A Matching Network for the DDA, CFA and Similar Antennas
Network Solutions Begin
After a little thought, the schematic in the diagram below developed. It allows the impedance of each section to be matched fairly easily, it is multi-band capable and easily modified. Just remember, this is an experimental device subject to many more improvements through more tests.
To start with, the input is split to two variable inductors. These can be either roller inductors or tapped inductors. In my case, I used two roller inductors out of two 50-year-old ARC-5 transmitters. They had to be modified because they were without a connection to the inductor to allow one end of the coil to be grounded. For this, I added a wire to the end of the coil and connected to the other end of the coil support. Now, the coil was properly configured. This end of the coil was next connected to the ground line leading from the coaxial connector input. This line was connected to the output connectors and also grounded to the station’s ground.
Coaxing?
Now, some will question the use of coax to feed the antenna, but this subject will be discussed in a later article. For now, if you are interested in building this particular matching unit, just wire it as shown, unless you do not ever intend to use coax to feed the antenna. I think for anyone living in the cold northern climates, tuning an antenna outside in the dead of winter would be a bit daunting. The same applies to the extremely hot parts of the world, like here in South Texas. Having the tuning unit inside the house makes tuning the antenna more comfortable when the weather is nasty.
Tuning to Frequency
Now, let’s go to the rest of the diagram. L1, C1 and L2, C2 are picked to tune to the frequency of interest. Although not shown on the diagram, I used very large inductors that happened to be in my packrat box. These came out of some 2 MHz, 1 kW transmitters. C1 and C2 are 7.5 to 500 uuf vacuum variables I have had for a long time, (same source, packrat box) and were used for this particular design. They were selected to provide very smooth and easy to adjust tuning capacitors. Normally, such hardware is not needed for the 100-watt power level most of us use. However, I do have a 1.5 kW amp that will be used later for testing at higher levels. Normal 100-watt power levels do not have these requirements and can be made of more modest components. A pair of 365pf broadcast variable and coils wound on PVC or another insulator will work. The original matching network was constructed with coils wound with #12 wire (AWG) on a PVC coil. Your coils can be constructed the same way and toroids are not required. At the 100-watt power level, the components should not get hot or warm.
After the network was constructed, some experimenting with a grid dip meter indicated the tuning range was from 1.7 MHz to slightly above 10.3 MHz by tapping down on the coils. I am sure this type of unit that will go to 10 meters without any problem. Provision for power monitoring to each output is easily added without any great effort. This will also be discussed in another article if it later proves to be an advantage.
Other Materials Okay
Looking at the photo, you will notice I used acrylic plastic for the base and panels. The reason for this is that I had a lot of plastic and a source of plastic scrap for construction purposes. You can use most any other material you may have on hand or prefer. Also, the coils are very large edge wound coils with the exception of the roller inductors.
Connections to the antenna in these early experiments are by wire conductors with the ends terminated in banana plugs to match the SO-239 connectors. This also serves to plug into the antenna connectors. This is the easiest way at this time to connect to the antenna and ideal for the required changes that may be used in future experiments to improve the overall system for maximum radiated signal.
At this time, the tuning procedure follows these steps and subject to change in the future.
- Set the rotary inductors to the middle of their ranges and then key your transmitter at low power, 5-20 watts, the lower the better.
- Tune either C1 or C2 for indication on your field strength meter and tune for maximum.
- Tune the other capacitor for an increase in field strength and for maximum indicated readings.
- Now tune both C1 and C2 alternately for maximum.
- When maximum is reached, then vary either of the roller inductors for an increase in signal strength, then tune for maximum on the field strength meter.
- When you can’t get any more signal strength from the roller inductor, then tune the other for maximum signal. You will have to alternate between matching inductors for maximum strength.
- After the matching inductors are tuned for maximum, then check the tuning capacitors for max again.
- This should put you on the air with maximum signal, although there will possibly be some variations in tuning show up as experimentation progresses.
Swapping Leads
If you are not able to get any signal strength out of the antenna, then swap the leads out of the coupler around and try again. The reason for this is that unless the correct phase is fed into the elements, the antenna will not radiate well, so try swapping the leads at the antenna or tuner, whichever is easiest.
This system will be used directly below the antenna, but later on will be set up for remote tuning via coax. This will switch the matching network to doing matching to the transmitter and the antenna matching will be done at the antenna. Tuning still will be done by this unit as well as matching to the transmitter. Actual antenna matching to the coax will take place at the base of the antenna.
This will give anyone wanting to experiment with the little DDA antenna a slightly more flexible antenna matching/tuning unit as well for the CFA and similar antennas. If it is built using open construction as this example is, then it will be easier to modify for future experiments.
Finding a Way
There remain many questions about the matching units used by the commercial antennas. Only through experimentation can these questions be answered. At this time there has not been any designs set in concrete for the matching unit, so it will be up to the individual experimenter to find out what is best.
Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF
Last Updated : 29th April 2024