This is one of a series of “Notes” I published on Facebook. Since Facebook has discontinued the Notes feature, I am publishing that series here on my blog.
I have been listening on 630 meters for over a year and I had operated a WSPR beacon two previous nights with very low power of about 15 milliwatts EIRP (Effective Isotropic Radiated Power). Even at that power level I was heard by more than 25 stations at distances to beyond 1000 miles, 1600 kilometers. But last night as winter descended upon the region with the first significant snowfall, I was transmitting with 22 watts TPO (Transmitter Power Output), resulting in about two watts EIRP. That is still four dB below our legal limit of five watts EIRP. Results were better than I expected for my first night, even though propagation was certainly not at its best!
At times I am a rabid contester, an avid DXer, an experienced builder of antennas and equipment. I love radio. I enjoy all of these activities but if I had to describe my pursuit of radio in one word it might be explorer. I love trying something new, venturing into little used territory, pushing limits, trying to beat the odds. I have a well equipped station. I could easily converse with amateur radio operators around the world most days just by turning on a radio and speaking into a microphone. Instead I am driven to pursue nearly impossible contacts on frequencies that do not easily go the distance. I especially enjoy pushing the limits of propagation, equipment and myself as an operator on modes where the human ear/brain is the decoder. Unfortunately that is becoming a rare thing in today’s world of digital modes where the computer does the decoding, all too often using only partial information received over the air, the rest filled in from a database or assumed because it was information previously known to the decoder. The continuing erosion of what represents a two-way amateur radio contact saddens me beyond words. Nevertheless I continue to find joy and excitement in the exploration of frontiers which, if not new to mankind, are new to me. I regret only that there will not be enough time to explore all there is to see and do in radio.
So it was that little more than a year ago I began my quest to conquer the two new low frequency bands, 630 meters (472 to 479 kHz) and 2200 meters (135.7 to 137.8 kHz). I spent the first winter learning about and building receiving equipment for those frequencies. There was a learning curve and I had to find what worked within my budget. Mission accomplished.
In the Spring I set my sights on building a transmitting station but quickly ran into a setback. The only way to put up a reasonably efficient antenna in the space I have would be to support it between my two existing towers. It didn’t take long to discover the older, weaker tower would not stand the strain of such an antenna! There was only one thing to do: replace the tower! The search for materials took all summer and the project wouldn’t have been possible at all without a great deal of help from friends. The work began in October and was not complete until the middle of November. That left little time before the onset of winter to build the antenna, a transmitter, and all the associated things that go with it. The antenna, a Marconi-T made of wire and strung between two towers, is shown in the cover photo.
One has to get into a very different mind set about antennas at these frequencies. Most of us are accustomed to “full size” antennas – dipoles, verticals, loops – which radiate nearly all the power we put into them. In many cases we use directional “gain” antennas that actually make our effective power more than we have coming out of the transmitter – in a favored direction. Very few if any will be able to reach anything approaching 100 per cent efficiency at 630 meters and no one will at 2200 meters. Forget about gain antennas! Horizontal polarization does not work well at these frequencies. A quarter wavelength vertical for 630 meters would be 490 feet (149 meters) tall and for 2200 meters, 1700 feet (520 meters)! Not only that, but unless located over salt water a vertical needs an extensive system of ground radials around it to be efficient. Most amateur antennas at 630 meters will radiate no more than a few per cent of the power fed to them, while many will be less efficient. At 2200 meters reaching one per cent efficiency will be very difficult for most, impossible for many. My 90 foot (27 meter) tall vertical with three 100 foot (30 meter) horizontal top loading wires sits over a radial system with more than 10,000 feet (3048 meters) of wire in it. Yet the best I can hope for is 3% efficiency on 630 meters and 0.25% efficiency at 2200 meters. The vast majority of power is eaten up by ground losses and losses in the large loading coils needed.
In fact, once we cancel out capacitive reactance due to the antenna being electrically short, the resistance we see isn’t really the antenna at all – it is dominated by loss resistance! Even at 630 meters the radiation resistance of a typical antenna will be an ohm or two, in many cases less. Even very good ground systems will be many ohms. My radiation resistance on 630 meters is 1.25 ohms. I measure 39 ohms when the antenna is resonant. Almost all of that is the ground system loss resistance, and that is where most of my power goes! This really is a different world from the higher frequencies.
The challenge doesn’t end there. After we manage to get a few watts radiated, we have to contend with the fact the ionosphere doesn’t propagate signals at low frequencies as well as it does higher ones, while both atmospheric and man made noise is much worse down here! It’s a wonder we manage to communicate at all. But we do. That’s the challenge, and that is what attracted me to this.
The variometer (left) is an adjustable inductor used to cancel out capacitive reactance in the antenna and resonate it on the desired freqeuncy. The matching transformer on the right steps up the resistance of the antenna system (in this case about 39 ohms) to the 50 ohm impedance of the feed line and transmitter.
By midday December 9 I was ready. The antenna was up, the variometer adjusted, matching transformer properly configured and my little 25 watt home made transmitter was ready to strut its stuff. I set it to transmit as a WSPR (Weak Signal Propagation Reporter) beacon, sending out a two minute standard WSPR message every ten minutes. In broad daylight, I received several reception reports after my very first transmission! WSPR reception reports are available on the Internet almost immediately and may be viewed as a list or displayed on a map.
The transmitter, consisting of a QRP Labs Ultimate 3S exciter (assembled from a kit) on the left and home made amplifier on the right.
I spent the first few hours nervously checking the transistor in the amplifier with my finger to see if it was running too hot, watching squiggly green lines on the oscilloscope for any sign the antenna was drifting off frequency or something breaking down outside. To my joy all continued to look good! It was not as if I had no reason to wonder. One transistor had died weeks earlier during initial testing of the transmitter. That occurred while running into a perfect resistive load, not an antenna that might be imperfect or change on a whim. These low frequency antennas which are, of necessity, electrically short actually can change on a whim! The capacitance and hence resonant frequency can change as they sway in the wind. The resistance can change with weather and season. With several inches of snow forecast for the night, I didn’t know what to expect. So I nervously watched over the system. After every transmission I checked to see where I had been heard.
As sunset approached and passed, the band naturally stretched out. I began to get stronger signal reports and was being heard at ever greater distances. After some time there was a report from a station in the Netherlands! My puny signal had made it all the way across the Atlantic on a frequency below the AM broadcast band! This was soon joined by reports from the Cayman Islands and Canary Islands. Africa! Toward morning I even got a reception report from Hawaii! Just before dawn I observed some variations in the antenna system. The two sine wave patters on the oscilloscope display began shifting slowly, almost rhythmically back and forth with respect to one another. This was caused by changes in antenna capacitance as it swayed slightly in a gentle breeze. I have heard the effect described “as if the antenna is a living, breathing entity” and I would have to concur with that description. It was fascinating to watch. I got very little sleep and didn’t even notice it was one of the longer nights of the year. I was having too much fun! I was exploring!
Oscilloscope patters from the “scopematch” monitor the antenna resonance and resistance. If the antenna were perfectly resonated on the transmitting freqeuncy and perfectly matched to 50 ohms these two sine wave traces would converge into one. Here, one is shifted to the left slightly, indicating an off resonance condition (in this case exhibiting inductive reactance), and they are not of the same amplitude (height), a sign of imperfect resistance match (here showing the resistance to be slighty high).
There are many challenges and new experiences yet to come on the low frequency frontier. One of my winter projects will be to build a transverter so that I can transmit modes suitable for two way contacts. But the bigger challenge is getting operational on the lower frequency band, 2200 meters. The coil and variometer will be much larger, creating not only issues in construction but in protection from weather. The antenna impedance will be far greater, resulting in very much higher RF voltage and the possibility of interesting but unwelcome events such as insulation breakdown, arcs and corona! Undoubtedly the antenna will “breathe” to a much greater extent. On 2200 meters I expect a lot of action on the scope when there is any breeze.