Introduction to Delta Match for Antennas
I had discussed about Gamma match and practical implementation of Gamma match earlier. A short mention of Beta match or Hairpin match was also there some time back. Now I have come across a Delta match for Yagi-Uda antennas when I was looking for a 6m antenna. Just as in the case of Gamma match, with Delta match also, driven element need not be cut in center to make two quarter wavelength segments like a usual dipole antenna. Delta match is typically used in a Yagi antenna, though it can be used in other types of antennas as well. Mechanical stability of the driven element is better when the matching is Gamma or Delta as it is a single continuous half wavelength element, mounted on the boom. This is more important for HF antennas where the driven element is quite long. As can be seen from the diagram, a Delta match looks like an inverted Delta alphabet when viewed from below.
In Delta match, the feeder is connected to two points away from the midpoint of the driven element. Usual dipole has a feedpoint impedance of about 70 ohms. When it is fed with a Delta match, the points of connection of the Delta can be adjusted to have 50 ohms feedpoint impedance. Still, it has been mentioned that Delta matched antenna has to be fed by a balanced feedline. Balanced feedlines have much higher impedance than 50 ohms, requiring impedance transformers. For example, the ladder line which I had homebrewed using pieces of drip irrigation pipes as ‘steps’ has a calculated impedance of about 460 ohms! Another option is to have a 4:1 coax balun at the lower part of the Delta for matching with a coaxial cable feedline.
We know that the radiofrequency current in a dipole is maximum at the center and minimum at the ends. RF voltage on the other hand is minimum at the center and maximum at the ends. When the distance of the feedpoints on either side of the Delta match is moved towards the ends symmetrically, the impedance will increase. That is because the RF voltage increases as you move away from the center of the driven element. Theoretically it is possible to match the impedance of the open wire balanced feedline with Delta sides of around 0.12λ.
But practically, I think it is likely to be a tough job as there is another matching involved at the radio end. Usually radio has an impedance of 50 ohms while the open wire balanced feedline has a much higher impedance. More over a balun is needed to connect between the unbalanced output of the radio and the balanced feedline. Using a 4:1 coax balun at the feedpoint avoids these problems, though open wire feedline has less losses than a coax. RG-58 coaxial cable loss is about 6.6 dB per 100 m at 30 MHz while 300 Ω twin-lead loses only 0.55 dB.
For further detailed reading, may I suggest two web pages: