2. ASR-8 System (Airport Surveillance Radar)
The air traffic control radar ASR 8 is a relocatable, all-weather radar with dual-channel, frequency
diversity(Frequency Diversity (FD) is a technique that significantly improves radar detection
performance in clear and under adverse weather conditions), remote operator controls, and a
(dual-beam) tower mounted antenna.
1. ground based surveillance radar.
2. detects aircraft within 60 nautical miles
Aircraft range and azimuth information allows air traffic controllers to issue
instructions through radio communications and thereby control aircraft flight
patterns within the airport terminal area.
ASR 8 used a klystron as transmitters power amplifier stage with a load of 79
kV and 40A.The two operational frequencies have a minimum separation of 60
MHz.it does not provide digital inputs to new terminal automation systems, nor
any storm motion information.
3. Maintenance
Except for the antenna, major assemblies are duplicated
for dual-channel operation. If one channel fails, the
radar operator can switch the alternate channel into
operation while maintenance is performed on the faulty
channel.
Except for major malfunction in the antenna or
transmitter requiring replacement of heavy components,
adjustment and maintenance can be performed by one
technician
4. Signal flow
Normal Channel
i. The waveguide system for the normal channel consists
of five high-power devices
i. waveguide switch(for each radar channel)
ii. Diplexer(which is common to both radar
channels)
iii.Circulator
5. Circulator
A Circulator is defined as a non-reciprocal, passive three ports, ferromagnetic
device in which power is transferred from one port to the next adjacent port
in a prescribed order.
The high-power circulator operates in conjunction with a T-R tube in the
receiver to form a duplexer for the normal channel input to each receiver
Waveguide switch
The waveguide switch in each radar channel connects the transmitter RF
output from the circulator to the antenna during operation or to a high-power
dummy load when that radar channel is in off-line, condition.
Diplexer
The high-power diplexer allows either single-channel operation using either
radar channel or dual- channel operation in frequency diversity mode
6. Ports of the high-power diplexer are tuned to the
assigned frequency of each channel.
Transmitter pulses from either or both radar channels
are conducted through the diplexer to the antenna.
Received signals from the antenna, from either single
channel or dual-channel operation, are connected
through the diplexer to the appropriate radar
channel receiver according to the radar channel
frequency.
7.
8. What is the difference between duplexer and diplexer?
Duplexer"" means the separation function of transmission and reception
Diplexer"" means the band switching function of multi-band respectively
What is the difference between diplexer and circulator?
The diplexers are usually chosen to work at different frequencies at the same time;
while circulators work at the same frequency at the same time.
What is TR limiter?
The TR limiter has traditionally been used for duplexer applications with a magnetron-
based RF source. Here the limiter typically sees a short pulse, low duty signal, but with
high peak power—often many hundreds of kilowatts at S-band or many tens of
kilowatts at X-band.
9. Passive Channel
The passive waveguide channel consists of a low-power
diplexer and a TR limiter in each input to the radar channel
receiver.
The low-power diplexer operates similar to the high-power
diplexer.
The TR limiter protect receiver inputs from transmitter
power coupled from the normal channel feed horn to the
passive channel feed horn at the antenna.
What is feed horn?
A feed horn (or feedhorn) is a small horn antenna used to
couple a waveguide to e.g. a parabolic dish antenna or offset
dish antenna for reception.
10. WAVEGUIDE COMPONENTS
The system consists of a high power (normal) and low power (passive)
waveguide run.
The high power run conducts the transmitter high power pulse to the
active or main beam input of the antenna and conducts received signals
from that beam to the receiver.
The low power waveguide stem conducts signals received by the
passive (upper) beam of the antenna to the receiver.
The high power portion of the waveguide is capable of withstanding
the maximum power output the transmitter when the waveguide is
filled with air at atmospheric pressure; however, a pressurizing
system has been provided to insure no arcing occurs.
11. Both radar channels, each operating at different frequencies, are
coupled to the same high power and passive antenna waveguides in
each radar channel and allows selection of either antenna or dummy
load termination for the high power waveguide.
When dummy load operation is selected a low power termination is
automatically connected by the waveguide switch to the input of the
high power diplexer. This provides proper termination of the
diplexer.
The decibel (symbol: dB) is a relative unit of measurement equal to
one tenth of a bel (B). It expresses the ratio of two values of a power
or root-power quantity on a logarithmic scale. Two signals
whose levels differ by one decibel have a power ratio of
101/10 (approximately 1.26) or root-power ratio of
101⁄20 (approximately 1.12).
12. Three, two port couplers end four signal couplers are
included in the RF system to provide test points and
signals required to monitor radar system operation.
Two of the two port couplers, W8 and W28,have a
90° connector, 30 DB pad(reduces power to one
thousandth) and crystal detector coming off their
reflected port (2) for use in arc detection.
A crystal detector used for rectifying or detecting
UHF and SHF signals, It consists of a metal contact
held against a piece of silicon in a particular
crystalline state
13. Dummy load
A dummy load is a device used to simulate an electrical load, usually for
testing purposes.
In radio a dummy antenna is connected to the output of a radio
transmitter and electrically simulates an antenna, to allow the transmitter to
be adjusted and tested without radiating radio waves.
14. ASR-8 circulator and dummy load
The circulator and dummy load in conjunction with the receiver TR tube form
a duplexer that accomplishes the following functions:
a. Couples the transmitter signal to the antenna and provides a minimum
isolation of 20 dB between the transmitter and receiver
B. Couples the received signal from the antenna to the receiver and provides
a minimum 22dB isolation between receiver and transmitter.
c. Provides at least 22 dB isolation between the antenna and transmitter so
that reflected power from mismatches in the waveguide and antenna will not
degrade the operation of the transmitter.
The reflected power is terminated in the matched dummy load (W3 or W23),
15. Low pass filters
• The output of the transmitter is coupled through a dissipative low pass filter.
• That attenuates the second harmonic by at least 40 dB
• the third harmonic by at least 30 dB
• and the fourth by 10 dB
• The microwave portion of the filter, is housed inside a pressure tube.
• The filter consists of a central waveguide that is slot coupled to an array of secondary
waveguides on all four walls of the waveguide
• Each secondary guide is terminated by a matched waveguide load.
• the harmonic widths of the secondary waveguides are small enough that they will not
propagate power in the 2700 to 2900 MHz band;
therefore, none of the desired radar power is coupled from the main waveguide
As the radar energy passes down the main waveguide a little of the harmonic energy is coupled
through each slot, travels down each secondary waveguide, and is absorbed in the internal
load.
16. Signal test Couples
One signal coupler is located in the high power and passive waveguide runs of
each radar channel. The couplers are used to inject noise and test signals into
the receiver to determine noise figure.
17. two-port couplers
One two-port coupler is placed between the high power and waveguide switch of
each radar channel and one is located at the output of the high power diplexers.
The couplers are used to measure reflected power to determine radar power
output and waveguide VSWR.
Voltage standing wave ratio (VSWR) is defined as the ratio between transmitted
and reflected voltage standing waves in a radio frequency (RF) electrical
transmission system. It is a measure of how efficiently RF power is transmitted
from the power source, through a transmission line, and into the load.
Signals can also be coupled into the incident port to determine system MDS. The
incident coupler has a coupling of approximately 55 dB and a minimum directivity
of 20 dB and the reflected power coupler has a coupling of approximately 45 dB
and a minimum directivity of 27 db.
18. Waveguide Switch
A four-port, E-plane Waveguide switch for each radar channel connects that channel to either the
antenna or to a high power dummy load.
When the dummy load is selected, a low power termination is also connects that channels input
to the high power diplexer.
This is necessary to present the effect of a high power waveguide section at the input to the
diplexer.
Such a section could store leakage energy from the other radar channel and result in possible
degraded diplexer operation or waveguide breakdown.
A central rotating element of the switch contain two 90 degree E-plane bends which form
waveguide paths through the switch.
Chokes complete RF paths through the switch and provide isolation between channels.
A small reversing motor drives the rotating section, Cam operated limit switches interrupt current
to the motor when the desired switch position is obtained and give readout indication to the
control circuit as to switch state.
19. Diplexer
A high power frequency diplexer is used in the high power waveguide run
and a low power diplexer in the passive run.
The purpose of these diplexers is to allow both radar channels to be
connected for diversity operation.
The assembly consists of two filters, a connecting tee, waveguide
pressure windows, and protective cover (not shown).
The low power diplexer is identical except it does not have the pressure
windows nor gas-fittings and the filter elements are designed to operate in
an air rather than a gaseous atmosphere. To withstand a peak power of 1.4
MW the high power diplexer must be filled with sulfur hexafluoride. The
unit is pressurized to 5 PSIG under normal conditions, however it will
provide satisfactory operation at atmospheric pressure if the unit is filled
with sulfur hexafluoride.
