Since Anita (AB1QB) and I both want to operate at the same time, we are planning to put two SteppIR DB36 Yagis on our tower. These antennas will be connected to a DX Engineering Stack Matching System so that they can be operated together as a 4 over 4 array. The DXE Stack Match can select either antenna individually and connect it to the feedline associated with the array but it does not provide breakout of both antennas onto separate feedlines. We designed and built a custom feedline breakout system to enable simultaneous breakout of both antennas to separate feedlines. This project involved the construction of both a tower mounted box to house a part of relays and a control box for the shack.
This device is inserted between the Stack Match and the antennas in line with the two phasing lines to each Yagi. It is critical that the breakout device provide identical impedance and phasing effects on both phasing lines if the array is to function correctly. To accomplish this, we selected a pair of Tohtsu Coaxial Relays (Model CX-800N) that have very low SWR impact in the HF bands. These were installed in an outdoor utility box that we got from DX Engineering. Only one relay is used to breakout the lower antenna to a separate feedline as the Stack Match can break out the upper antenna to the main feedline for the array. The reason that two relays are needed is to ensure that RF performance of both phasing lines to the two antennas is identical.
The relays require a 24V source to energize them. I built a simple control box for the shack to provide the needed control voltage. The controller includes three switches so that it can be used for additional 24V relay applications in the future.
I wanted to be sure that the Breakout System had good isolation characteristics between the two phasing lines so that the device did not allow a transmitter using one antenna to interfere or possibly damage a transceiver using the other antenna.. The relays we choose have good isolation characteristics which is a good start. To ensure that we have good isolation at a system level, I used an ArraySolutions Vector Network Analyzer (VNA) 2180 to measure the isolation between the various input and output connections in the Breakout System. The ArraySolutions VNA 2180 uses a PC and software to control a measurement unit which can perform one and two port SWR, impedance, loss and phase measurements (many other measurements are possible as well). In this case, we are making a port to port loss measurement.
The VNA 2180 has a dynamic range of about 100 dB which means that it can measure isolation up to this level. As you can see from the following screen shot taken with the VNA software, the isolation of the Breakout System is very close to the limits that the VNA can measure. The worst case isolation measurement is about -97 dB on the 6m band. We also use Bandpass Filters when we are both operating and these filters provide an additional 55 dB or more of isolation which means we have a total of about 150 dB of isolation through this path. In the real world, the antennas themselves will likely have much less isolation between them than this so the isolation performance of the Breakout System should be more than adequate.
We are making good progress towards the planned installation of three of our Yagis on the tower next week. I will provide some additional posts over the next several days covering additional aspects of the preparation for next week.
– Fred (AB1OC)