# 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:

 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 overlap 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:
• Diameter=(K*lambda)/hpbw (where k depends on the edge taper .. typically k=1.2)
The hpbws are different for the linefeed and yagi so their illumination patterns are  different.

The near/far field transition occurs when the path difference for a ray from the center of the dish and the edge of the dish to a point P have a phase difference that is some fraction of lambda. The picture below shows the path difference dl vs the H and the illuminated Radius:

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

• H=(k*lambda/hpbw)^2/ (8.*dl)
• Setting dl=lamba/8  gives: H=(k*lambda/hpbw)^2/(lambda)
 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..).

 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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