The New DDA and Network Test Results
More testing of the antenneX DDA has been conducted during the past month and some interesting results have been obtained. Several different configurations were built and tested, with varying signal reports results ranging from mediocre to decent. However, these designs all suffered from other problems that put them into the trash bin. Some of them could not be matched properly no matter what was tried and others could be matched but would not radiate well enough to satisfy the field strength requirements that I was looking for.
The spacing dimensions were altered and found not to work well with the solid disks, as compared to the disks with holes cut in them corresponding to the formula given in a previous article. No other dimensions were changed, as the allotted time was inadequate to re-cut the cylinder to new dimensions, redo all the connections and other variables that are present with this antenna in order to adequately test each configuration. On the surface, this looks easy but I can assure you that it is not that easy if it is to be done correctly. The final design has not been found yet, and this is a matter of continued research. Once we reach this point, then building one of these devices will be a lot easier to do.
Again I want to state the antenna I built was not to any set formula, but with the materials I had on hand at the time. I got lucky with my first try in that the components were happy together and I had good signals right away. There is not enough data to make very many dimensional conclusions about this antenna at this time. Nothing about this antenna has yet to be set in concrete—nothing at all, except that it works when set up correctly.
Formula determination for optimum performance will take place as more experiments are completed. One thing is clear and that it is definitely an antenna that cries for more research because it has proven to be more easily tamed and more versatile than previous experiments with the CFA have shown..
80 Meter Version
One version built by Osmo Tiinamen, OH8MT, who used the information from last month’s article Introducing the Duo-Disk Antenna! to construct his 80-meter DDA, enjoyed excellent results. At a height of about 20 feet (6 meters) signal strengths were 1-2 S units below the inverted V that had its center at 52.5 feet. (16 meters). These reports were from 150-600 km. This is an exceptional report to get on a compact antenna like the DDA, particularly since it was located at a lower height than the V and was 82 feet (25 meters) from the inverted V. Osmo is very pleased with his antenneX DDA and said in his E-mail that reports from it was 3 to 5 times higher than from the CFA that he built before. He intends to do more experiments with this antenna.
A very small (2-1/2” or 6.3cm) experimental version of the antenneX DDA has been constructed and it will be used to experiment and determine the lowest frequency where it can be used effectively. If successful enough, more will be reported about this “little giant”. As of this time, no minimum size has been found for the antenneX DDA, so this is definitely an area for lots of experimenting and should provide for some very interesting projects.
One of the most unusual aspects of this antenna is that the signals on transmit and receive do not seem to have as much QSB as they do on a wire or other type of antenna. Even the versions that had difficulties all showed the same phenomena. I have yet to understand why this occurs, but it certainly is an interesting characteristic. There are other things about this antenna that will be documented as experiments are completed and they can be repeated reliably.
Another thing that has been reported is that when the antenna is mounted with the cylinder in the horizontal position, the polarization becomes vertical, identical to a vertical-mounted small transmitting loop, or a figure 8 when viewed from above. The antenna is then directional, as is the loop. When the cylinder is mounted in the vertical plane, the pattern is the same as for a 1/4-wave vertical, a circular pattern with a null overhead. What this would do on 160 meters would be really a fine thing to behold for the 160-meter gang.
Network Changes
The network was set up in several different configurations, some of which did not do well at all. The current configuration is illustrated below in Figure 1.
The main change that made a very noticeable difference was the addition of the 1:1 balun at the input. This stabilized the tuning and also eliminated the RF that had shown up on the coax feedline. Before the addition of the balun, tuning was affected by hand capacity on one side but not the other. There are several other variations of this circuit to be tested yet, and this particular design is not by any means the final configuration. Much more will be done to both antenna and network and will be reported here in antenneX in a continuing series over the next several months.
Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF
Last Updated : 16th May 2024