Determining the focal length of a parabolic dish (axi-symmetric, circular)

-

Determining the focal length of a parabolic dish (axi-symmetric, circular)
Focal length = f
Depth = c
Diameter = D
f = ( D * D ) / ( 16 * c )
Measure the depth using a tight fishing line across the dish and a rule to measure depth c. 

Parabolic dish showing measurements needed to determine focal length
Parabolic dish showing measurements needed to determine focal length.
If the f/D ratio is low, say 0.25 to 0.35 then the feed will be close to the dish and needs to spread its power at a wide angle to efficiently illuminate the dish.  The feed therefore needs to be of small diameter.   If the f/D is 0.25 the feed is level with the dish aperture, which may make it difficult to make a satisfactory feed.
If the f/D is large like 0.5 to 0.75 then the feed will be further away from the dish and needs to project its power into a narrower angle.  The feed needs to be of larger diameter.  
With a open circular waveguide antenna feed (scalar feed) the focal length will be the distance from the reflector to a phase centre point just inside the circular feed wave guide aperture.
If the feed is a horn,. the phase centre of the feed may be further back inside the horn.  The wider the angle of the horn, the deeper inside the phase centre moves. If it is unknown make four accurate signal quality measurements at carefully measured distances, say 10mm apart.  Then interpolate for the best focal distance. 
The power at the edge of the dish should be about 10 dB lower than at the centre for best gain or about -16 dB lower for best discrimination against adjacent satellite interference.  The axially symmetric dish shown above suffers from beam blockage due to the LNB at the focal point blocking the beam.   This reduces gain and increases off-axis interference.
The rim of the dish above must be accurately flat.  For a receive only dish aim for maximum error of about 1/10th of a wavelength or about 6mm at C band and 2.5mm at Ku band.  You can use several fishing lines stretched across the dish to test the flatness of the rim of a parabolic dish. They should all just touch where they cross on another.

Comments

Post a Comment

Popular posts from this blog

Symbol rate, Bandwidth and Bit rate Calculation

-
In satellite communication bandwidth (BW), symbol rate and bit rate are the most vital fact for proper signal transmission. There are many commercial software for BW, Symbol rate and bit rate calculation for TV signal transmission. But the equations of these parameter calculation are as follows. BW= (1+Roll of factor)* Symbol rate In general Roll of factor is 35% Symbol Rate =BW / (1+Roll of factor) and Bit Rate = Symbol rate * Modulation* FEC* (188/204)                                     [when 188 is encoder packet length] or Bit Rate = Symbol rate * Modulation* FEC* (204/188)                                     [when 204 is encoder packet length] Here, FEC = Forward Error Correction. In general FEC is in the range from 1/2 to 7/8. 188/204 = constant Unit ** BW= Hz, KHz, MHz, GHZ ** Sym...
Read more

Azimuth and Elevation Angle for Satellite tracking

-
Image
Azimuth (Az) The earth station needs to know where the satellite is in the orbit. Then the earth station engineer needs to calculate some angles to track the satellite correctly. These angles are called antenna look angle. The look angles for the ground station antenna are the azimuth and elevation angles required at the antenna so that it points directly at the satellite. With the geostationary orbit the situation is much simpler than any other orbit. As the antenna beam width is very narrow and tracking mechanism is required to compensate for the movement of the satellite about the nominal geostationary position. Three pieces of information that are needed to determine the look angles for the geostationary orbit are a. Earth station latitude b. Earth station longitude c. Satellite orbital position Using these information antenna look angle can be calculated using Napier’s rule (solving spherical triangle). Azimuth angle denotes the horizontal angle measured a...
Read more

Ethernet (UTP) Cable

-
Image
For computer networking purpose most commonly used cable is ethernet cable. Ethernet cable is mainly two types, UTP and STP. UTP stands for Unshielded Twisted Pair and STP stands for Shielded Twisted Pair. Among those UTP is used for most of the networking purpose. Depending on the network devices, UTP cable is two types- straight and cross. Here is a short description on this network cable. Figure: Network Cable (UTP - Unshielded Twisted Pair) Generally mainly used two types of UTP cable. 1. Category 5 (Cat 5)  2. Category 6 (Cat 6) In UTP, there are total eight (8) cables in four (4) pairs. Those are Internationally there are two types of standard to make straight and cross cable. 1. T-568 A (generally called  standard A) 2. T-568 B (generally called  standard B) Note: From the above design it can be observed that , if  interchange 1,3 and 2,6 then A change into B or B change into A. Or simply interchange between Green and O...
Read more

Followers