Beam Steering in Combat or How to Use Software Radio and an Array of Short Verticals in a 160 Meter Contest This past weekend, I used a new secret weapon in the 12th Stew Perry Topband Distance Challenge Warm-Up Event. This secret weapon consists of 8 short verticals, 4 Softrock v6 receivers, and a Delta 1010LT sound card plus computer running my Phasor module, DttSP and SDR-Shell. The verticals were configured as a 4 element broadside array of end-fire arrayed verticals. See http://k1lt.com. Since the phased array must be used with a computer to form an entire receiving system, I had to integrate this new technology with a traditional contest station. I did this by routing the audio from the computer/radio/antenna system to the left channel of my headphones and the audio from the main station to the right channel. Since I didn't have time to build a real SO2R (single-op, 2 radio) control box, I used a Ramsey 25B stereo FM transmitter and a Radio Shack Stereo FM Headset receiver. This headset has separate level controls for each ear (for balance) besides an overall volume control (nominally, the volume control). Thus I could use the audio control on the IC765 for "traditional" (Beverage antennas and a triple conversion receiver) audio, and either KDE's kmix audio level pop-up control or the left earpiece level control on the headset to control "high technology" (phased array and software radio) audio. I set up my station a few hours before the first expected signals. The contest is already on-going at this point, but nobody bothers with ground-wave contacts in this event. As I putter about the shack, I can monitor the waterfall display provided by SDR-Shell for signals. Since my monitor is 1600 pixels wide, I can see about 36 kHz of spectrum at a time, which is plenty for most 160 meter contests. (It would not be enough for the ARRL 160 or the CQ WW 160 contests, which occupy most of the bottom 60-80 kHz of the band.) (SDR-Shell must use 2 pixels per FFT bin.) I guess I should buy a 1920 pixel monitor before then :-) The first problem becomes immediately apparent when I call the first station: the software radio provides the monitor of my transmitter, since its always receiving, but the audio is delayed by 0.021 (2048/96000) seconds, which is enough to make sending via the paddle difficult. I thought about quickly hacking the antenna phasing module to block the audio in the presence of a signal stronger than -40 dBm, but the thought of hacking software during a contest was a bit too much like work. Also, blocking the audio in the phasing module would likely add severe clicks and would disable the waterfall display. The waterfall is useful in that I can see where I'm transmitting in relation to the signal I was just receiving. Several solutions suggest themselves: mute the audio in DttSP using the push-to-talk signal from the transmitter, or even better, use an additional pair of ADC channels to sample the transmitted signal and subtract it from the received signal. Alternately, separate processing could delay the audio from the traditional radio by 21 milliseconds, although this would not provide for the monitoring of one's own sending, even though is does cure the weird echoing sound when listening to the same signal through both ears. In years past, I've used SDR-Shell and DttSP to monitor the 160 meter band during a contest. Although there are almost always too many signals visible at one time to tell if one of them is new, there are a couple of uses for the waterfall display. First, a pile-up is usually an indicator of an interesting station, and a pile-up is plainly visible. Second, holes stand out, which makes finding a new run frequency much easier than tuning the band. However, my own transmitter usually paints the entire display white with the distortion products from overload. Therefore, one has to stop transmitting for a while to make use of the display. With the phased array, I noticed that I can steer the array for a nice compromise between listening in the direction of Europe, and attenuating my own transmitted signal. Of course, the transmit antenna is much too close to the array for a proper null. But by steering the array a mere 6 degrees north of the ideal European bearing, I could reduce the apparent strength of my own transmitter from -40 dBm to -48 dBm, which is enough to reduce the extent of the waterfall "white-washing" to about 25 kHz and allow me to see some other signals despite my own transmitter. Thus is born the idea of subtracting the transmitted signal from each of the array receivers. This trick requires another pair of ADCs phase (word) locked to the existing 8 ADCs (by using another Delta 1010) and the same software that I'm already trying to use to "calibrate" the phased array. So this is an area of future experimentation. The second problem became apparent when numerous weaker European stations were calling: not enough (any) knobs! When trying to copy a barely audible weak CW signal, this operator tweaks the incremental tuning, the pass-band shape, and the antenna direction to try an find the best copy. Often, these settings change during the course of a difficult QSO as propagation changes (fading). So, a mouse or keyboard based user interface does not make for rapid tweaking. Maybe we can find a bank of rotary encoders that can be interrogated via a USB port, and interface that to SDR-Shell. Also, I think I shall update the phasing module to allow "push-button tuning" to select a number of preset beam headings, perhaps by mapping the edge keys of the numeric keypad as if they were a compose rose. I use the same tactic to select from a set of 12 Beverage antenna direction using a telephone style keypad. Also, I might have to add a personal modification to SDR-Shell to disable the escape key. (The function of the escape key is nominally completely correct, except for my own silly situation). My station has two IBM model M keyboards side by side, the left keyboard for the Linux machine that runs DttSP and the rest, and the right keyboard for the Windows machine that runs the NA logging program. When the rate gets slow, I get up and wander around while the CQ machine runs. When someone finally calls, I rush back to the keyboard and bang the escape key to stop the transmitter. Several times during the contest, I hit the wrong escape key, which shuts down a receiver when I most want to receive! There is nothing like real user tests to find these kinds of issues :-) During the first two thirds of the contest, when signals may be arriving from Europe, I kept the phased array focused on Europe, and used the Beverages to aim west. That way, I could take calls from either coast without having to hunt through all the Beverages as I have in the past. I still need to check the southern Beverages for calls from Florida and Texas, but that is still better than the old way. In previous contests after each CQ, I would switch through 4 or 5 Beverages to make sure I didn't miss a weak signal. During the hours from 0400-0600Z, which have the most European contacts, I kept both the phased array and the Beverages focused on Europe. Although I could usually hear a European caller on both antennas, the copy was nearly always better on the phased array. The phased array seemed to have a slightly better SNR although its very difficult to make critical quality judgments while trying to copy a weak signal. Note that NEC modeling suggests the phased array should be about 3 dB better than my best European Beverage. C52C in The Gambia, Africa, was on-the-air during the contest, although he was not participating. I did note that the phased array could distinguish him from the Europeans in that his bearing seemed to be about 20 degrees to the south of the Europeans. Also, I worked several Scandinavians, and they seemed to be about 10 degrees north of the typical European bearings. Note that I have the phasing module set up to change direction in 10 degree steps by default, and in 1 degree steps with extra key presses. Also note that propagation often smears the apparent bearing of many stations outside of North America, so take these observations with several grains of salt. But the results seem promising. After sunrise in Ireland, I took a nap and came back around 0930Z to begin listening for Oceania and Japan. However, I was severely drained of motivation by that point, and also the propagation did not seem to be there, so I did not gain much insight about the phased array performance to the west. DX to the west from my location has always been difficult for several reasons, including geography, local topography, and personal chronometry (I have a hard time being awake very early). So, I didn't listen for long, and I may have missed DX and certainly missed the opportunity to evaluate the phased array to the west. My quick perception is that the array was noisy. Finally, I need to record the output of the 4 Softrock v6's, so that I can analyze this stuff after the contest. For example, I could go back and figure out why I think I heard ZS0X in grid square JO99, when ZS is a South African prefix, and JO99 is in the Baltic Sea. (Maybe the call was 7S0X, although I swear I heard "Z" and not "7".) Or, I could figure out why TI7H came in from the south, even though he gave me a stateside grid square. I suspect there are JACK plug-ins that could do multi-channel recording, and if not, such a capability would be fairly simple to add to my Phasor module. The bottom line is that the phased array works, even though I have not finished optimizing it, nor have I a decent way of calibrating it.