feed for shallow parabolic dish at 408 Mhz

I want to make a feed (frequency 408 Mhz) for 4m parabolic dish with f/d of .42

can any one suggest what type of feed i can make with HPBW of 120deg in both e and h plane.

thanks in advance..............

To obtain HPBW ~120 degrees, the easiest way is to use a waveguide radiator. For 408 MHz, you can scale up its cross section approximately to 60x60 cm. For feeding such antenna, check ARRL Radio Amateurs' Handbook, there are various waveguide devices described.
Such waveguide antenna can be then used as a primary feed for your antenna, 4 m diameter.

Is this for a low noise application (antenna looks to sky)? If so, you need to consider spillover also. When you optimize for maximum gain, the spillover will increase the noise temperature of the antenna.

@jiripolivka...thank you, but the main purpose of the antenna is to observe the radio sources in the sky feed of that size may blog the radiation.......
if you can suggest any feed which is smaller in size and circular polarised ........

---------- Post added at 10:42 ---------- Previous post was at 10:35 ----------

thanks for ur reply .........yes it is for low noise application ......... I have designed a quad antenna with HPBW of 75deg considering the spillover but it can barely detect the sun which is a strong radio source.........
I need a better feed which is smaller in size and use the all antenna surface for getting maximum gain......
any sugessions......?

Hello,

For your low noise application, you should consider the antenna-LNA combination. When you have a relative noisy LNA, you can optimize for maximum gain, resulting in spillover and therefore higher antenna noise temperature.

I assume you would like to maximize G/T ratio. You may know that when you have a very good LNA, a short run of cable may increase the noise temperature significantly. An LNA with F= 0.5 dB has noise temperature of about 36 K.

When looking from a transmitter perspective.
When n% of the TX power is lost in a structure with temperature of Ta (absolute temperature), that structure contributes n*Ta/100 to the overall noise temperature.

A piece of cable (at 300K) with 0.2 dB loss, "consumes" 4.5% of the transmitter power (assuming good VSWR). In case of reception, this would increase the noise temperature with 0.045*300 = 13.5K. Same reasoning is valid for input filtering, every 0.1 dB counts.

Just for my interest, how do you determine that the illuminator is at the best position towards the dish?

You can be lucky with the size, an illuminator with larger beam width will have less size. As jiripolivka suggested, info from radio amateurs (HAM) is a good starting point (it is available for free).

If you have access to simulation tools, it is good to simulate the illuminator that looks fine for your application. Gain (in the preferred polarization), radiation efficiency and phase change versus orientation is of importance.

An antenna may have certain total gain, but the gain in the preferred polarization is of importance as gain in the non-preferred directions may cancel after reflection at the dish surface.

Phase change versus orientation w.r.t. main beam direction is of importance as this may require you to position the illuminator somewhat away from the optical focal point. I hope you have a good range to test these issues experimentally.

If you want to maximize G/T ratio you will not be using all the antenna surface from a gain point of view. The feed horn radiation pattern and how it illuminates the dish will also depend on how low noise your preamp is. LNA should be mounted on feed to minimize noise temp contribution of transmission line.

Some sidelobes will be pointed at the relatively hot earth (300 deg K) so they can quickly ruin the antenna noise temp. At 400 MHz you should be able to get a preamp in the order of 10-15 deg K noise temp range. At 400 MHz and 4 m dish you are going to have a hard time getting a noise temp of antenna better then maybe 70-90 deg K. Dish is just too small to illuminate optimally at 400 MHz in practical construction. 400 MHz also has a lot of terrestrial noise. Hope you are not located too close to a high power UHF TV station. You likely have to worry about intermod from UHF TV on your preamp.

Most common method for circularly polarized feed is two perpendicular quadrature phase probe feeds in a cylindrical can. You can put some scalar choke collars around the mouth of circular feed horn to shape the pattern somewhat and balance the E/H field symmetry.