21cm Hydrogen Line Feedhorn Assembly for Radio Astronomy

I am excited to finally be able to say that my radio telescope is starting to come together. This morning I worked on another major component of my receiving system, a 1.420Ghz hydrogen line feedhorn. This has been a project that I have been wanting to build for roughly 16 years and now that things are moving along I am hoping to have a system together ready for testing within a month or so.

Now for the feed, I decided to go with a rectangular design for my feed instead of a circular one for simplicity of assembly. I went back and forth many times on which design I should use but ultimately ended up deciding on the rectangular feed for several reasons. It is easier to assemble (90 degree cuts are easy to mill), it's based on a standard size, and the material was cheap. Rectangular waveguides are polarized, although for radio astronomy purposes this should not matter as any natural occurring signals would in theory have random polarization. Here is the final assembled version:

While not really a feedhorn as of yet (I have not built the horn) it is a nice waveguide to coax adapter that will be used as a feed at the focal point of my dish. As for the horn, I will have to check to see if I will have any benefit of using one. A horn can provide additional gain from the dish, but it also blocks off surface area of the dish in its shadow. A choke ring on a circular feed would have the same effect in blocking the signal, this is just something I need to research more.

The feed itself is assembled out of 1/4" 6061 aluminum stock that I cut and milled down to size. The dimensions of the opening are 6.5" x 3.25" which is the exact spec of the industry standard WR-650 waveguide which is designed for frequencies between 1.12Ghz and 1.70Ghz. The Hydrogen line of 1.420Ghz fits nearly perfect between these two limits which makes this specific size ideal for radio astronomy. I drilled and tapped 22 holes which have stainless steel hex head screws holding it together. I was very pleased with the final assembly as it has a nice tight fit.

The probe consists of a 4mm section of copper wire which is exactly 1/4 wavelength of 1.420Ghz long and positioned 1/4 wavelength from the back of the feed. The probe terminates to an SMA connector mounted to the top of the feed. I had to mill a small slot into the top of the feed to allow the bottom section of the SMA panel mount jack to lie flush with the inside of the feed.

One note on the WR-650 standard itself. There are commercial feeds available as it is a standard waveguide size, but the cost is extremely high since this this specific size of waveguide does not show up in the surplus market very often. Smaller waveguide standards for higher frequencies like WR-90, WR-42, etc, do show up but it has been extremely hard to find anything WR-650 available for cheap. My total cost to build this feed is about $50.

I have already tested this with my HP 8614A signal generator set at 1.420Ghz and have verified it does indeed work very well. Next steps are to add the mounting brackets to it which will allow me to mount it at the focal point of my dish and also add the additional RF amps, filters, and my downconverter to the back section of the feed. I still also will need to calculate total system gain and noise once completed.