Why are commercial satellite dishes oval - not round? 2

[blueandwhiteg3]

Basically all small and medium satellite dishes (1 meter and less) are not quite round. Some dishes are oval and horizontally wider, vertically taller for multiple feeds, wider angles, etc. Others are vertically taller.

I have several dishes here, from the little 18" DBS dishes to big 1 meter dishes. Every single one is slightly distorted one way or the other enough that it's clearly not a manufacturing defect.

I suspect the goal is to optimize for an offset feed, which is quite common with satellites. However, the implication of this is unclear to me.

Does this design modification preclude the dish from being useful with a traditional centered parabolic feed approach? Would it improve the ability to use multiple feeds on a single dish with the same remote focal point?

 

[VEBill]

Most consumer dishes are offset feed. This is primarily to allow the dish (when installed in the lower 48 of the USA) to remain more or less vertical (neat and tidy against the house, and self cleaning) when installed. It also eliminates self-blocking. I assume that the reflective surface has been optimised for the offset, and therefore wouldn't be optimal for center feed. But I've not done the math. But on larger dishes the top half of the dish is very obviously different shape than the bottom half.


Most consumer dishes are now designed for two feeds (side by side) aimed to two orbital slots, normally 9 degrees apart. It is quite common to retrofit multiple feeds to existing single-feed dishes, this implies that the dish shape need not be optimised for adjacent feeds.

An exception is StarChoice in Canada where their orbital slots are more closely spaced than normal (about 4 degrees) but the dish is extended horizontally into an obvious oval shape.


You've already met the toroidal dishes. Their distortions are obvious. One common model puts most of the funny business onto a subreflector, and the primary surface is kept to a subtly simpler shape. But such double-reflectors require 'reversed' LNAs when used with circular polarization.

 

[blueandwhiteg3]

Can you give me a brief introduction to the math or estimating the losses when working with an offset feed dish as if it were a center-fed dish?

From what I can tell, offset feeding is less effective in terms of dBi gain per cm^2, hence my interest in a traditional parabolic.

 

[VEBill]

It's not really as simple as "per cm^2". The illumination of the dish by the feed (thinking about transmit mode since it's easier to think about) is not evenly distributed. The general rule of thumb is that the power density at the edge of the dish is about 10dB down from the middle. The reason is that your feed doesn't have a sharp cut-off. So you have to pick a number. If your feed is too narrow, then you're not really using the edge of your dish. If your feed is too wide, then your feed is looking past the edge of the dish too much.

Traditional center feed dishes block the sweetest part of the reflector. Which isn't a big deal if the dish is 10-feet diameter. But when they're 18-inches across then it's much simpler and cheaper to offset the feed than to miniaturize it.

I wouldn't even attempt to answer the first question.

 

[blueandwhiteg3]

VE1BLL:

Thank you for the reply. Yes, I understand that the dish effectiveness is not uniform. However, I look at the total dBi gain versus the total area, and calculate the average gain per unit of area. This is a good way to look at size and material cost versus signal performance. It is not useful as a design tool per se.

In the 5.2-5.9 GHz range, the feeds I have on traditional parabolic designs are pretty darn small. One feed for a 39" diameter parabolic dish is circular and only 2.5 cm in diameter.

Based on the assumption of a centered feed already optimized for a similar sized dish and that degree of blockage of incoming signal, how could one begin to estimate the performance of a dish like the 100 cm Fortec?

 

[VEBill]

If I understand the basic concept, an offset dish is simply an offset section extracted from a larger parabolic shape. If this basic design concept is true, then you could create a mathematical model of any offset dish based on a few key measurements. And then try modeling it with the new feed point.

If you're doing this professionally, it might be cheaper to simply find a company to stamp out a thousand dishes of the design you actually want.

 

[Higgler]

the simplest way to picture a loss in gain between offset vs. not offset reflector is projected area of the reflector aperture.

Picture the reflector being perfectly vertical and the offset beam bounced upwards theta degrees above the horizon. The reflectors cross sectional area is then reduced by the cos(theta), which you would call a loss mechanism.

Hence, projected area depends on cos(theta offset) and gain is linearly proportional to projected area for reflectors.

or 10 log(cos(theta offset)) loss in dB. Not much for small offsets.
offset reflectors don't have the blockage loss for energy bouncing off the feed, so this will reduce the relative loss even more.

kch

 

[blueandwhiteg3]

Higgler, yes, that is a good way to think about it.

Also, I just realized I didn't include the 1 meter Fortec dish URL:

http://www.fortecstar.com/estore/storefront/default.asp?ProductID=27&CategoryID=8

Given that has an offset angle of 24.42°, I would not expect a huge loss of gain from what you are saying.

Can anybody show me an example set of calculations to solve for the performance of a dish design like that? I would want to re-solve for different frequencies, etc.

We will simply assume we are using the existing feed, even though I know a different feed would be required, but I can work that data out later.

 

[VEBill]

OP: "...preclude the [offset] dish from being useful with a traditional centered parabolic feed approach?"

If I understand the OP's question, he's asking if he can use a dish that was designed to be offset, and try to force it into service as a center fed.

Short answer - wrong shape.

Medium answer - Each parabolic shape has one point of focus and it is where it is.


He's not actually trading off the cos(theta) factor to decide between one option and the other.


Perhaps the OP should confirm the sort of project that he's working on...

 

[blueandwhiteg3]

Yes, the point of focus is off and probably less effective. I am trying to determine by how much. Actually, more than determine - I can do that mostly by testing - I want to calculate it, or at least numerically estimate it.

These are being tested for more point to point links, with a remote site being located higher than the local site, but not nearly as high as most satellites' orbit.

Sidelobes and oddities in the radiation pattern are not too big a deal if we're hitting a nice sharp point going out in one direction at least.