Antenna Transmission Line Power Splitters

(they are also combiners)


Dave Clingerman -W6OAL

            In this offering, I’ll address several ways of splitting power to feed two like (50 ohm terminal impedance) antennas. The combining of two antennas will increase the overall antenna system gain and provide a larger aperture than just a single antenna thus better directivity, better reception. At first glance doubling the size of an antenna system should double the effected radiated power (theoretically). This in theory is true however, any addition in a transmission line distribution system is going to have some losses – be it a physical power splitter, a coaxial power splitter of just connectors in general. So, count on an increase of ~2.8 rather than the suggested theoretical 3.0 dB gain.

I’ve mentioned the physical power splitter, these are commercially made or can be “home brewed”. Information on their construction can be found in a myriad of technical publications including the ARRL Handbooks. Physically they are made of a quarter (or half) wavelength of square or round aluminum or copper tubing or pipe with an appropriate size copper or brass center conductor. They can exist in two configurations of a 2 X or 4 X (I‘ll address the 2X); the “war club” – two connectors at one end and a single one at the opposite end, or the “dog bone” – a connector at each end of a half wavelength of square or round material and one in the middle. The two way and four way splitters can be made in the same configurations.

Power splitters can be constructed using one quarter or three quarter wavelengths of 75 ohm transmission line. Paralleling a pair of 75 ohm quarter wavelength sections results in a combined impedance of 37.5 ohms or a 37.5 ohm transmission line. Why do we want 37.5 ohms? This, because it is approximately the geometric mean of two paralleled 50 ohm antennas (resulting in 25 ohms) and a 50 ohm transmission line system, thus: Zo = √25 X 50 = 35.36 ohms (~5.7% discrepancy). The two center conductors at one end of the paralleled pair are then soldered to the center conductor of a coaxial connector (solder the shield to the shell). The opposite end of the quarter wavelength section the two center conductors will be soldered to the two center pins of a pair of coaxial connectors (solder grounds to the shells). This produces a coaxial line power splitter. They aren’t real pretty but they function very nicely and save the better part of a 100 dollar bill (if the commercially produced one is out of budget).

Another popular power splitting method to achieve proper impedance matching from transmission line to a pair of like antennas is to use two quarter (or three quarter) wavelength lengths of 75 ohm transmission line and a coaxial “T” adapter. At the “T” 50 ohms is desirable therefore we have to assume two 100 ohm impedances paralleled at that point. Assuming that the two like antennas are 50 ohm terminal impedance, then again employing the geometric mean equation Zo = √50 x 100 = 70.7 ohms (close enough to use 75 ohm line). Thus 75 ohm coaxial cable will suffice with very little return loss or VSWR in the matching system. Mind the Velocity Factor (Vp) of the coax used. Run-of-the-mill RG-59 has a polyethylene dielectric which has a Vp of ~66% or 0.66 of a free space wavelength. The two quarter wavelength lengths with the “T” at the center is too short for proper spacing (0.66 wavelength) of a pair of short yagi antennas (4 elements). This can be affected by adding to both sides odd quarter wavelength multiples of line sections equally to the basic coaxial power splitter.

A second method of using 75 ohm transmission line to split power between two like antennas is to use uneven lengths (1/4 X 3/4) and a coaxial “T”. This method affords an extra 1/2 wavelength of phasing line and result in the proper spacing between a pair of short yagis without having to add any multiples of quarter wavelength sections. This added half wave length of coax or 90° must be dealt with accordingly. When equal lengths of coax are used in each leg the gamma matches must be on the same side or the mast if the antennas are horizontally mounted, vertical – both on top or both on bottom of the booms. Using the (1/4 X 3/4) method the gamma matches must be on opposite sides of the mast if horizontally mounted and if vertically mounted, one gamma arm up and one down. Hopefully this instruction will be helpful in two antenna phasing.