(Part 1 - overview)

Martin Farmer, G7MRF, has spent the the first half of the millenium year (and some time before that) rebuilding his earlier 24GHz and 47GHz portable equipment. The result is the fine system you can read about and see below. What is really impressive is the mechanical engineering of the system so that swapping from one band to another, using the same dish, can be achieved in a matter of a minute or so. Even more impressive are the precision home-made waveguide switches, waveguide transitions, cassegrain feed system and feedhorns.

The cassegrain feed is described in more detail in the Proceedings of Microwave Update 2000,available from the ARRL.

The photo left shows the complete setup, with David, then M5AFG (now M0VZT) in the background. He has an identical system except for his dish, which is smaller than the Procom one shown here. This photo clearly shows the rear mounted transceiver unit which is actually two large Eddystone diecast aluminium box bolted together. One box houses the 24GHz transverter while the other contains the 47GHz system. The cassegrain reflector on the dish is also homemade, the dimensions being calculated by means of an Excel spreadsheet which produces the necessary coordinates for a CNC lathe.

This second photo shows the RF units  in more detail. The two boxes, bolted together are mounted by means of three locating legs into a metal frame fixed to the rear of the dish and, at the same time, to a sturdy tripod mount. In the photo it is the 47GHz transverter that is feeding the dish. The 24GHz tubular feed can be seen projecting from the rear of the upper box. The three locating legs for the 24GHz system are also visible. Each band's antenna feed projects through a hole in the centre of the dish, such that individual, dual mode feedhorns can easily be slid into place.

47GHz dual mode horn

24GHz dual mode horn

These horns have been accurately milled to suit the cassegrain reflector, which in effect doubles the 0.38 f/D of the Procom dish to around 0.8. During initial antenna engineering, simple laser pointers were used to line up the centre of the cassegrain reflector with the centre of each feedhorn .... no "guesstimates" here!

With using such a large dish,  initial alignment when arriving onto the portable site is done by fitting  a rifle site into the sub reflector boss on the front of the dish so that it looks back though a hole in the dish (where the feed fits) and by optically aligning the dish onto a visible landmark and then working out the beam heading (-180 degrees of course) to this point so as to accurately set the compass scale on the neck of the tripod. After an initial contact refinement to the compass can be made.

For a closer look at this interesting equipment click here: