If I were to ask the average ham to tell me about the spectrum below the AM broadcast band, probably the most common response would be that the bands are noisy and require big antennas. They might also say that signals will only go down the street and operating there is generally a waste of time. I’ve heard all these responses before. After some discussion, I have been relatively successful at changing some minds and attitudes. Curiously enough, similar thinking was implied in the early days of DXing on 160-meters as reported in Jeff Brigg’s (K1ZM) book, DXing on the Edge: The Thrill of 160 meters where operators might listen for days or weeks without hearing a signal. Fortunately for us, technology has improved tremendously, giving us receivers with DSP that have greatly improved minimum discernible signal (MDS) levels, more compact directional receive antennas, knowledge of and experience with propagation, and numerous modulation techniques and modes that allow signal levels on par with what EME operators experience to yield QSO’s.
Lets examine a few key areas that seem to get a lot of attention when we talk about 630-meters:
Antennas are probably number one on the list of misunderstood aspects of operating on 630-meters. In fact there seems to be an endless stream of comments that point out that a half-wave dipole is almost 1000-feet long and “won’t fit in my back yard”. While this might be a fact, it’s important to point out that 1) dipoles generally don’t function as dipoles at these frequencies due to proximity to ground and 2) under normal band conditions, vertical polarization offers the best power coupling to the ionosphere. As a result, both size and these facts rule out the half-wave dipole as a practical antenna. There has been some very good work done, however, in the field of full-sized half-wave dipoles by Brian Pease, W1IR / WG2XPJ, of Vermont. Using a dipole approximately two-meters above the ground and center fed, the results suggest a low angle vertical component off the ends of the dipole. In fact, the dipole is functioning as a loop, with RF currents flowing through the ground, and is somewhat analogous to traditional terminated ground-loop antennas that have seen some testing in Europe on 630-meters and 2200-meters. Some of these effects are seen with “NVIS” antennas also and are highly dependent on ground conditions.
To satisfy the second point of vertical polarization for optimal ionospheric coupling, there are two potential options and, much like the debate between Chevy and Ford about which is better, the discussion tends to bring out passionate support for each. The most obvious choice is the vertical antenna. Of course, we can’t forget that a quarter wave vertical will require radials, which is probably the least of our worries since a quarter-wave radiator at these frequencies is nearly 500-feet tall. But there is hope! There has been a lot of discussion over the past few years within the Topband community about the performance of short verticals, where care has been taken to control loading and ground losses. The consensus seems to be that short verticals can perform very well compared to their full-sized counterparts. As evidence to this, in January of 2014, I performed a series of very non-scientific 630-meter experiments that sought to evaluate micro-antennas, many of which were simply amateur-style antennas that had been retrofitted with loading devices and matched to a feed line. The results were very surprising, in fact, the best antenna tested was a short G5RV base loaded as a Marconi-T (where both sides of the feed line are tied together at ground level and attached to the top of the loading coil) and fed against only a small number of short radials. Using WSPR as an evaluation method and 100-watts to the coax (estimate about 0.75 watt ERP), this arrangement was reporting only about 3 to 6db down from the main antenna at my station, an 80-foot tall, asymmetric Marconi-T over nearly 3-miles of radials at 1-watt ERP at the time. From a transmitting standpoint, digital mode and aural CW contacts would certainly be possible with this modified G5RV over a variety of distances and paths. It should also be noted that the test location was a standard quarter-acre lot with above-average ground conditions.
So just how small could an antenna be on 630-meters and still allow the operator to find some success?
Rather than stepping down incrementally to a smaller antenna from the modified G5RV, I chose to skip to the very bottom end of a minimalist vertical setup: an 8-foot tall, base-resonated, pipe with no top loading and radial system of questionable level of repair – it certainly doesn’t get much dicier than this. Surprisingly, yet again, the antenna did, in fact, radiate a bit using WSPR as an evaluation method and 100-watts to the coax. While the signal-to-noise numbers were not as promising as with the modified G5RV, someone that wants to get their feet wet on medium wave from a small lot would certainly be able to do so. Regional digital mode coverage with this small antenna is probably reasonable to expect, with relatively consistent S/N levels in the low to mid-20db range under “normal” noise conditions and standard 2.8 kHz bandwidth for modes like JT9. Local aural CW coverage is also a reasonable expectation.
The second antenna option which has yielded good performance for many medium wave and long wave operators is the vertically polarized loop. Like its horizontal counterpart, the vertical loop comes in a variety of shapes and sizes but has little ground dependence compared to the vertical antenna. Often times operators using these loops will bundle a number of parallel conductors to minimize conductor losses due to high currents and resonate at the feed point with a vacuum variable capacitor. These loops require two tall supports that are in the clear in order to support each side and maintain the vertical orientation.
There are smaller loop options available and interested parties should research the Edginton quarter wave loop. While the 630-meter version is far smaller than a quarter wave, some very good recent work by Jim Voll, WB5WPA / WH2XQC, has shown that a resonant, transformer-coupled vertical loop that fits in a standard-sized back yard can radiate an effective signal and still leave room for other activities. There is also the additional feature of having transmit and receive directivity on 630-meters, where the small loop could conceivably be engineered to rotate.
For many operators, the idea of using separate transmit and receive antennas is foreign. Typically only larger rigs offer separate receive antenna ports and often times only operators that are interested in the low bands investigate their utilization. The importance of improving signal-to-noise ratio cannot be overemphasized for the effective medium wave and long wave operator, particularly in this day of the ever increasing noise floor. Receive only antennas might conjure visions of 1000-foot long beverage antennas. Fortunately it does not have to be that complicated or taxing on real estate. K9AY loops, short, close-spaced vertical arrays and rotatable resonant loops have become standard for many operators on 160-meters, many of which live on small lots with both transmit and receive antenna residing very closely together. In these close-quartered environments, a simple relay tied to the PTT line can detune the transmit antenna while receiving in order to preserve the null and pattern. Doing so further improves signal-to-noise ratio. Also careful attention to impedance matching and controlling common mode currents on feed lines can go a long way to improving receive performance over what one might experience by listening with the transmit antenna.
Modulation and Power
A discussion of RF power generation would be difficult without a discussion of modulation types currently in use on 630-meters. Due to bandwidth limitations for the 7 kHz-wide band, it is expected that emissions will be limited to narrow band digital modes and CW. In fact, work done over the past several years suggests that 630-meters will be a haven for would-be weak signal digital and CW operators!
Why does the emission type matter when it comes to signal and power generation? It’s because of linearity versus non-linearity. When using modes that vary in amplitude and / or phase, a linear amplifier is required in order to minimize distortion products that can otherwise be generated, radiated and cause unintended interference to other users. Linear modes include PSK, AM, and SSB. Non-linear modes, on the other hand, include CW, FSK, and MSK and don’t require linearity in order to produce a low distortion signal. The process for generating a linear signal involves changes in amplifier bias which results in a decrease in overall amplifier efficiency, in many cases as low as 40%. This means less power output along with a significant increase in the heat generated by the amplification process. In other words, you need more input power to make less output power. Contrast that with the non-linear modes and its possible to achieve 80 – 90+% efficiency on many class-D and class-E systems that can be constructed with minimal components and effort today.
Many of the newer HF rigs on the market today have been configured to generate a +0dbm (1 milliwatt) linear signal to drive an external amplifier and low pass filter. These external amplifiers can be linear or non-linear but it’s important to note that if using a non-linear external amplifier, the operator should limit their operation to non-linear modes to avoid generating and subsequently transmitting distortion products that can manifest as sidebands. These sidebands can ruin a off-frequency, weak signal contact much in the same way that key clicks impact operators on 160-meters.
Today, it is possible to hear and contact stations using a variety of linear and non-linear modes, including CW, FSK (often in the form of WSPR, JT9, and Domino-EX), MSK (as GMSK using MMVARI), and PSK-31. My personal on-air experience has been that modes like FSK and MSK have a clear advantage when compared to PSK due to recovery time in potentially noisy environments where the noise floor may fluctuate many decibels over the course of a few seconds. That’s not to say that PSK is not usable on 630-meters only that other modes should be further explored to determine what might be a good fit under the given operating conditions.
CW contacts have been plentiful as well. Since September of 2012, I have amassed a logbook full of CW contacts with stations like WG2XJM (NO3M in PA), WH2XGP (W7IUV in WA), WH2XHY (WD8DAS in WI), WD2XSH/20 (N6LF in OR), WD2XSH/6 (W5THT in MS), WG2XNI (AB0CW in CO) and WD2XSH/12 (AI8Z in CO) just to name a few. For an 18 – 20 wpm CW signal, the required signal-to-noise ratio is typically on the order of -10 db in 2.8 kHz bandwidth. There are exceptions to this and it’s not uncommon to hear CW in the noise for signals in the -14 db to -16 db S/N range. That does not mean it would be a comfortable rag chew but completing a contact is not out of the question.
Finally, lets discuss JT9 versus JT65. JT65 variants have found a very significant following on the HF bands. The weak signal decoding capabilities have allowed many little pistol stations to achieve DXCC using minimalist stations, even on 160-meters. JT65, however, uses a rather wide bandwidth compared to its counterpart mode, JT9 which only uses 16-Hz in the same 1-minute transmit and receive cycles as JT65. By utilizing a more narrow bandwidth, power density increases for the same amount of power, resulting in a deeper detection limit. In short, not only will more signals fit into a smaller chunk of spectrum but they should also have a better chance of being decoded. It’s my belief that JT9 will be the default “rapid” QSO mode on 630-meters. If you are a JT65 operator on HF, please make the switch to JT9 for 630-meters!
There is a commonly held myth that signals below the broadcast band can only propagate via ground wave. This could not be further from the truth. While a very robust ground wave component can exist in areas where the ground conditions and physical obstructions permit, skywave is the primary mode of propagation near and during darkness and also during periods of minimal solar angle such as during winter. In fact, numerous skywave contacts using JT9 have been completed at solar noon during the winter with WG2XJM in PA, almost 1800 km away from my station in North Texas. Other anomalous propagation phenomenon are seen, many of which are common on 160-meters, such as ducting, gray line, and solar event onset enhancements. While noise is the primary limiting factor on 630-meters, propagation to somewhere exists all year long. Trans-Atlantic and Trans-Pacific signals are regularly reported and it’s expected to only be a matter of time before two-way contacts are completed. Typical 160-meter operating protocols apply on 630-meters (you have to be in the seat to have a chance to work ’em).
High voltage safety is high voltage safety. However, over nearly 30-years as a ham I have personally observed a number of amateurs that have taken liberties while working on HF antennas that might have gotten them killed at medium wave or long wave frequencies. Because of the nature of the physically small vertical antennas in use on 630-meters, large loading coils are often used to cancel the capacitive component and resonate. In many ways, these coils are like large transformers and loosely similar to Tesla coils, generating a tremendous amount of voltage and current at the tops of the coils. John Molnar, WA3ETD / WG2XKA, uses the rule of thumb that the voltage at the top of the coil is roughly the Q of the coil multiplied by the voltage on the coax. For even a poorly constructed coil with a Q of 100, 100-watts on the coax into a 50 ohm load equates to about 71 volts which means the top of the coil is about 7100 volts at a sizable (and lethal!) current. So please be careful and resist the temptation to load up the rain gutter or adjust taps with RF on the system. Most of us that have been on 630-meters for the past few years have started fires in one way or another with our antennas or matching systems and I think that gives a new appreciation for high voltage safety. We were lucky and learned lessons from what happened and got another chance. Make sure you have an opportunity to play another day by not taking chances and thinking carefully before doing something related to the high voltage and high current components in your system.
Being able to operate on 630-meters has been one of the greatest operating joys that I have ever experienced. Not only has it given me the chance to develop a strong sense of camaraderie with a group of very fine operators, but it has also created an opportunity to further the primary body of knowledge associated with operating below the broadcast band and the ancillary disciplines that are closely attached to it. We have learned a lot through these amazing experiences on this amazing band but those experiences are only drops in the bucket and in the grander scheme of things we know very little. There are so many areas that are left unexplored and having a new generation of amateurs exploring and studying propagation or developing new modulation techniques and receivers is an exciting proposition.
Welcome to 630-meters.