CH2 Flashcards
coaxial cables
a better way to carry radio-frequency signals
contains a center conductor surrounded by insulation and then by a layer Of shielding conductor (and finally another layer of insulation). Coaxial cable works well at many radio frequencies.
At most radar frequencies, however, even coaxial cable is too lossy. To avoid these losses, another kind Of conductor was invented which is even more efficient at carrying radar signals. This conductor is called
waveguide
waveguide
It is usually a hollow, rectangular (but sometimes round) metal conductor whose interior dimensions depend u ‘on the wavelength Of the signals being carried
Long pieces of waveguide arc connel ed together by
special joints to connect the transmitter/rec iver and the antenna. This allows thc transmitter and recei er to be located at one place while the antenna is mounted e ewhere, usually up on a tower for conventional, ground-bas d weather radar.
Since there is seldom a straighl line bet-ueen the transmitter and the antenna, waveguide al: has to be able to conduct its signals
around corners
The signal conducted inside the waveguide con tsts of both
an electric and a magnetic component
The cross section of wavcguid is usualty
rectangular rather than square
, The shorter dime’ ‘ion is the
direction Of the electric field
the longer di ection is the
direction of the magnetic field
Waveguide ca bend in either of two directions:
in the dircction Of the ele tric field (called an Eplane bend) or in the direction Of th’ magnetic field (an Hplane bend).
Given a choice, it is bett r to use
E-plane bends rather than H-plane bends since the Iosses in Il-plane bends arc greater than those Of E-plane ben
Another kind Of waveguide s flexible waveguide. This consists ofa
waveguide that iss rt of like a goose-neck lamp. The metal part ofthe wavegw le can bend to accommodate slight misalignment in wav guide or to allow for slight movement between adjacent :nponents_ The outside of flexible waveguide is usually rul ber coated to make it and watertight.
A final special form of wave uide is
rotary joint
A final special form of wave uide is rotary joint. Antennas on weather radars must
rotate so the antenna can scan horizontally (i.e., azimuthally); Id in elevation.
A final special form of wave uide is rotary joint. Antennas on weather radars must rotate so the antenna can scan horizontally (i.e., azimuthally); Id in elevation. Rotary joints are used between
the waveguide fixed to the radar tower and the waveguide fixed to th, antenna
A second rotary joint is used so the antenna can
scan up and down.
The shape Of the waveguide in a rotary joint is usually
circular in cross section rather than rectangular.
Waveguide comes in …………. pieces
straight pieces which must be assembled into the final run of waveguide.
Waveguide comes in straight pieces which must be assembled into the final run of waveguide. The manufacturer usually puts all the waveguide together when the
radar IS installed at its final site
………………………………………. connectors must be attached to the waveguide to allow the pieces to be connected together
When the distance between two points is too far to reach with a single piece of waveguide or when a long piece must be cut for a short distance of waveguide
the ends that are attached together, Both a
flange joint (also called fiat joint) and a choke joint
shows the ends that are attached together, Both a flange joint (also called fiat joint) and a choke joint must be used to
connect the waveguide properly
. If two flat joints are connected and even a very tiny crack exists
energy will be lost. To avoid that the combination Of a flat and a flange joint solve this problems . The choke joint contains two grooves outside is simply a grove for a rubbe int solve this problem. s. The gmove near the O-ring gasket to make the waveguide air tight.
The inner groove is made a
quarter of a wavelength long
The inner groove is made a quarter of a wavelength long. This length causes
energy entering the groove to be reflected back into the waveguidc exactly in phase with the energy passing that •oint. Thu’ rcsult is that the joint looks to the radar waves as fit is a perfect conductor and effectively prevents the los of energy.
. If you ever assemble waveguide, bc very carefu that you
do not connect two choke joints together.
. If you ever assemble waveguide, bc very carefu that you do not connect two choke joints together. Doing th t
makes the equivalent of a half-wave groove
bc very carefu that you do not connect two choke joints together. Doing th t makes the equivalent of a half-wave groove. That is like
short circuit to radar waves
Wavnzguide pieces are held to ether Wii h
screws
Wavnzguide pieces are held to ether Wii h screws. The flat joint is
threaded
Wavnzguide pieces are held to ether Wii h screws. The flat joint is threaded while the chok joint
is not
Wavnzguide pieces are held to ether Wii h screws. The flat joint is threaded while the chok joint is not. By using this convention, it becomes
impossil le to inadvertently connect two fiats or two chokes togethe
Waveguide is an excellent c, nductor of
microwave signals
Waveguide is an excellent c, nductor of microwave signals, but
it is not perfect
Waveguide is an excellent c, nductor of microwave signals, but it is not perfect. Each waveguide components introduces
losses
shows the losses Of waveguide as a function Of
frequency
each waveguid size fits a range of
frequencies
also note that there is a limited number of
waveguid sizes available for WSR-88D radars
the antena is typicaly ………………….. from the transmitter/reciever cabinet
60 to 70 ft
Antenna
The antenna is the device which sends the radar’s sig+ nal into the atmosphere. Most antennas used with radars are directional; that is, they focus the energy into aparticular direction and not in Other directions
One Of the great advantages of radar is its ability to
determine the direction of a target from the radar It is the ability of a radar’s antenna to aim energy in one direction that makes it possible to locate targets in space
isotropic antenna
An antenna that sends radiation equally in all directions
An antenna that sends radiation equally in all directions is called an isotropic antenna It can be compared to
the light from a candle. A candle’s Iight is approximately the same brightness in all directions, except, of course, directly below the candle. For weather rad r, transmitting a signal equally in all directions would usuaIlly not he very useful. Instead, radars are more like flashlights_ Flashlights have a shiny reflector behind the light bul to direct the light in a specific direction.
The real antenna is the
radiating element which transmits the radar sig into the atmosphere toward the reflector
the real antena is radiating element which transmits the radar sig into the atmosphere toward the reflector
the reflector then
reflects and directs the Signal away from the radar.
Most weather radars use a
feedhorn as the true antenna although some use dipoles or Other radiating elements
The shape Of the reflector determines
the shape Of the antenna beam pattern
The shape Of the reflector determines the shape Of the antenna beam pattern. Most meteorological radars have
reflectors which are parabohc in cross-section and circular when viewed from the front or back; naturally, they are called circular parabolic reflectors.
The shape Of the reflector determines the shape Of the antenna beam pattern. Most meteorological radars have reflectors which are parabohc in cross-section and circular when viewed from the front or back; naturally, they are called circular parabolic reflectors. The beam pattern formed by a circular parabolic reflector is
conical and usually quite narrow, typically 1o in which for the mainlobe of the pattern
The bigger the reflector, the
better it is able to direct the signal and the narrower the of the antenna.
Cassegrain-feed antenna. In this design, thè actual antenna is located at
the end of the tube coming out of the center Of the main parabolic reflector
a Cassegrain-feed antenna. In this design, thè actual antenna is located at the end of the tube coming out of the center Of the main parabolic reflector. The signal is aimed at
at a hyperbolic-shaped subreflector: The signal then reflects back to the main parabolic reflector and then out into space
Cassegrain-feed antenna. In this design, thè actual antenna is located at the end of the tube coming out of the center Of the main parabolic reflector. The signal is aimed at a hyperbolic-shaped subreflector: The signal then reflects back to the main parabolic reflector and then out into space. The received signal follows the
reciprocal path from space, to the main reflector, to the subrefiector, to the feedhorn, through the waveguidc and into the receiver
One Of the advantages of the Cassegrain feed is that they
usually have better sidelobes for a given size antenna.
There are a number Of things we need to know about an antenna. One is the
wavelength it is designed for.
There are a number Of things we need to know about an antenna. One is the wavelength it is designed for. The…………………. determines this parameter
radar transmitter
There are a number Of things we need to know about an antenna. One is the wavelength it is designed for. The radar transmitter determines this parameter; the antenna must match the
the transmitter’s wavelength.
A second parameter of interest is the
size Of the refleclor
A second parameter of interest is the size Of the refleclor. For circular parabolic reflectors, this its
diameter
A second parameter of interest is the size Of the refleclor. For circular parabolic reflectors, this its diameter. Antennas on weather radars range from
I ft (0.3 m) to as much as 30 (9 m) in diameter,
Another measure of importance to radar antennas is
the gain of the antenna
