CQ DX for a SWL
By definition "CQ DX" means calling for a long distance contact; the amateur radio is usually a DXer chasing far or new entities. That also means that a SWL (short wave listener) has the opportunity to follow him and to add new entities to his palmares as well.
It is in this context that this article has been written to the attention of SWLs, although methods and tricks that will be described applied of course to licensed amateurs too, who are first listeners before working a DX station.
How to listen in the best conditions to amateur DX radios stations ? To tell me to switch on my receiver (or my WebSDR sofware) and listening is not enough. You will quickly understand that it is not obvious to answer simply and in a few words to this question.
Indeed, before working a DX station an amateur must first of all be able to listen his correspondent before to say like some do without listening at their contact "59, QSL ?"...
However the problem is harder to solve for a SWL as he is passive; he can only listening to ham bands, waiting that an amateur calls "CQ DX" and in hope that someone will reply to his call.
In listening amateurs using a dipole or even a small vertical 6 m high, you will note that we receive quite weakly (RS ~55) signals from DX stations located over 10000 or 12000 km away. It is already a good performance. But knowing that you can hear them louder and till go further in optimizing your working conditions, let's try to understand why we received them weakly, and try to find a way to improve our working conditions to solve this problem.
When listening amateurs in various conditions, you quickly note that many factors could affect and ruin all your chances to listen to or work DX stations, although both situations are not exactly reciprocal. We will come back on this difference when we will review wire antennas for listeners.
The following recommendations are based on my humble and own experience of amateur radio (and listener) that extends over several decades, and after have participated in a few contests and won many awards.
How optimizing your DX chase ?
If it is quite easy for a licensed amateur radio working in HF to send a "CQ DX" to a far country and waiting for a contact, for a listener there is no other solution than scanning continuously bands and regularly QSY in search of the most wanted call signs, with or without the assistance of clusters. Of course licensed amateurs are also confronted to this problem.
Indeed, it is not enough to call "CQ DX" if there is nobody to answer you on the other side of the Earth or if your antenna system does not allow you to pick up efficiently the signal of your remote correspondent !
There are thus several ways to optimize your chase to DX stations and to pick up rare stations. Here is a non-exhaustive list of factors that will impact your resultats, either restricting or increasing your chances to hear DX stations :
We are going to review each factor and how it influences your chances to hear a DX station. We will describe first how to mitigate or remove factors that reduce or prevent you to hear correctly DX stations, then we will insist on factors and solutions that can improve your working conditions.
Performances of your receiver
Shortwave receivers (as those of transceivers of course) don't show all the same performances. For a listener the most important parameters of a receiver are its sensitivity and selectivity.
The sensitivity represents the minimum input signal required to produce a specified output signal. Under this threshold you listen only to the electronic noise of components (thermal, shot, flicker and burst noises) or, at best, the electromagnetic noise from the environment (artificial and natural noise) on the frequency but no readable signal.
The second quality factor is the selectivity of your receiver. It represents its ability to respond to a tuned station and to reject any nearby undesired interference or noise, or both. It is probably the first factor where your receiver will show its performances. The next one will be the performance of its digital signal processing (DSP) that can also depends on the quality of the IF filter (roofing filter).
If you want more information, I suggest you to read the page How to select a HF transceiver and specially the paragraph about the selectivity as it plays a very important role in receive.
Your antenna system
If the Sun is the major source of QRN on frequencies after the ionospheric perturbations, whatever the index of the solar activity, your chances to capture amateurs transmissions depend first of all on the type of antenna used, its length, its properties (radiation pattern, gain, coupling, etc) without to mention propagation conditions.
As we told, to hear and to work a DX station are two very different activities. A licensed ham using a directive antenna needs to concentrate all the input power in a narrow beam to be sure to reach his or her correspondent in good conditions.
But for a listener, often using a poor antenna system (an indoor antenna, a dipole tight 5 m high outdoor or a small vertical), the antenna plays a passive role, except for the few SWLs than can pay for a directional antenna (beam or quad).
However, there is one rule to remember : the longer will be your antenna driven element the better will be the receive. Additionally, its height over the ground is also important.
In fact, your receiver will be more or less able to capture weak signals according to your antenna design and its radiation pattern. For example, a long wire cut for the 20 m band placed too low over ground (say 2 m high) or showing a length of 1λ only will show a bidirectional pattern and two main lobes while it will show many additional lobes if it is tight 7 or 10 m high or if its length exceeds 5λ as displayed below right. A similar change occurs for a vertical depending on whether it is cut at 1/4λ or 5/8λ as displayed below left.
An amateur using a very directional beam placed 15 m above ground will work in conditions far better that the amateur using a short vertical. The first installation will react much differently from the second to the ground effect and interferences (mainly true for emissions).
Directional antennas are the most suited for DX chasing due to their directivity and ability to reject noise (using a beams or a quad, a front-to-back ratio exceeding 20 dB is common). But if you cannot avoid or reduce noise and RFI, sometimes you have no other choice than waiting that the night falls to hear the world traffic from Canada to Chile via Australia and South Africa among other DX. This is mainly true at spring and summer during the maximum of the solar activity.
Practically, if I check my log during a cycle of high solar activity, I was practically forced to interrupt listenings HF bands at daytime (8 a.m.-6 p.m local time) on summer due to the QRN. Of course when propagation was widely open in summer, by 3-5 p.m. local time in ON or LX I already heard loud and clear far stations from countries like DU, JA or Z2 on the 20 m or 15 m band, but compared to the number of stations I heard in the summer at night, at daytime it is sometimes like working near a pneumatic drill !
But the Sun is not the only responsible of all these noises and many interferences have a man-made origin, from computers to power lines or transformers. We will take time to discuss of this problem in another page dealing with RFI.
If you are unable to modify your antenna system (for technical or financial reasons, or both), you can still optimize your chances to hear DX stations because other factors have to be taken into account.
Your location and working conditions
HF shortwaves at wavelenghts between about 40 and 10 meters need free space to propagate; that means that due their "size" they can be influenced by hills or small valleys.
From a wave propagation point of view, it is obvious that you will have troubles in trying to listen to far DX stations from the bottom of a steep-sided valley. If your HF antenna is unable to capture waves travelling a few hundreds of meters higher than the roof of your house, it is useless to install a shack at this location. Better to go mobile and to reach the top of the hill to capture these shortwaves.
Of course, the longer will be the working wavelength (80 or 160 m), the easier will be its capability to jump over obstacles and reach your shack located at the bottom of the valley or behind a hill.
Contrary to a beam polarized horizontally, a vertical antenna is vertically polarized. This latter is thus not entirely able to discriminate signals from the noise, and will capture all local noises as strong as the signals from far emitters. As explained above, during high solar activities and the summer months, you should wait until the evening and the sunset to hear your favorite DX stations. However, it will be the contrary for them : these stations located at the antipodes will be on the sun side and will experiment much QRN.
At last, you can also change your working mode and listen to amateurs working in CW (even if you need a Morse translator to understand the traffic). Not many amateurs like this mode but it is still the best to pass through QRM or QRN and to capture (or reach) the farest DX stations.
Solar and geomagnetic activities
You will find in the French pages of this site devoted to the solar system a thorough study of the Sun activity thanks to the collaboration of NASA and SOHO scientists. In a few words we can say that the 11-year sunspot cycle determines radio amateurs activities on bands, and thus your chances to hear DX stations.
When the Sun is in a period of high activity like between 2012 and 2014, the low frequencies (below 10 MHz) become less usable due to absorption while higher ones become more suited for DXing. Conversely, when the Sun is in a quiet period six years later, with practically no sunspots on its surface, at daytime lower frequencies are more longer open to DXers. So in 2014, at the paroxysm of the solar activity, where the Sun displayed over 300 individual sunspots some of them visible naked eye like on this picture showing two sunspots groups ~65000 km wide, thanks to an intense solar flux (SFI of 155, SSN of 110) ionizing upper bands (21-30 MHz), many stations 10000 km away were accessible without too much difficulties 24 hours a day.
Effects of the solar and geomagentic activities on the ionosphere are very complex. Tens of perturbations have been identified. Not only the solar flux affects directly the concentration and height of ionospheric layers, but solar particle emissions (CME, X-flare with fast electrons and heavy protons, ions, etc) are also involved in geomagnetic disturbances like fading, blackouts, sudden ionospheric disturbances (SID), polar cap absorption (PCA) and the famous auroras and associated events. All these disturbances affect sky waves propagation and thus the ability of your antenna to pick-up signals of DX stations. See the previous hyperlink for more detail.
The band status
When the solar flux is high with low geomagnetic activity, bands are usually wide open to low powers. In such conditions you will discover that radio amateurs work 24 hours a day 365 days a year on most bands, including on the top band (160 m) at night.
With over one million active radio amateurs around the world (for more the three millions licensed), you have many opportunities to hear them here and there. But some bands are more crowded than others, and weekends more than week days.
Even if that seems trivial, do select the most active band according to the season and the time of the day of your listening. Indeed, it should be an absurdity to listen a band that all hams known to be close, e.g. the 20 m band in winter late in the afternoon when the Sun is quiet or during blackouts at spring. For sure you will hear some QSOs, probably local or some powerful emitters working with 1 kW and gain antennas, but the rate of QSOs in that band will be most that probably very low compared to the listening of the same band during the summer eve's and in a time of solar maximum activity.
The skip distance
There is also another reason that could explain that you don't hear many amateurs on the air : the skip distance. It is a silent zone that essentially affects short distance contacts (mainly between 300-2000 km or so). If you do not take care to the way that shortwaves travel in the ionosphere, according the distance to your correspondent, your working frequency and propagation conditions you will be able or not able to hear QSOs between your neighbours; signals enter the ionosphere and escape to space or, if they reflect to the ground, you can experiment a signal loss over 25 dB or even a complete extinction. In fact there is no skip distance as long as the working frequency is less than the critical frequency of the concerned layer. In addition the skip distance shows the same variations associated to that critical frequency (diurnal, seasonal and solar cycle).
Hopefully, in the lower bands of 80 and 160 m, the waves are so long that there are no skip distance or silent zone around any emitter. In the 40 m band there is no skip distance excepted at night where it can extents up to 500 km. In the 30 m band the skip distance reaches 300 km at daytime and about 1000 km at night. Therefore in small states QSOs with border countries can be hard to work. In the 20 m band the skip distance reaches 700 km at daytime and can exceeds 1600 km at night. On the 17 m band the skip distance reaches 1200 km at daytime, it is of 1300 km on 15 m at daytime, 1600 km on 12 m and up to 2000 km on 10 m, only at daytime.
This phenomenon explains why all local QSOs and mobiles activities are most practiced in the lower bands of 40, 75, and 80 m, where you can easily heard your friends living a few tens or hundreds of km away, work castels, lighthouses and other national islands, but even in these bands first dedicated to ragchewing in local QSOs, DX can be worked in winter, at night or using the grayline as we will see later.
The day and time of the year
To select the "right band" supposes that you take into account the cycle of light and darkness as well as the cycle of seasons to increase you chances to capture DX entities. The 24-hour cycle produces a major effect on all HF shortwaves.
A good rule is to say that both extremes of the HF spectrum are very sensitive to the time of the day. As listed in the next table, we can say that wavelengths above 30 m (frequencies from 10 to 1.8 MHz) are best at night and wavelengths below 30 m (frequencies from 11 to 30 MHz) are best at daytime.
This division between bands does not mean that wavelenghts just in the middle of the HF spectrum like the very active 20-m band (14 MHz) is opened 24 hours a day; most of the time it is opened to one or two continents during 4 to 8 hours, but rarely to all five at a specific time of the day. Check e.g. the DXHeat cluster to get an overview of propagation conditions for each band and the five continents. We will come back later on DX clusters.
You have probably also discovered that from Europe you cannot hear W6 stations, located on the Western U.S. coast, when you hear quite well K1 stations in NJ and sometimes the ones located in the Midwest; at other occasions you can hear ZL and VK on mornings using the gray line (see next page) but with much more difficulties a few hours later; at last when you can hear PY it is very difficult to listen to UA9 stations. The problem is still worse in the lower bands : when bands are wide open up to 10 m, the lower ones, 40 m and below can stay close. In fact this factor is mainly directed by the degree of ionization of the D and E-layers, thus related to the Sun position over the Earth.
Do observe openings in the propagation during a few days in summer. First bands open to UA0, then JA, ZL, VK, UA9, VU, ON, ZS, VY, K1, PY, W6, KH,...). Ask to an amateur using a directional antenna to try calling a JA station when the opening is toward VK, there is few chance that he succeeds. Ask him to call a VK when the Sun is over PY : he will work VK with much difficulties or will fail. But when it is noon in VK he can work him quite easily. So you have a confirmation that openings follow strictly the Sun position over the Earth. So, if your hear some JA, UA9 or VU stations you know now that you will not hear at the same time K1, YV or PY stations.
Then the time of the year affects the amount of light or darkness on the earth (except at equatorial latitudes). In winter as the days are much shorter, wavelenghts above 40 m (3.5 and 1.8 MHz) open more and more earlier and gather most of amateurs. If you can also hear DX stations on upper frequencies (between 21-30 MHz) during winter time and high solar activity, these bands will unfortunately close very soon, following the sunset (at 4:30 or 5 pm local time).
Conversely, in summer while the days lengthen propagation allows to work on shorter wavelengths, all night long if necessary, always during periods of high solar activity. Your working frequency will thus change according to the time and season of the year.
This practice is also used by broadcasters transmitting in HF bands; depending on the time of the year these emitters change their frequencies so that they can reach their audience at best, with a strong signal and more than 90 % of reliability. It is for this reason that engineers from the "Voice of America" radio station developed the VOACAP propagation prediction program in the 90's.
At last, as we introduced above, the possibility to hear or to work a station is determined by the critical frequency of ionospheric layers, and especially by the Maximum Usable Frequency, MUF.