mathews yagi on the 430ch

18mar06

Sections:
Measurements.
Near, far field and the sky ovverlap at 100 km.
Relative strengths of linefeed/yagi sidelobe detections.
 


Measurements:  (top)

      A yagi has been place near the linefeed. It will be used for meteor detections. The system was measured on nov05 (before the last move of the feed). The results were:
 
Data measures 06nov05 before last move.
dipole linefeed
Tsys(za=15) 167 100
gain(za=15) .6 K/Jy 10.5 K/Jy
sefd(za=15) 3C48 270 Jy 9 Jy
beam width 18 Amin 9.5 Amin
az pnt err (avg)
zapnt err (avg)
1.0 amin
-.7 amin
-.25 Amin
-.1  Amin
az pnt offset used:
ch pnt offset used
 0.4833 deg
-1.5483
az pnt offset measured
za pnt offset measured
0.467 deg
-1.540

 
Near, far field and the sky ovverlap at 100 km.  (top)

    The illuminated area on the dish (and the edge taper) determine the half power beam width (HPBW) at a given frequency.The relationship is:     The hpbws are different for the linefeed and yagi so their illumination patterns are  different.

    The path difference for rays from the opposite edges of the dish to a point P is non zero in the near field and tends to zero (rays parallel) in the far field. The picture below shows this difference:


 

Using the hpbw to define the illuminated area (d=k*lambda/hpbw)  and solving for H gives:

linefeed, yagi optical specs
lineFd Yagi
HPBW (far field) 9.5 amin 18 amin
Illuminated Diam=1.2*.7/(hpbw) 304 meters 160 meter
Near/farField Transition: D^2/lambda 132 km 37 km

    Meteors occur around 100km. At this height, the linefeed is still in the near field while the yagi is in the far field.  The angles and distances of the two beams at 100 km are shown below (assuming the far field..).
 
Angular/spatial separation of lf,yagi at 100 km
lf
yagi
pnt Offset relative to Lf 
(farfield)
0 1.622 deg
Offset  len @100km 0 2.83 km
angle , distance angle,distance
1st sdLb: angle,dist@100km 14.25', .41 km 27', .78 km
2nd sdLb:angle,dist@100km 23.75', .69 km 45', 1.3 km
3rd sdlb: angle,dist@100km 33.25', .97 km 63', 1.83 km
4th sdlb: angle,dist@100km 42.75', 1.25 km 81',2.36 km.
5th sdlb: angle,dist@100km 52.25', 1.52 km 99',2.88 km.

    The yagi sidelobes move by about .53 km per sidelobe at 100 km. The lf sidelobes move by .28 km/sdlb at 100 km (if they were in the far field). The offset direction of the yagi relative to the linefeed is determined by the az,za pointing offsets.


Relative strengths of linefeed/yagi sidelobe detections.  (top)

    If you see a meteor in the linefeed and the yagi it must be close to the plane defined by the linefeed, yagi.  The combination of linefeed tx sidelobe and yagi  rcv sidelobe must combine to give a horizontal offset of  about 2.8 km. The relative strengths of the different combinations (using sin(x)/x ^2 for the sidelobe falloff) are:
 
lf sidelb/dist yagi sidelb,dist total
distance
 strength
rel to lf mainbm
strength
rel to max
0,0 5,1.52 2.88 3.36e-3 1
1,.41 4,1.25 2.77 2.37e-3 .071
3,.97 3,.97 2.8 6.95e-5 .021
5,1.52 2,.69 2.82 5.5e-5 .016
 
    So the strongest detection is the linefeed main beam and the 5th sidelobe of the yagi. It is about 300 times weaker than the mainbeam tx, mainbeam rx of the linefeed.
 
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