Caution - DDA at Work! STARC
On November 13, 1999 the South Texas Amateur Radio Club, (STARC) held a one-day hamfest at Corpus Christi, Texas, complete, with a barbecue pit and other fun things. I had been invited to attend on the condition I would demonstrate a Duo-Disk Antenna (DDA), an experimental device, which has been written about in previous issues of antenneX. I agreed to do so and took the 40-meter DDA along with the new Quad Phaser network to the hamfest. The network is the one that Harold Allen, W4MMC had wanted to us to test. The DDA is the same one used in past experiments with the cylinder constructed of brass tubing.
The demo plan was to mount the DDA on a PVC mast that would have placed the DDA at approximately 25 feet (7.6m) high. Unfortunately, I was not able to erect the mast due to the ground being extremely dry and hard from lack of rain for several months. Instead, I had to place the antenna only a few feet (about four feet or 1.2m) off the ground on the back of a pickup with a plywood truck bed.
Contacts Made
Several contacts were made from that position, but the antenna had to be relocated because of concerns someone might trip over the coax to the antenna. This being the case, the antenna was moved to a flatbed trailer and placed on a 5-gallon bucket, which had served as a radome for the antenna in the past. As can bee seen from the Photo above, the DDA was in a fairly flimsy situation at best. This made for quite a test demo! More contacts were made with the same stations as before and signal strength reports were exchanged. At this stage, I decided to take some photos of the antenna where it was located. I took one picture and before I could take any more, the antenna fell off of the bucket, due to a sudden gust of wind, and broke off its support legs. That ended the photo shoot and any further on-the-air demos.
Accident, notwithstanding, the results obtained from this brief test were most encouraging, with signal reports ranging from 1 dB below the reference dipole to weak but readable at the most extreme range, as that station was mobile up near the Texas and Oklahoma border and panhandle regions. Other stations were not that far, with most being in the range of 150-400 miles (240-645 Km), with the exception of one station that was about 800 miles (1,300 Km) away.
Everyone observing from the crowd agreed this antenna design possessed potential and wanted to find out more about the antenna. They were directed to the antenneX site for more detailed information. I explained as much as I could about what we knew about this young antenna. Many of the observers were interested in making a DDA for 160 meters. I told all concerned parties that the 160 and 80 meter ones would follow, as soon as the networks could be developed individually for each band. This is to ensure that maximum efficiency will be obtained and the network duplicated easily.
Band Conditions
I found the location where we had set up the station was extremely noisy with all sorts of motor noise from a nearby swimming pool pump and various other noise sources in that busy area. The noise level was running from 15-25dB/9 at best, with occasional crashes to 40/9. This was observed with the dipole antenna connected to the Kenwood TS-440S being used. But, amazingly, switching to the DDA, the noise dropped dramatically and the signals fell off too, but not as dramatically. This told me that the DDA seems to act as a magnetic field antenna similar to a loop. For those who do not know, static is composed of an electrostatic and an electromagnetic field. If your antenna is of the small magnetic field type like the small transmitting loop, it will favor the electromagnetic component of the radiated signal. This holds true for both natural electromagnetic and manmade electromagnetic signals. So the DDA will receive the electromagnetic component of any signal better than the electrostatic component. There were not too many signals present on the band at this time and I am sure that the extremely noisy band had something to do with it. Others on the air had made comments about the excessive noise too.
One of the more interesting phenomena noticed about this antenna is that there is very little QSB on received and transmitted signals. At this demo, this characteristic was again noticed on the received signals and also noted on my transmitted signal by several of the receiving stations. As of yet there has not been any real good explanation for this phenomenon. Also the tuning did not change when the antenna was moved and nor when placed with the cylinder horizontal. Now there are those who will say the reason for this is that the matching network was acting like a dummy load, but this is not the first time that the antenna was moved and no tuning changes observed.
Matching Network
The diagram of the network is the one designed by Harold Allen, W4MMC, which included an input isolation 1:1 transformer option being exercised to verify the DDA to be earth-ground independent. It worked very well. Further experimentations will continue be carried out to come up with more network designs since this is believed to be an important area to focus on and find solutions to make tuning these devices even easier. In this design, you will notice there is only air wound coils and toroids have not been used anywhere. One reason for this is that in some places toroids are not easy to obtain, so the good old air-wound coils are used, but toroids may be used if preferred and available to you.
In this particular network, the two coils are bifiliar, and are on the same form, insulated from each other. C1 and C2 are identical. You should choose values that will tune the band of interest. The input transformer is a pair of 1:1 coils, in this case wound from #12 house wire and insulated from each other. Four (4) turns were used for each winding and they were secured by hot glue. Small alligator clips were used for the taps on both coils. The photo on the right shows an example of a bifiliar coil.
The tap for the cylinder ended up being near the hot end of the coil and the feed tap was placed just below the middle of the coil. The disk connection was made above the middle of its coil by several turns. All changes were slightly interactive. The changing of taps on each coil did require some tuning every time a tap was changed. Interaction was slight between coils but some amount of tuning was always required. A field strength meter is essential beyond a doubt, as there were several tap locations that gave low VSWR but not much field strength.
There will be much more experimenting to perform on the network design, as many are still trying to devise a more efficient network for the CFA, DDA, DLA-type antennas that are being extensively experimented with at the present. More new devices will be discussed in future issues of antenneX. As they say, stay tuned!
Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF
Last Updated : 2nd May 2024