Using N1MM+ Serial (OTRSP) Data for Two-radio Antenna Switching

I have been investigating ways to automatically switch antennas in a 2-Radio (SO2R) station for use in Contesting. Using multiple antennas is nearly a must for HF contesting, including Yagis, Veticals, and any of a variety of wire antennas. When working a Contest, Band Switching is a key time-waster, and steps need to be taken to make it as quick as possible. Serious HF Contesters usually have instantaneous Band Switching, or something close, that involves little or no human intervention.

HF Radios Provide BAND DATA

Most modern HF radios provide some form of “Band Data” through an external connector, to tell antenna control equipment what Band is currently in use. In a two radio setup, each radio provides updated Band Data any time Bands are changed. The physical interface from the radio varies, from 4-bit parallel BCD (Binary Coded Decimal) signal lines typical of an Elecraft K3, to a variable analog voltage (typical of ICOMs like the IC-7600). It is possible to convert these signals to antenna selections to drive something like a 2X6 (2 Radios, 6 antennas) antenna switch (one commercial version called a “six pack”). So it may be necessary to deal with 2 different physical interfaces to drive the 2X6 switch.

Using this approach, each time you change Bands on a Radio, the corresponding Band Data line states change to the new band. This information is sent downstream to the antenna control equipment. This seems to imply a one-to-one Band-to-antenna mapping. Since that is rarely the desired case, other means are necessary to allow things like selecting from multiple antennas for a single Band, or using a single antenna for multiple Bands (like using a Tribander for 10, 15, and 20 meters, or a Dipole or Loop for 160 and 80 meters).

This all sounds messy to me. If you happen to have a pair of K3 Radios and some money to spend, you can buy all the right cables and a six-pack, put it all together, and off you go. Or, at least that appears to be the case. Since I do not have these things, I will not be doing that. I have a Yaesu FTDX-3000 and an ICOM IC-7600. I control my station with a DX Doubler SO2R box, and N1MM+ software (probably the best free software I have ever encountered). So I need a different approach.


N1MM+ provides a rich set of serial and Ethernet data for use in a Contest Station. Serial data is provided by N1MM+ through a COM Port using the Open Two Radio Switching Protocol (OTRSP). A key element of the N1MM+ OTRSP data stream is Antenna Select Data (ASD). The basis of ASD is an Antenna Table in the N1MM+ Configuration dialogue’s Antenna Tab. This looks very different from the Band Data switching approach. You can read about the N1MM+ Antenna Tab in the great documentation provided by the N1MM+ Team.

The N1MM+ Antenna Configuration Tab defines a Table of Band-to-Antenna mapping. In its simplest form, you can configure the Table for a straight one-to-one Band-to-Antenna mapping just like the Band Data from a pair of Radios. So there would be 4 output lines for each Radio. But much more can be done with ASD. The Antenna Table can be configured to allow multiple antennas per Band, and/or multiple Bands per antenna. This approach looks best for my situation. When I select a Band in N1MM+, the appropriate ASD will be sent to my Antenna Control system. N1MM+ allows you to use the ALT-F9 key to cycle through all antennas that have been associated with a Band. I am not sure if this can even be done with the BAND DATA approach, but I would be interested in hearing about it.

Converting OTRSP Antenna Selection Data to Band Decoder Signals

ASD in the N1MM+ OTRSP data stream does not appear to be well-documented, but testing shows that every time there is a Band change, the serial data stream outputs a Radio Number followed by an Antenna code value (0 – 15). I have already built an OTRSP decoding Unit (the DXDUSB) that extracts OTRSP data, and drives a set of parallel digital output lines. My plan is to adapt this hardware and software to the task of extracting the ASD from the OTRSP data stream and driving a pair of 4-bit BCD output lines. Each 4-bit signal will be used in place of Band Data that would have been provided by a Radio.

The mapping of Bands to antennas will be completely dependent upon the Table. It can be configured to emulate Band Data from a K3 (and drive commercial Band Decoders sold for use with K3 radios), or it may be configured for a very different mapping, depending upon the set of antennas available at the Station. I have a friend who is going to provide me with a 2X6 antenna switch to test this concept. The switch is normally used with K3 radios and I will see how easy it is (or is not) to adapt it to my station.

The KG3V AntDecoder Project

This project will be referred to as the AntDecoder. It will use a new PC layout modification of the DXDUSB hardware, and new software based upon the existing DXDUSB Code that decodes the N1MM+ OTRSP data stream. The goal of the project is to build a complete SO2R antenna-switching capability for my two radios and my antennas using a standard 2X6 switch, without requiring direct connections or signal conversions from Band Data connectors on the radios. This probably will not be as easy as it looks, but that will make it a challenge, and more educational.

As always, this project will be undertaken in my “spare time,” so the pace will not be as fast as I might like.


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