K5TRA

OFFSET-FEED DISH

CALCULATIONS

1T.Apel

K5TRA

Parabola

T.Apel 2

0.00

0.50

1.00

1.50

2.00

2.50

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

• All parabolas are similar

• Scaling parameter determines:– Relative curvature

– f/D

– How ‘shallow’ or ‘deep’ the dish

x

yP

K5TRA

Paraboloid

3T.Apel

• Plane waves are focused IN-PHASE to the focal point

• A paraboloid surface is a parabola rotated about the focal axis

• Dish reflectors are usually defined by the intersection of a parapolid and a plane

• Axial feed: intersecting plane is perpendicular to focal axis

• Offset feed: intersecting plane cuts through the origin (end of focal axis)

K5TRA

Single Reflector Dish Feeds

AXIAL FEED OFFSET FEED

T.Apel 4

• AXIAL FEED HORN BLOCKS DISH APERTURE

• AXIAL FEED MATCH IS AFFECTED BY DISH REFLECTION

• OFFSET FEED HAS A LARGER EFFECTIVE f/D

K5TRA

Assumptions

• Analysis based on measurements along vertical cross-section of dish

• Bottom of dish is origin of full parabola

• Two measurements determine solution:

– Vertical width of dish

– Depth of dish at center

5T.Apel

K5TRA

Deep-Dish Example

T.Apel 6

0.00

0.50

1.00

1.50

2.00

2.50

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

Yp

0.00

0.50

1.00

1.50

2.00

2.50

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5

Yp

Yl

K5TRA

Shallow-Dish Example

T.Apel 7

0.000.100.200.300.400.500.600.700.800.901.001.101.201.301.401.50

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

Yp

0.000.100.200.300.400.500.600.700.800.901.001.101.201.301.401.50

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5

Yp

Yl

K5TRA

HughesNet Dish

T.Apel 8

0.00

5.00

10.00

15.00

20.00

25.00

-25.0 -20.0 -15.0 -10.0 -5.0 0.0 5.0 10.0 15.0 20.0 25.0

Yp

0.00

5.00

10.00

15.00

20.00

25.00

0.0 5.0 10.0 15.0 20.0 25.0

Yp

Yl

K5TRA

Offset-Feed Geometry

9T.Apel

FOCUS

F

Fe

hD Ymax

Y(Xe)

q

f

½ f

XeX=0

YL

YP

½ Xe

K5TRA

Analysis

10T.Apel

K5TRA

Analysis

11T.Apel

K5TRA

HughesNet Dish Solution

12T.Apel

MEASUREMENTS

SOLUTION