Progress in the IBA-4 modification for the Project BIG-DISH

Mike Watanabe, JH1KRC
Feb. 13, 2007

Yesterday we tested a small dish feed and two-band loop feeds for the IBA-4 big antennna modifications. See the phots in the photo pages. I already named this modification as the Project Big-and-Smass-Dish a la carte. I wonder if you would understand what I like to say.

The SWR of the 2.4-m small dish with a round septum feed, was very good, ca. 1 to 1.1 or less at 1296 MHz. We all wondered if the SWR meter was not correctly working, but it was OK.

This feed was designed and built up by Czech group, mainly OM6AA Rasto and OM2ATT Milan. As Rasto said, this small dish should have a front gain of 28.5 dBi on 1296 MHz, which is well-enough to illuminate the subreflector of the IBA-4 cassegrain dish, in order to achieve 48.5dBi gain in total on 1296 MHz.

The loop feeds for 144 and 432 MHz bands were calculated and simulated into details by Rasto's working group also.

Even though it needed a lot of time for me to build it up in a good shape, and did not use any metal materials other than the loops themselves, the coax cables and the connectors, it still needs some modification in the constructions for the mechanical reason.

Yesterday we comfirmed the loop feeds also work fine, in front of a steel door of the IBA-4 antenna pedestal where there is the radio shack. It was an easy simulation and an experiment before installation.

For this experiment, we estimated a little wider distances to be made, from the designed distance between the feeds and the vertex of the IBA-4 subreflector.

The result was fine; it measured 50 ohms impedances with some propper distances which we almost expected from the Rasto's design for the IBA-4 modifications.
They wait for the moment of installation on to the BIG-DISH soon.

Because of some mechanical reasons and of possible multiband activities on the same days during the possible operating periods,
we did not take the marionette yagi feed, even though it sounds interesting. You would have put a yagi beam at a certain distance between the subreflector vertex and the prime feed horn of the IBA-4 cassegrain, possibly using ropes. It sounds like a marionette theater show....

We are going to hook up a 144-432 two-band loop feed (vertical pol), and a 2.4m dish feed with septum for 1296 (circular pol.)
next weekend, Feb. 17 or 18. Hope that the weather would be fine and not windy...
We will put up these feeds onto the IBA-4 dish with a 50-m class heavy crane!!