compare this with the results of the return loss measurements. Comment on the
results.
Note:
The directional coupler in this experiment is not perfect. The return loss
measurement results below about -18dB should not be taken as accurate. Hence any
value below -18dB should just be considered as some value less than -18dB and
the exact value should be ignored.
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