Microwave antennae and feeders

This section is intended to be a mix of theory, brief tutorials and projects in the microwave antennae field. The sections inside do not necessarily follow a specific order, and in fact we will continue to add material every time we have to tell something. Without pretending to be rigorously scientific on the subject, we want to shed some light in the mysteries of microwave antennae and suggest some ideas for your own designs. Special thanks to Paul Wade W1GHZ and his online Microwave Antenna Book, that has been (and still is) a great source of learning: we strongly recommend to read and study his publications.

Build a Feedhorn from a SAT-TV LNB

Using the same approach of LNB reuse, proven successful to build an effective 10GHz transverter, we found a way to give a second life to the many LNB chassis left in the bin without the precious PCB that was taken for other uses. Almost all universal LNBs are made by diecast aluminum, shaped in a structure that integrates a circular corrugated horn and a box where electronic PCB is protected. Once the electronic is removed, the remaining horn is well suited to build a feed for an offset dish, simply because it was designed to do this. What we have to do is to modify it to to have an SMA connector and let it better work in our 3cm band. There are many projects around using the same idea, and here we describe a successful implementation we did.

Practically all LNBs sold today have a corrugated horn that well illuminates common offset dishes that have an f/D around 0.7. So we have just to find one that needs minimum modifications. The one described here has a long circular waveguide between the corrugated horn and the large section where the PCB sits. After cutting by hacksaw the waveguide section along the red line, you will have enough waveguide length for the SMA launcher and a tuning screw. Remove burrs and flatten the rim of the waveguide opening, because it must make a good continuous contact with the shorting plate we will place later. Better is to patiently sweep the rim over a fine-grain sandpaper sheet put on a flat surface. Then draw a straight line along the waveguide axe to have a reference line for the holes we will drill.
Drill the hole for the SMA inner pin: distance of the hole center from the open must be 7mm. Hole diameter should be equal to the diameter of the SMA teflon insulator. A second hole is for the tuning screw, 10mm away from the SMA center towards the horn. This one will be tapped, so hole diameter depends on the screw diameter: for a M3 screw the hole should be 2.2mm The two holes to hold in place the SMA will be done after flattening a small area of the waveguide outer surface where the SMA will fit. Use a flat file, and stop when the residual metal thickness around the SMA hole will be about 0.7mm. Perhaps the resulting flat area will be smaller than the SMA flange, but it's OK anyway. More important is a good flatness and enough room to drill the holding screws holes that must be blind ones (only the tuning screw is allowed to enter inside the guide!). Last holes will be very small, along the rim of the waveguide, for the micro screws that will hold the shorting plate. At least three of them are needed.
I made the shorting plate with a piece of FR4 single-layer copper clad. Also metal sheet (or a suitable coin) will work, but I prefer FR4 because it doesn't bend while cutting and filing it: it is important that the plate makes a sound contact all around the waveguide rim. Some SMA models have a center pin long enough to act as a launcher: the one I had wasn't, so I prolonged it using a pin taken from a DIN audio plug. The probe internal lenght should be around 6mm, so consider to leave some extra length to allow for tuning. I started from 7mm, ending at 6.2mm after tuning with a file. Teflon insulator should be cut flush with the inner surface of the waveguide. Tuning screw should not go deeper than 2-3mm inside the waveguide
After mounting the SMA and the tuning screw, the feedhorn is almost complete. Return loss must be checked with the shorting plate in place, but you will need to remove and replace it every time you try a new probe length, so initially don't tighten its screws too much. For each probe length you try (0.1mm steps) move the tuning screw over its full span looking for the resonance. I stopped reducing probe length after reaching 18dB return loss, ending with a 6.2mm probe and about 2mm tuning screw.
Such a construction has been a simple and short way to obtain a proper illumination on my 40cm and 80cm offset dishes, both thoroughly tested on the field. I hope to make some more accurate tests using sun noise as soon as I will collect all the necessary equipments.

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Build a Feedhorn from a SAT TV LNB