How to achieve 4 degree Beamwidth in case of a Parabolic Reflector

Hello, Everyone....I have a confusion regarding the beam width concept...According to a specification I need to achieve 4 degree beam width using parabolic reflector and my question is what should be the diameter of the dish to achieve this 4 degree beam width ?Is there a formula or theory which talks about this relation between the diameter of dish and beam width....

Thanks for your help!

well as a starting point for you..... beamwidth for a given size dish is frequency dependant ie. as you increase the freq, the beamwidth narrows.
an example from my microwave playing over recent years.....

a 1 metre dish has ~ 10 deg beamwidth at 10GHz ie 5 deg either side of the centreline
same 1 metre dish has ~ 4 deg beamwidth at 24GHz ie 2 deg either side of the centreline

qualify that by saying thats for the 3dB point ( where I swing the dish offline by 5deg (10GHz) the signal drops by 3dB)

now it works the other way as well for a given freq (you havent stated your freq of interest) as you increase the size of the dish the beamwidth drops.

here's something that should help you..... Table of Contents - W1GHZ Microwave Antenna Book ONLINE
scroll down the contents to appendix 6A Dish Gain & Beamwidth

You will discover that it is more normal to talk about the 3dB beamwidth of a Para. reflect (or any antenna) rather than the referring to beamwidth in degrees

Cheers
Dave
VK2TDN

Thank You for the reply Dave....But I was asked to achieve this 4 degree beamwidth and my antenna would be operating in the X Band....So I am completely confused how to get such a sharp beam at the X band frequency....

OK,
X band ~ 8 -12 GHz lets say 10GHz for a centre point. I think you will still need to ask the person presenting you with the problem to state the Power Point at that 4 deg point. Just saying a 4 deg beamwidth without any other information is meaningless.
as commented in my previous post it really needs to be related to a power point
so 4 deg beamwidth at 10 dB power point, or the 5dB one, or the 3 dB one, or the 30dB one ?

your parameters must be defined, else you are not going to come up with a meaningful answer
if this is a college/university etc lecturer that is giving you this problem to solve, surely he/she must realise that ?

Dave

---------- Post added at 10:01 ---------- Previous post was at 09:39 ----------

You have to understand why I'm stressing the relationship between the 2 beamwidths.
As every radiator has a front to back ratio (measured in dB) even a parabolic dish. So even having a tight beamwidth in degrees, there is still going to be a leakage of some of the power to the rear of the dish. Not much, it may even be 30 - 40dB down on the radiated power going foreward from the dish. The point is, its there and thats why the standard of a 3dB beamwidth (1/2 power level) power point is used. Cuz we all know there is still power at some level being radiated outside that defined beamwidth area.

I'm suprised others haven't chimed in and maybe added some clearer definitions
(I dont claim to always be able to put thoughts into words in the clearest way haha :) )

Thank you once again for the reply Dave....Well I think I should ask my Professor once again to specify at what power point he is expecting the beam width to be 4 degrees...Thank you once again.....I shall keep a post if I get any other doubt....