How to Combine Signal Transmission Lines
A sudden change in impedance in a transmission line creates an impedance bump or SWR. You lose power as a result, when part of the power reflects back as a standard wave and interferes with the transmission. And the transmitter may not like the impedance bump, since it is dealing with 100 watts out and 50 or more watts back. If the SWR (standing wave ratio) is too high, the outgoing signal plus the SWR can burn out your transistors.
Then how can you then make the transition combining signal transmission lines, say from a 50 ohm line to a 72 ohm line, without this problem? The solution is changing the impedance slowly to prevent an SWR reflection. There are several ways to accomplish this.
How to Combine Signal Transmission Lines with Limited SWR
A classic solution for preventing SWR when combining signal transmission lines is the quarter wave power divider.
In the case of a 50 ohm line, there are two 75 ohm lines to split the power forwarded one to two 50 ohm loads. These power dividers can combine the input of two amplifiers, combine two separate antennas or combine the output of two different amplifiers. If the loads aren’t well matched, you can add a 100 ohm balancing resistor to offset the difference.
The quarter wave power divider results in well balanced reception across 10% of the bandwidth while it is usable with degraded performance over 20% of the bandwidth. When you combine signals from two wideband antennas like log periodic antennas, you need a good impedance match across a wide range of frequencies. Tapered lines become even more valuable in this case at minimizing SWR. How could you do that?
The ideal way to handle a wide frequency range with minimal SWR is to taper the diameter of the central conductor. If you can’t do that, you can taper the shield by filing it down so that the taper is at least a quarter wave of the lowest frequency on which you intend to use the power divider.
For a six inch or less line, you can take out the central conductor from the line. The hole shrinks some. If you’ve pulled out a nineteen gauge wire, for example, you could push a 24-gauge wire in. You have to work fast before the conductor returns to its old shape. However, you end up with a smaller center conductor. For 19 gauge (0.040”) to 24 gauge (0.022”) wire, this process turns a 50 ohm coax to a 75 ohm one.
Another option is buying pre-made antennas like the one show here that already come with the tapered power divider so you only have to hook up the antenna and go. The quad patch antenna shown here is one such example. The line generates 100 ohms from each antenna to the central line. That handles 50 ohms at the broad ends before tapering down toward the middle where it delivers a 100 ohms load. Where all the signals meet, it delivers a combined 50 ohm signal.
Another option is phase matching lines using phase matching mixers or frequency mixers; this is the better choice when you’re working with very high frequencies.
Kent Britain, WA5VJB, was interviewed as a source of information in this article, along with other sources. He is also the designer of the quad patch antennas shown here.