Investigation on Patch Antenna Characterisations
Investigation on Patch Antenna Characterisations
Patch antenna (Rectangular Antenna) is a popular antenna type shown in Figure 1. It shows
a wider beam angle, can be broadband (4 - 9% of centre bandwidth), is robust and easier to
design.
Figure 1 Patch Antenna
Its name is attributed to the fact that it consists of a single metal patch suspended over a
ground plane. Patch antennas are simple to fabricate and easy to modify and customise.
They are based on the original microstrip patch antenna as described by Howell [1]. The
radiation mechanism arises from discontinuities at each truncated edge of the microstrip
transmission line [2]. The radiation at the edges causes the antenna to be slightly larger than
its physical dimension electrically. In order to obtain a resonant condition at the antenna
driving point, a shorter than a one-half wavelength section of microstrip transmission line is
used. A patch antenna is generally constructed on a dielectric substrate, usually employing
the same sort of lithographic patterning used to fabricate printed circuit boards. The patch
antennas used here have a line feed through the ground plane to the front radiator
Configuration
The simplest patch antenna uses a patch which is one half-wavelength-long with the dielectric
loading included over a larger ground plane separated by a constant thickness. Electrically
large ground planes produce stable patterns and lower environmental sensitivity but of course
make the antenna bigger. It isn’t uncommon for the ground plane to be only modestly larger
than the active patch. When a ground plane is close to the size of the radiator it can couple
and produce currents along the edges of the ground plane which also radiate. The antenna
pattern becomes the combination of the two sets of radiators.
The current flow is along the direction of the feed wire, so the magnetic vector potential and
thus the electric field follow the current. A simple patch antenna of this type radiates a linearly
polarized wave. The radiation can be regarded as being produced by the ‘’radiating slots’’ at
top and bottom, or equivalently as a result of the current flowing on the patch and the ground
plane. Experimentally, how would you verify this? Can you do this is part 2 of following
experiment?
Gain
The gain of a rectangular patch antenna with air dielectric can be very roughly estimated as
follows. Since the length of the patch, half a wavelength, is about the same as the length of a
resonant dipole, one can acheive approximately 2 dB of gain from the directivity relative to the
vertical axis of the patch.
If the patch is square, the pattern in the horizontal plane will be directional, somewhat as if the
patch were a pair of dipoles separated by a half-wave; this counts for another 2-3 dB gain.
Finally, the addition of the ground plane cuts off most or all radiation behind the antenna
(Front-back ratio), reducing the power averaged over all directions by a factor of 2 (and thus
increasing the gain by 3 dB). Adding this all up, one can obtain approximately 7-9 dB for a
Dr. Sandra Dudley
Mr. Oladimeji Onalaja
Mr. Alan Nigrin
Mr. Ya Bao
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Investigation on Patch Antenna Characterisations
square patch, in good agreement with more sophisticated approaches (see Balanis, p. 841,
for more details).
Please note that throughout this work the formulas use Metric units.
Most Wi-Fi products operate in the 5.7 GHz (or 2.4 GHz) ISM band or the 5.7 GHz UNII band.
These frequency bands are free (unlicensed band) to be used by many users in the same
geographic area. While spread spectrum technology can support this type of use, it is not
100% immune from interference. Then antennas here are designed to work at 2.4GHz.
These interfering signals can be from other users, or from other links installed by the
same user. A professional site analysis would normally require the use of a spectrum
analyzer and directional antenna to sweep the area and look for unwanted or potentially
interfering signals. However, a simple approach is to use one of the radio products with a
typical antenna and look at its Received Signal Strength Indicator (RSSI) output. If the RSSI
shows low signal levels in the direction of interest, then the frequency is clear enough to use.
For best performance, the RSSI should show a signal level lower than the operating signal
level of the link. The operational signal level of the link is calculated below.
Path Analysis (the RF signal strength)
The path analysis calculates the expected receive signal strength by taking into account the
transmit power on the far end, adding antenna gains and subtracting signal loss through
space.
At the end of this section, a specific example will be used to illustrate this procedure. The
expected Receive Signal Strength (RSS) is calculated from the following formula.
RSS = Tx Output Power + Tx Antenna Gain – Path Loss + Rx antenna Gain.
The Path Loss (L) is calculated using the following formula.
L = 32.5 + 20 log (distance in km) + 20 log(frequency in MHz)
For example, the expected receive signal level using a known transmit antenna system
system (transmit output power is 24 dBm) with 24 dBi antennas and a path length of 10 km at
2.4 GHz is calculated as follows.
Path Loss at 10 km at 2.4 GHz = 32.5 + 20 log(10) + 20 log (2400) = 120 dB
The expected Receive Signal Strength is calculated next.
RSS = 24 dBm +24 dBi – 120 dB + 24 dBi = – 48 dBm
The same link at 5.7 GHz would have an expected receive signal level calculated as follows:
Path Loss at 10 km at 5.7 GHz = 32.5 + 20 log (10) + 20 log (5700) = 127.6 dB
RSS = 24 dBm +24 dBi – 127.6 dB + 24 dBi = – 55.6 dBm
Q. 1. Which system has the greater loss and why do you think this is the case?
For industry a rule of thumb is normally RSS should be around 20dB higher than the
sensitivity of the system, to enable loss occurrence which would not mean that the link was
left open. You will see this explained in WCAN lecturers. Of course with software background
Dr. Sandra Dudley
Mr. Oladimeji Onalaja
Mr. Alan Nigrin
Mr. Ya Bao
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Investigation on Patch Antenna Characterisations
capabilities, the RSS can be much lower sometimes as low as 1dB. But path fading will be
critical in this case.
The system looks as that shown in Figure 2 below.
Figure 2: Experimental set-up
Initial Experiment Setup
1)
Ensure that the black cable connected to the receiving antenna is NOT plugged into
the wireless adaptor of PC2.
2)
Check the retort stands to ensure that both antennas are secured onto them and they
are aligned and as close as possible without touching.
3)
Switch on PC1 and PC2, if they are NOT already on.
4)
On PC1, locate the insider software on the desktop and double click on it to start it
up. Once started, locate the start button on the top right hand corner of the insider GUI. (Ask
demonstrator for assistance if necessary).
5)
On PC1 click on the start button you had earlier identified in step 4 above.
6)
At this stage you should see all the network connections in range. Pay attention to
just one of them named ‘labwork’ as we you will be using this connection for the duration of
your experiment. It may be at the bottom of the list as this will be the most recent.
7)
At the preamble of every visible connection, there is a tick, untick all apart from the
most recent ‘labwork’. This will be near the bottom of the list.
8)
At the bottom of the GUI, you have some screen options namely ‘News’, ‘Time
Graph’, ‘2.4 GHz channels’, 5 GHz channels’, ‘Filters’ and ‘GPS’. Locate ‘Time Graph’ and
click on it.
9)
At this point, you should be able to see Amplitude vs. Time graph. On the tabs you
will see, mac address, RSSI etc, use these for your experiment.
10)
Before you connect the cable to PC 2, make a note of the RF level and the RSSI.
What does RSSI mean?
Procedure 1
Dr. Sandra Dudley
Mr. Oladimeji Onalaja
Mr. Alan Nigrin
Mr. Ya Bao
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Investigation on Patch Antenna Characterisations
The experimental set-up is as follows
One PC is acting as the access point and the other acting as the receiver. The AP has
access to the exchange via a network cable; the receiver only has access to the exchange via
the AP.
1) Initially, using the ruler given, set antennas 5cm apart and facing each other. Make sure
they are aligned properly.
2) Using the inSSIDer software panel available on the screen of PC1 you will see the noise
floor before connection and the RSSI. Using the noise floor value you can now calculate the
SNR when the cable is connected. Make a note of this value.
3). Now connect the Black cable to PC2’s wireless card. You will now need to note the power
of the new signal. Carry this out either using excel and place this sheet in your log book or by
performing the calculations by hand and place in log book, but either way make sure
calculations are clearly shown in your log book.
4) Repeat 2 and 3 until you reach 100cm by measuring in 5cm steps. The graph should
clearly show RSSI signal against distance (m) so you can work out the path loss over the
distance of 100cm.
5) Using the equation for RSSI, can you work out the Path Loss for this system at 30cm?
Take the antennas to have a gain of 7dBi each.
RSSI = Tx Output Power + Tx Antenna Gain – Path Loss + Rx antenna Gain.
What does RSS mean and what is its significance in wireless systems.
6) Using the Path Loss equation previously expressed, does this agree with that found in the
RSSI equation?
Procedure 2
Radiation pattern, antenna polarisation and front back isolation ratio
1) Place antennas 15cm apart and make sure they are aligned.
2) Using protractor on receive antenna, turn antenna on pivot 90 degrees from left (0) to right
(90) away from you in the z axis and take signal measurement for each 10 degree
movement. Repeat this until you reach 90 degrees.
3) Now move antenna back to zero and perform the same measurements going from right to
left (towards you).
4) Create a radiation pattern graph for your results using excel or by hand; it should be
somewhat similar to Figure 3.
5) Now move antenna back to zero keeping the antennas at least 20 cm apart. Keeping the
receive antenna facing the transmit antenna, turn receive antenna in an anticlockwise
direction until is at 90 degrees to its original position. It will still face the transmitter. Take the
signal strength again here. If in doubt, please ask demonstrator to show you. Take this
measurement and compare it to the signal strength when both antennas are “upright” at the
same distance apart.
Dr. Sandra Dudley
Mr. Oladimeji Onalaja
Mr. Alan Nigrin
Mr. Ya Bao
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Investigation on Patch Antenna Characterisations
6) From the introduction section can you tell which way is the antenna polarised, or is it equal
in all directions? What is the ratio?
7) Using the same distance in 5, turn receive antenna 180 degrees so its back is facing the
front of the Transmit antenna and reread the signal strength. Take the ratio of 0 and 180
measurement and this is you Front-back isolation ratio. Show this calculation in your log book
and explain its importance in antenna propagation usage.
Figure 3: Typical Patch antenna radiation pattern
For both Procedure 1 and 2 write appropriate conclusions regarding your results and your
answers to the questions within the experimental sheet.
References
1. J. Q. Howell “Microstrip Antennas," IEEE International Symposium on Antennas and
Propagation, Williamsburg Virginia, pp. 177-180, 1972.
2 C. A. Balanis. “Antenna Theory: Analysis and Design, 3rd Edition,” John Wiley & Sons,
2005.
3. D. M. Pozar “Microstrip Antennas,” Edited by D. M. Pozar and D. H. Schaubert, IEEE
Press, 1995.
Dr. Sandra Dudley
Mr. Oladimeji Onalaja
Mr. Alan Nigrin
Mr. Ya Bao
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