Deep Dish - Low f/d ratio feed

[Joolz]

I am wondering who in the forum has a C Band dish with low f/d ratio feed ( 0.3 or less ) and if the increased depth helps with 5G / TI ?

I understand a f/d of 0.25 needs the feed to cover 180° and many of the original US C band dishes (Winegard etc) had an f/d as low as 0.28. If you didnt try to see all the way to the edge of the dish (by adjusting the amount of the dish visable to the feed) the "unused" edge would act as a screen that would give you less gain but also less noise in the C/N ratio??? I understand this all depends on what angle the "noise" is coming from but it would be good to get some real world feedback.

 

[Satfaca]

I have this Orbitron SX-10 with 0.30 ratio, currently "in the box", but do have serious plans for the summer to put it back up.
I've also managed to find a very rare ADL CP-400 RP2 feedhorn which fits perfectly with this dish so expectations are high. Let's see...

 

[Joolz]

I have this Orbitron SX-10 with 0.30 ratio, currently "in the box", but do have serious plans for the summer to put it back up.
I've also managed to find a very rare ADL CP-400 RP2 feedhorn which fits perfectly with this dish so expectations are high. Let's see...

That sounds like a great combination.

 

[Channel Hopper]

A 0.25 f/d ratio dish has an efficiency of little more than 50% even with a matched feed.

The gain will be reduced slightly further when receiving LH/RH signals as a result of the dielectric plate creating its own phase changes in the feed port (I believe vertical polarised signals will also be lower than horizontal ones but it was a long time ago since I carried out tests).

Elsewhere on the forum I have mentioned fitting a microwave skirt/sleeve on the outer rim of the reflector to lower the effects of 5G at the feed, on a deep dish this will mean moving the feed away from the true focal point by a small distance to retain overall performance.

 

[Channel Hopper]

For such a deep dish (and one of a reasonable diameter) it might be worth considering making a convex sub-reflector for it.

 

[Joolz]

For such a deep dish (and one of a reasonable diameter) it might be worth considering making a convex sub-reflector for it.

Thanks for the feedback - the sub-reflector sounds a good solution.

 

[Channel Hopper]

Thanks for the feedback - the sub-reflector sounds a good solution.

For a parabolic reflector you will need to create a matching hyperbolic mirror, though at C-band the surface distortion can be a few mm off .

https://ocw.mit.edu/courses/2-71-optics-spring-2009/21d0d2d809a12ea75f386d65df3b3af8_MIT2_71S09_lec03.pdf