SO2R What it is, and how to Get Started

By Dave Pruett, K8CC

SO2R – probably no other term in amateur radio contesting is more talked about, and less often understood.  Some contesters want to implement it, while others want it delegated to its own category in contests.  This article will talk about what SO2R is, what you need to implement it, and what you might want to do with it.

First, what is SO2R?  SO2R means “Single-Operator, Two Radios”.  I would expand that definition to “a single operator using a second receiver to listen and search for new stations to work while his first radio is transmitting”.  (OK, so that mouthful doesn’t fit the four-letter acronym.)  However, this describes the fundamental advantage to SO2R, which solves the perpetual dilemma for the contester: whether to call CQ hoping to attract a station to work, or to search for one.  With SO2R, there is no dilemma because the contester can do both at once.  While the computer or other automated device calls CQ, the contester can devote their attention to finding new stations to work on the second receiver enabled by the SO2R system.

SO2R is NOT transmitting on two radios simultaneously.  To the best of my knowledge, every major contest sponsor restricts single-operators to a single transmitted signal at any instant in time, and an SO2R setup which meets this criteria will have some sort of hardware circuit and/or software logic in the logging program to ensure that this requirement is met.

Next, what do you need to do SO2R?  The most fundamental item is a second transceiver.  It might seem that a second receiver is all that is needed, but after finding a new station on the second receiver, working that new station (which is the point, after all) is much easier if the second receiver comes with its own transmitter.  Some people (and at least one transceiver manufacturer) claim that SO2R is possible with a single transceiver with a sub-receiver.  Ten-Tec floated this claim when their Orion transceiver came to market some time ago because it has separate antenna connections for the sub-receiver.  The Yaesu FT-1000D which is the basis of my K8CC contest station has the same thing, but both the FT-1000D and the Orion fail in that most basic SO2R precept: “receiving on the second receiver while the transmitter transmits“.

The next requirement for SO2R is having enough antennas to provide for the desired band combinations of transmitting and receiving on the second receiver.  This is both a hardware question and a strategy question which can be contest and geographically dependent  If your station has all mono-band antennas, a relay switching matrix like the KK1L box or the Array Solutions Six-Pack will allow either radio to be on any band, allowing all band combinations.  But most stations have one or more multi-band antennas and so some compromises are necessary.  For example, in Sweepstakes 40M is the most important band so I’ve done SS SO2R with one radio dedicated to 40M, and the other radio to the remaining bands.  If you have a tri-band beam and dipoles for 80M and 40M, such an arrangement is possible as long as the 40M dipole has its own feedline.

Another consideration regarding antennas for an SO2R station is the isolation between one antenna being used to transmit and another antenna being used to receive at the same time.  If too much transmitted energy is picked up an antenna being used to receive, the receiver can be damaged.  Designing the antenna farm to prevent this is a technical problem beyond the scope of this basic article, but common sense and intuition can go a long way.  The two most effective tools to enhance isolation between two antennas is distance and cross polarization.  Each time the distance between two antennas is doubled, the energy coupled between them is reduced by 9 dB.  The isolation between cross polarized antennas  such as a horizontal dipole and a vertical monopole is approximately -20 dB, but both antennas have to be as close to straight as possible.  For example, an inverted-vee and a vertical will have less isolation because the sloping wires of the inverted-vee radiate with both vertical and horizontal energy.  But the ultimate tool for augmenting isolation between antennas in an SO2R station are bandpass filters such as those sold by Dunestar and Array Solutions which are inserted in the feedline.  Such filters restrict the energy flowing on the coax to only the band of interest, which reduces the undesired energy which makes it to your receiver and can cause damage to it.

Another factor in the layout of an SO2R station relates to the “competitive goodness” of your two radios.  If you have two “equivalently good” radios, then either can be used for running stations (what I refer to as “symmetrical” SO2R) and the 40M/non-40M strategy described in the previous paragraph makes sense.  But if you have one “good” radio and one “not-so-good” radio, you might want to put all of your best antennas on the “good” radio for running stations, and let the “non-so-good” radio make due with a lesser antenna like a trap vertical.  A less-capable station might not be adequate for running stations but can be perfectly adequate for answering other stations, particularly in domestic contests.

One last point about transceiver selection for SO2R.  It is a tremendous advantage IMHO to have two identical radios in terms of manufacturer, models, IF filter selections, etc.  One of the most difficult SO2R setups I’ve ever used was a Drake C-Line and a Kenwood TS-830.  The two radios were completely dissimilar in terms of features and control layouts.  The VFO knobs even spun in opposite directions!  Each time I switched radios my brain had to adapt to the different controls.  About drove me crazy by the end of the contest.

The final requirement for SO2R is to have switching hardware to select which radio the operator is listening to (or both simultaneously) and which radio gets the voicekeyer/mic/ptt/CW keying signal.  The listening/transmitting selections must be independent, and as noted earlier simultaneous operation of both transmitters must be prevented .  Homebrew switchboxes are easy to build and can be customized to your own SO2R strategy and equipment layout.  The TopTen Devices DXDoubler and the YCCC SO2R Box Plus are examples of commercially available switchboxes.  Many modern logging programs offer SO2R control capabilities, but then you are locked into adopting their style of SO2R so choose carefully.

Finally, you need to decide what you’re going to use SO2R for?  At the least, SO2R can be used to check activity on another band to help make band changing decisions (SO2R Lite?)  Maximum effort SO2R goes to the second receiver each time the transmitter calls CQ/QRZ or sends a contest exchange (Full-gonzo SO2R?).  The more you use your SO2R capability, the more potential there is for improvement to your score.  But your brain will also be working harder, and you may be more fatigued at the end of the contest, or even have a headache!

Hopefully, this brief article will help to get you a started in your implementation of SO2R.