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I am doing a project where I will be re-purposing an old TV satellite dish as a directional Wi-Fi antenna. The dish is circular in shape, not oval. The dish has an arm coming from the bottom that makes a 90 degree angle with the dish. I will be adding a second arm to the end of the dish's arm that will allow me to mount an SMA antenna connected with RG-59 cable precisely where it will utilize the dish's geometry. I need to figure out the proper position of the antenna on the dish. I want the dish to receive signal directionally where there is a zero signal elevation angle. From my previous understanding, dishes receive their signal from space where there is an angle of elevation from the transmitter and the receiver part of the dish.

I created a diagram that illustrates my plan with dimensions and calculations. I need some verification with my diagram and suggestions on how to maximize the reception strength.

I think that the antennae should be placed somewhere along line C in the diagram but I want to be sure before I start modifying the dish.

Diagram

EDIT: I've calculated the focal point on the dish to be approximately 320 mm.

PS:I just realized that the 75 ohm impedance of the RG-59 cable is not optimal for the Wi-Fi adapter. I only need about 6 inches of cable so I will try it with and without the jumper cable and compare signal levels.

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Unfortunately, your drawing is not correct.

To find the focal point of a symmetric parabola, measure the diameter of the open rim of the dish, call this D. Measure the depth of the dish at its center, call this c.

Calculate the focal point distance from the bottom of the dish as:

$$ f = \frac{D^2}{16c} $$

Note that the focal point is in the center of the dish, it is not offset as in your drawing. So your receive antenna will slightly shadow aperture of the dish.

Due to the physical aperture of your receive antenna, you may need to adjust it in or out from the focal point in order to fully illuminate the dipole or other element.

If the dish is an asymmetric parabola, the focal point will be offset from the center in order to avoid blocking the aperture. This is the type of parabola that nearly all modern home satellite dishes use. They are identified by an LNA support arm that extends from the bottom of the dish and the dish apparent elevation angle appears to be lower than the satellite capture elevation.

In the case of an asymmetric dish, the existing LNA arm should be used as a starting point to find the appropriate focal point for the dipole or other antenna element since the focal point is largely frequency independent.

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  • $\begingroup$ Can you clarify what you mean by slightly shadow aperture of the dish? I appreciate your input. $\endgroup$ – Jake Psimos Jun 24 '17 at 3:33
  • $\begingroup$ Since you antenna focal point will be in the dead center of your dish, your antenna will block some of the signal from entering/leaving the dish. This is described as shadowing or shading. $\endgroup$ – Glenn W9IQ Jun 24 '17 at 3:37
  • $\begingroup$ Have fun with the project! $\endgroup$ – Glenn W9IQ Jun 24 '17 at 3:53
  • $\begingroup$ Can't the feed be offset? There's no reason the focal point needs to be right in the center. $\endgroup$ – Phil Frost - W8II Jun 24 '17 at 11:08
  • $\begingroup$ If it is an asymmetric parabola, then yes. $\endgroup$ – Glenn W9IQ Jun 24 '17 at 11:12
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Please post a photo of the dish. Ku band satellite TV dishes are always offset feed, whatever their shape.

The direction of firing is roughly along the feed arm. The dish is "minimally offset" (my term) to avoid feed blockage but not much more. So if you draw a line from the bottom edge of the dish, over the feed, that's about the direction. When aimed at the horizon, the dish will look like it's pointing about 30 degrees down towards the ground.

I've made many feeds for 2.4 and 5.8 GHz, using a three element Yagi. They work very well, you get about 50% efficiency from the dish. A patch antenna might also work, or a short CanTenna.

Unless you have some fancy test gear, the absolute best plan is to use the original feed arm and LNB holder. Then you only have one axis to optimise, the in-out position of the feed.

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  • $\begingroup$ "Ku band satellite TV dishes are always offset feed, whatever their shape." - not quite true, dishes from 100cm upwards are increasingly more often prime focus, especially old ones. As a rule of thumb, f/D for offset dishes tends to be 0.6 or 0.7, for prime focus ones about 0.35. $\endgroup$ – Radovan Garabík Jun 27 '17 at 6:49

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