The T and Delta Matching Systems
A recent question came via E-mail about the delta and t match systems, which were used to match open line feeders to an antenna. This could be a dipole or the driven element of a beam. If you plan to use either one of these matching systems, and are using coax, you must first use a balun to step up the impedance from the coax to the impedance of the open line feeders.
Delta Match
In Figure 1, the delta match is illustrated. It is simply two wires that are connected to the driven element of the antenna. To obtain a match, the wires are moved equidistant from the center of the antenna until a match is found. It is possible to lower the VSWR to 1:1 by slowly sliding the adjustable wires out from center and taking the VSWR reading as adjustments are made. Both the spacing of the wire connection and the length of the delta must be determined by experimentation in most cases.
T-Match
The T-Match is also used to match open and it used to be quite common for beams to be matched by this method. In Figure 2, the distance from the center of the antenna as well as the spacing determine the amount of impedance change that takes place as the connection to the antenna is moved out from the center. The most change is caused by the movement of the connections to the antenna. However, for each antenna there is a certain distance where the impedance will began to decrease as the distance from center is increased.
The second part of Figure 2 is that of a T-match with variable capacitors placed in line to tune any residual reactance out. A maximum value of 150 pF. in each section, should be enough to do the job at 14 MHz.
Both of these matching systems utilize the inherent inductance, capacitance and resistance of the antennas to accomplish matching. By proper selection of the spacing and distance from the center of an antenna, a satisfactory match can be obtained in nearly all cases. Remember that these systems are for open line feeders and as stated can be used with coaxial cable only if baluns are used.
This has been a brief description of the delta and T-matching systems and there is more detail written in the ARRL antenna book as well as others. This was only intended to be a short description of these methods as they are not in very wide usage today.
Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF
Last Updated : 15th March 2024