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VSWR MeasurementWhen a transmission line (cable) is terminated
by impedance that does not match the characteristic impedance of
the transmission line, not all of the power is absorbed by the
termination. Part of the power is reflected back down the
transmission line. The forward (or incident) signal mixes with the
reverse (or reflected) signal to cause a voltage standing wave
pattern on the transmission line. The ratio of the maximum to
minimum voltage is known as VSWR, or Voltage Standing Wave Ratio.
Signal reflection occurs when the RF signal reflects back due to an
impedance mismatch or change in impedance caused by excessive
kinking or bending of the transmission line. The best possible
value is 1:1 which means that all of the power generated by the
transmitter is radiated from the Antenna.
Where to MeasureIt is important to know that for accurate VSWR
measurements of devices, the VSWR should be measured at the input
of the device in question (antenna etc). Any cable loss or
attenuation, will make the VSWR at the input of the cable appear
much better than at the load or termination. The reason is that the
cable loss or attenuation increases the return loss.VSWR of the
feeder cable is measured with the help of site master.
Open/Short/Load 50 ohm is required for calibrating the site master
for accurate result. At the BTS site RF cable (jumper cable
connected to feeder cable) is removed from BTS and connected to
site master RF Out/Reflection port. Other end of the RF cable is
connected with sector Antenna. Site master is operated with 12.5 V
dc. At tune on the site master runs through a series of quick
checks to ensure the system is functioning properly. The Anritsu
site Master is capable of up to 1.5 Hours of continuous operation
from a fully charged, field replaceable battery. It can also be
operated from an external DC source using AC-DC adapter. If the
battery is low, or if the ambient temperature is not within the
specified operational range, self test will fail. Site master must
be re-calibrated whenever the setup frequency changes, the
temperature exceeds the calibration temperature range or when the
test port extension cable is removed or replaced. If the cal off
message is displayed, or the test port cable has been changed, a
new calibration is required.
How to Operate
Press the ON/OFF key on the Site Master. Press the MODE key. Use
the Up/Down arrow key to select the Freq-SWR mode and press enter
Press the Freq/Dist key. Press the F1 soft key as displayed on
screen. Enter the desired start frequency using the key pad or the
up/down arrow key. Press enter to set F1 to the desired frequency.
Press the F2 soft key as displayed on screen. Enter the desired
stop frequency using the keypad or the up/down arrow key. Press
Enter to set F2 to the desired frequency Check that the start and
stop frequencies displayed match the desired measurement range.
Press the Start -Cal key The message Connect OPEN to RF out port
will appear in the message box. Connect the calibrated OPEN and
press the ENTER key. The messages Measuring OPEN and connect SHORT
to RF Out will appear. Remove the OPEN, connect the calibrated
SHORT and press the ENTER key. The message Measuring SHORT and
Connect LOAD to RF Out will appear. Remove the SHORT and connect
the LOAD. An audible tone will sound when the calibration is
complete. Cal On message is now displayed Press Mode key, select
Freq-SWR mode at display with up/down arrow key then press ENTER
key. Press Amplitude key, select top and bottom amplitude of VSWR
Press Meas/Disp key to display VSWR reading Connect the device (RF
cable terminating at Duplexer or terminating at anteena port) under
test to the site master. RF out port will be displayed on the
screen. Press Marker key, press M1 and press On/Off to turn on
Marker-1
Distance-To-Fault (DTF)The Distance to Fault transmission line
test verifies the performance of the transmission line assembly and
its components and identifies the fault locations in the
transmission line system. After pressing the Freq/Dist press D1 to
enter the start distance and press Enter, D2 to enter the stop
distance and press Enter. Calibration is done again if the
frequency (F1 and F2) of operation is changed. By pressing the DFT
Aid soft key, select the appropriate cable type to set the correct
propagation velocity and attenuation factor. Markers can be set as
per requirement. The display will show distance and VSWR in front
of M1 and M2 at the bottom of the screen. It can be saved and
printout can be taken.
Procedure Connect one end of the RF cable terminating at
Duplexer or terminating at anteena port to the RF Out/Reflection
port in Site Master. Press the Mode key. Select the DTF-SWR using
the Up/Down arrow key and press Enter. Press the Freq/Dist key, set
the distance by pressing D1 and D2 soft key as per measurement
distance. Press Meas/Disp key. Distance and VSWR with Marker will
be display in the trace. Press Save Display, name the file and
Enter.
Power MeasurementsThe Site Master can be used for making power
measurements with broadband RF detectors, such as the 10 MHz to 20
GHz Anritsu 560-7N50B. The power monitor displays the measured
power results in dBm or Watts.Note: Always make it sure while doing
power measurement that all device under test should not be
withdrawing supply while connecting it to Site MasterRequired
Equipment Site Master model S331D with Power Monitor option
Broadband RF detector, Anritsu P/N 5400-71N50B 30 dB, 50 Watt,
bi-directional, DC 18 GHz, N(m) N(f), Attenuator, 42N50A-30
Procedure Press the ON/OFF key on the Site Master. Press the
MODE key. Use the Up/Down arrow key to select the Power Monitor
mode and press enter
Note: DO NOT connect Site Master directly to DUT as it can
damage Site Master. Always remember to connect attenuater in
between DUT and Site Master. Attenuater will be connected to sensor
cord and sensor cord will be connected to RF port of Site
Master.Zeroing the Power Moniter With no power applied to the DUT
(Device under Test), press the Zero soft key. Wait for a few
seconds while the Site Master accumulates data samples of the
quiescent power. When complete, Zero Adj: On is displayed in the
message area.
Measuring High Input Power Levels Insert an attenuator between
the DUT and the RF detector to protect the Site Master so that the
input power level is less than or equal to 20 dBm. Press the Offset
dB soft key. Enter the attenuation in dB using the keypad. Press
the ENTER key to complete the entry. The message area will show
Relative: On along with the entered value in dB.
Note: Attenuation value that we insert while doing power
measurement depands upon capacity of attanuater that we are using
in between DUT and Site Master.(Output Power=Actual Power + Offset
Power)
Displaying Power in dBm and Watts Press the Units soft key to
display power in Watts.
Displaying Relative Power With the desired base power level
input to the Site Master, press the REL soft key. The message area
will show Relative: On and the power reading will indicate 100%.
Press the Units soft key to display power in dBm. Since Relative is
ON, the power reading will be in dBr relative to the base power
level.
Cognitel Training Services Pvt. Ltd., Website: www.cognitel.in
Phone no.: +91 124 4119339 (1)