"A Newcomer�s Guide to LF Weak Signal Receiving Techniques"
A Newcomer’s Guide to LF Weak Signal Receiving Techniques The very weak received signal levels found in U.S. “Lowfer” (160-190 kHz) and European amateur (136 kHz) operation require special transmitting and receiving techniques if large distances are involved. This article focuses on U.S. Lowfer issues, but applies equally to amateur operation, though higher transmit power is permitted. Noise is the enemy we fight in LF communication. Unlike operation at VHF and higher, the noise generated by our own receiving equipment is usually not the primary issue. The real culprits are atmospheric noise, harmonics of the power line frequency, and a whole host of emissions produced by electronic equipment. The chief component of atmospheric noise is static crashes due to lightning, and that will be locally worse during the summer. Power line “crud” is based on either 60 or 50 Hz, depending on your location, and consists of harmonics that are generated in equipment plugged into power lines, with those lines then acting as antennas. Electronic garbage at LF is frequently generated by switching-type power supplies. Additional LF receiving problems come from discrete frequencies radiated by power line carriers, Loran-C, and LF broadcast stations. Power line carriers (PLC’s) are intentional RF signals transmitted by power companies over their HV lines to coordinate switching and load management. Loran-C is a world-wide navigation system that broadcasts fast rise-time pulses on 100 kHz. Both PLC and Loran operations produce stable carriers that can frequently be simply avoided by choosing a “clear” frequency. LF broadcast operation with high power levels is common outside of the Americas, and the European AM signals with carrier and sidebands can be quite loud in the Eastern U.S. on a winter night. The bandwidth of these signals may cause you to avoid several kHz of spectrum on either side of the carrier. Daytime electrical noise levels are frequently lower than those at night. This makes the daylight hours better for “local” communication, which could be 200 miles or more with slow-speed techniques. As with the AM broadcast band, “skywave” propagation is possible at night, and the lower atmospheric noise levels and longer nights of winter favor setting distance records at that time of year. For those of us in North America, the very best signal to noise ratios (both day and night) happen around the time of the winter solstice, producing a lot of activity around the Christmas season. The difference with the summer can be very dramatic. Conventional CW is effective to the point where the brain has trouble separating the desired signal from the noise. Even when copying by ear, slower CW speeds permit accurate copy. As the signal becomes weaker, you can slow things down until you reach a point where the brain can’t separate dots from dashes, which is about the same area where the audible signal gets lost in the noise. For many years, this was considered the threshold of Lowfer communication. DSP (digital signal processing) has changed all of this. By greatly lowering the data rate and narrowing the bandwidth, signals that are 40 dB or better down in the noise can be identified. This has really expanded the distances at which we can hear each other, though at the expense of very limited data rate. Since most Lowfers were just sending 2 or 3 letter call signs as beacons, over and over, we may now have to wait for 30 minutes to identify a station! Still, there is a real thrill in seeing such weak signals clearly visible on a computer screen when the receiver’s speaker is just producing roaring noises! Early DSP-based efforts depended on homebrew analog-to-digital converter boards that accepted the receiver’s audio, and fed the resultant data through a serial port on the computer. The easy availability of sound cards, and the compatibility dictated by the various Windows operating systems, has now made it possible to feed the receiver audio to the line input of a sound card without any external interface. The DSP work is then done totally within the computer. While CW-based slow speed communication is not the only approach, we will start there, and spend most of this article discussing it. We will mention a variety of other alternatives toward the end of the article, but feel that you should master the CW techniques first. The application of DSP to CW provides screens that are calibrated in frequency from top to bottom, and in time from right to left. CW signals are then displayed as dots and dashes, with at least several letters being possible along a straight line from left to right. “QRS” is the Q-signal to request someone to send more slowly. The term “QRSS” was coined to represent a very slow CW transmission, where the speed is best measured by the number of seconds it takes to send each dot. As a form of shorthand, “QRSS3” indicates that each dot requires three seconds. Common steps in QRSS operation are at 3, 10, 30, 60, 90 and 120 seconds, though endless variations are possible. For purposes of this article, saying that a particular computer screen display is set for QRSS30 simply means that detection bandwidth and speed that signals move across the screen are optimized for signals with the shortest element at 30 seconds. You would be able to copy somewhat faster and slower signals on such a screen, though there are practical limits which will be mentioned later. Let’s talk for a bit about the hardware and software needed to receive QRSS or other slow speed LF signals. Receiver: Many ham-band and communications receivers tune down to 100 kHz or lower. In almost all cases, the sensitivity of the receiver is much less at LF than at MF. This is less of an issue than you might think, as with some thought to the antenna and possibly an external preamp, you can keep even the weakest signals above the noise floor of the receiver. This isn’t VHF or UHF, and gain is cheap! Actually, the two of the most critical issues in selecting an LF receiver for slow-speed work are frequency settability and stability. Ideally, you should be able to tune the receiver in 1 Hz steps, though 10 Hz will work with some small mental gymnastics. 100 Hz steps are pretty coarse for this application, but you can still work around the issue. Once the receiver is tuned to a given frequency, it must stay there, or you’ll get pretty seasick watching the computer screen. All of this tends to rule out “boat anchor” receivers in this application. Stick with synthesized oscillators. If you are doing an hour-long reception on 177.777 kHz, it would be nice to know that the receiver is tuned within the few Hertz displayed from top to bottom on the screen! One alternative to conventional receivers are signal level meters originally designed for telephone company use. Some of these can be set very accurately in frequency, and they have a wide dynamic range. IF filtering can be less of an issue than it is for aural CW work. The really sharp filtering is done in DSP in your computer, resulting in much smaller bandwidths than you can achieve in the receiver itself. On the other hand, a narrow CW filter in the receiver may prevent nearby signals from pumping the AGC. Just remember that Lowfer or LF ham-band operation is very different than working pileups on 20 CW. If you want to keep your LF activity from encroaching on the MF or HF part of the shack, consider buying a separate receiver. For $600-700, you can set yourself up with a really fine receiver without the compromises that result from putting a transmitter in the same box. Antenna: We will not dwell on the intricacies of LF receiving antennas in this article. Many types will work, and folks tend to get passionate about their favorite antennas. Here are some pointers to guide your research: Wire antennas are usually not very long in terms of wavelength at LF. Vertical and horizontal wires will work, but remember that the ground is invariably part of the “circuit.” You will want to experiment with isolating the antenna ground from the receiver/AC ground. Such antennas can be good performers in a quiet location, perhaps with the tuning done away from the shack, and a coax feed connected back to the receiver. E-field “whip” antennas can be excellent performers at LF. They should be placed as high as possible, and attention should be paid to grounding paths. These antennas have the advantage of being untuned, but they do require a preamp at the base of the whip. The dynamic range of the preamp must be quite good. The AMRAD E-field antenna described in September, 2001 QST is an excellent performer with a “cast iron” preamp. Loop antennas, ranging from a couple of feet to 50 feet in diameter, are used by many Lowfers. The smaller versions can be easily rotated, and the bidirectional response of the loop can be used to peak desired signals, or null undesired ones. Most loop antennas are tuned, with a preamp at the antenna, and are located as far as possible from local noise sources. Computer: This isn’t too complicated. Stick with Pentium-class computers, clocked at 133 MHz, minimum. You really should use something faster, but give whatever you have a try. Some users report “glitch” lines on the screens of under-powered computers, but usually can decode the characters as well as they could on a faster machine. 800x600 screen resolution is sufficient, and the monitor can be anything that makes you happy. The available software is generally capable of running on Windows 95 or higher operating systems. RAM is always your friend, particularly if you are running other applications while the computer is digesting the output of your receiver. The computer and monitor may be a source of electromagnetic interference at LF. You may want to experiment with the routing of the LF antenna cable, and network or audio cables running from the computer. Some users report that laptop computers are RF-quieter than desktop machines with separate monitors. Sound Card: The latest software, such as Argo (below), is much less hardware specific. Still, many users are sticking with sound cards made by Creative Labs, in order to insure “Soundblaster” compatibility. If possible, find a sound card with separate microphone and line level inputs. The signal level from your receiver would have to be padded down to avoid overloading a microphone input. Software: This article focuses on one piece of software, Argo, a freeware program written by Alberto DiBene, I2PHD. You can download this software from Alberto’s site: http://www.weaksignals.com . Other software exists for copying QRSS signals, but Argo is very powerful, and easy to use. Once you are comfortable with it, you can experiment with the rest and begin to learn why Alberto nailed so many things down so they couldn’t be fouled up! Now, let’s talk about connecting the receiver to the sound card. When you copy CW by ear on a receiver, you typically are listening to a beat note between 500 and 1000 Hz. Your receiver’s CW mode may in fact be set to display the correct incoming frequency when the beat note is 700 Hz or something similar. Unless you have a compelling reason to keep the receiver in SSB mode, we suggest that you stick with CW to make the readout correspond more closely with the actual frequency, and to take advantage of any narrow CW filters you may have installed. Some receivers allow you to program (or adjust) the BFO offset. The default audio frequency for much Lowfer-related software is 800 Hz. While this is a little higher than some of us like for regular CW work, do remember that you will be using your eyes more than your ears! So, for purposes of this article, we will assume that you have set up the receiver for this 800 Hz target frequency, or that you are doing the necessary mental gymnastics. Some receivers provide a “recording” output that is independent of the AF gain control. By all means, use that if it is available. The rest will have to content themselves with using the receiver’s headphone jack or speaker terminals. You will just have to remember that turning down the AF gain will kill the feed to the sound card. The nominal audio level should be around 1 volt peak-to-peak (as seen on an oscilloscope) when the receiver is tuned to a moderately strong signal. There’s nothing critical about this – we just give it as a typical level. Assuming that your sound card has a line-level input, you can directly connect the audio from the receiver to the sound card. The sound card input is probably stereo, and you can choose to feed either channel or both. Typically, the “tip” of the sound card jack is the left channel, the “ring” is the right channel, and the “sleeve” is the common connection. Some of us choose to put a 1:1 (600ohm to 600ohm) audio transformer in that audio path to break up any possibility of a hum-inducing ground loop. In most cases, the improvement would be small. If your sound card has only a microphone-level input, you have some extra work to do. First, the audio level from the receiver will be about 30 dB too “hot.” Second, the sound card may be set up to power an electret microphone, and that DC voltage will have to be blocked. Here is a diagram of a suitable interface: <<<diagram of pad and blocking capacitor>>>