It is important to define what constitutes a comprehensive receive antenna distribution system in the context of an SO2R station. SO2R implies two radios, thus at least two receivers. A proper receive switching system needs to provide a way for each receiver position to independently select and, if necessary, properly share an antenna. This is further complicated by an operating technique I use - I call it "SO2R-3RX" - where audio from three receivers is streamed to the headphones. In this situation, at least three receivers need independent - and potentially shared - antenna routing.

In addition a reasonable, expandable number of antenna terminations are required. This permits adding receive antennas without creating undue complexity or having to redesign the switching system.

Existing Designs

Off-the-shelf SO2R antenna switches intended for transmit antennas are sufficient for situations requiring receive antenna distribution to a pair of receivers in the same radio (i.e. SO2V), where antenna sharing is not really needed. For true SO2R, though, they do not provide the ability to properly share antennas among multiple receivers, since they lack any kind of splitting devices that are important for isolation and impedance preservation. While external splitting could be implemented, the situation becomes cumbersome when you need additional flexibility, such as expanding the number of antenna terminations beyond the basic switchbox.

There are numerous documented solutions for switching receive antennas, but they also lack inherent splitting abilities, especially for same-band operation and are severely limited in the number of antenna terminations. A design I found to be the most promising for further development was published by Stu Mitchell, W7IY. While his current implementation is still limited in the number of antenna terminations, the modular approach he has taken could be easily expanded to essentially an unlimited number with further development.

Starting in the fall of 2010 I began developing the system I currently have in place. It addresses all the requirements I've outlined. It has a generous number of antenna ports, is easily expandable and is capable of proper antenna sharing among up to four receivers. Numerous features and capabilities provided by the user controllers in the shack also make the system a powerful tool for managing the receive antenna farm.

Switch Matrix

The remote switching matrix is based on W7IY's concept of individual antenna routing boards for each antenna. Each board is basically a four-way switch - one antenna input and four receiver outputs. By connecting the respective receiver outputs of eight of these boards together, an additional splitter board, and a microprocessor board, a complete 8x4 switching unit can be built. It is capable of providing independent selection of any antenna to any receiver as well as proper splitting of an antenna, even to all four receivers at the same time.

When a particular antenna is unselected, the antenna can be terminated directly to ground, through a resistor (50 or 75 ohm) to ground, or left open. This feature is useful for reversible Beverages that have two feed lines, where the unused one is terminated into 50 or 75 ohms.

The onboard splitter is based on the ubiquitous Magic-T and provides approximately 30 dB of port-to-port isolation in addition to maintaining the characteristic system impedance at each port. Typical signal loss on the order of 3 dB results from splitting. This onboard splitter can be used to share the antenna between "opposite" radio receivers.

Receiver ports are connected via SPST coaxial reed relays. The use of this type of relay was based on RF isolation testing that W7IY performed with various relays. I used COTO 2200-2302 relays, because of their small, narrow form factor, and I was able to obtain them cheaply at the time. You could use and SPST relay that has suitable RF isolation, however. Receiver ports A and B share a common connection on one side of their respective SPST relays, considered to be "same radio, adjacent receiver" ports. This simplifies the onboard splitting. However, if receivers A and B need to share an antenna, only the receiver A port is connected to the antenna, and the signal is split between A and B using an additional splitting board (discussed later). The same applies for the C and D receiver ports.

For the complete version of this article as published in the NCJ, view the pdf version.