A friend of mine recently passed her FCC Technician Amateur Radio exam but has been riddled with questions ever since. I’ve been attempting to help but I fear that I haven’t been able to give adequate time to properly answer some "basic" questions that many new hams have. With that in mind, I’m going to attempt to answer some questions in written form so I can refer back to these notes later and can hopefully build on these answers as well.
How do I work DX?
Listening and talking with DX, also defined as far away or foreign stations, is an activity that brings many people into the amateur radio hobby. Although not as popular as it once was, listening to shortwave radio opened up the world to many people and now with an amateur radio license in hand the new ham would like to be able to try to "work the world".
Given the necessary conditions and equipment, talking with someone several hundred or thousand miles away isn’t that difficult. Most of what is needed to complete the QSO is likely already in your head…
Frequencies
Generally speaking, a high frequency (or HF) radio is necessary to work DX. Yes, you can use very high frequencies (VHF) and ultra high frequencies (UHF) with satellites, moon bounce, and other methods for working the world but for now lets just stick with the basics. The FCC allows the following HF frequencies to be utilized by a Technician licensee:
| Band | Spectrum allowance (MHz) | Mode |
|---|---|---|
80m |
3.525 - 3.600 |
CW |
40m |
7.025 - 7.125 |
CW |
15m |
21.025 - 21.200 |
CW |
10m |
28.000 - 28.300 |
CW & Digital |
10m |
28.300 - 28.500 |
SSB, CW, & Image |
Of these bands, it’s generally going to be easier to find and work DX on the higher bands (15m and 10m). That’s not to say that you can’t work foreign stations on 80m and 40m but the antenna requirements at those frequencies are so much more intensive that it makes it much more difficult to get a good signal at a distance.
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Note
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When I was first licensed as KF4OTN the solar cycle was peaking and 10m was very active. Listening to stations from all over Europe and South America was the impetus I needed to buckle down and get my 5 WPM Morse Code exam completed so I could upgrade to a Technician Plus and obtain HF privledges. I worked many stations on 10m all over the world including Australia! |
It’s also important to note that the ionosphere changes every instance of every day over every portion of the earth and, thus, affects what band will support communications to particular parts of the globe at any given time. Generally speaking, one can predict, with some degree of accuracy, which bands are going to be "open" to certain parts of the world at certain times of the day. Propagation tools, like VOACAP, determine the best time and frequency to use when attempting to communicate to a certain place.
The antenna
HF propagation is generally built on the principle of bouncing your signal off the ionosphere. The angle at which your signal hits the ionosphere, the angle of incidence, will predict where your signal will come back to earth. A higher angle of incidence will yield a shorter distance covered while a lower angle or incidence will yield a farther distance covered. Because we want our signal to come back to earth far from where it started, we should design an antenna that will radiate our signal as low to the horizon as possible. The best antenna for this is a vertical antenna as it radiates in all directions at a relatively low angle. Of course there are other antennas that can also do this magic as well.
Vertical antennas
Vertical antennas are generally made of a quarter-wave radiator set vertically with a ground plane of some sort. A quarter-wave on VHF is generally manageable but as the frequency decreases the length increases!
| Band | Length (m) | Length (ft) |
|---|---|---|
10m |
2.5 |
~8.2 |
15m |
3.75 |
~12.3 |
40m |
10 |
32.8 |
80m |
20 |
65.6 |
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Important
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The measurements in the table are made based on the band and not any specific frequency. When building an antenna always design your antennas on expected operating frequencies and not random numbers. It is also important to model your antenna to verify impedence and SWR as tweaking one part of the antenna can often affect other aspects of the antenna. |
There are ways to make the antenna shorter but those can affect efficiency and reduce performance. It is also important to note that the ground plane is what actually "controls" the angle of radiation and affects the efficiency of the antenna greatly. There is lots of literature available both on the web and in libraries and bookstores so I won’t elaborate on this further (for now).
Dipole antennas
Dipole antennas seem to be the defacto standard HF antenna. They are simple to build and erect and work well if installed correctly. At a height of one wavelength, power is concentrated in the 5 to 15 degree lobes making the antenna work well for DX. Lowering the antenna makes the antenna radiate higher making the expected range decrease.
One can see that using a single antenna for multiple bands means the wavelength height is different for each band making the antenna have a different expected range for each band.
Delta Loop antennas
Delta Loop antennas are simple loop antennas that can pack a punch. When setup with a vertical polarization, this antenna will yield the low angle of radiation needed for DX.
Mode
Many times overlooked, mode of operation makes a big difference when trying to work the weak signals that are (generally) DX. While phone (voice) is traditionally the most used mode on the air today, CW (Morse code) and other digital modes offer much better signal-to-noise ratio with the ability of a "small" signal to be heard much further.
How do I work stations that are close to me?
If you’ve read the How to work DX section then you might have caught on to some key ingredients, specifically with the antenna, that make communications possible over a long distance. Of course if you’re not looking to work a station thousands of miles away but rather a few hundred miles away (say, within 300 miles) we’ll need to talk about Near Vertical Incidence Skywave or NVIS. Basically, this is a technique for using an antenna that is close to the ground so the angle of radiation is high. Because the angle is high, the angle of incidence is also high yielding a signal range that is fairly close.
NVIS generally requires lower frequencies to work properly (think 7MHz and down) due to absorption and reflection properties of the ionosphere. If you select a frequency that is too high the RF energy can pass straight through the ionosphere without being reflected at all. Of course selecting a frequency too low will yield absorption of the RF energy by the ionosphere and your signal will also never make it back to Earth. Generally speaking, 40m and 60m is best for daytime use while 60m and 75/80m is best for nighttime use.
So, following what we learned in the How to work DX section, we just need to use an antenna that is bad for DX. Something that radiates up instead of out. If we look at the dipole antenna we see that we should have it elevated to one wavelength above the ground for good DX. Lowering that antenna to almost ground level (an eighth to a quarter wavelength above the ground) makes the radiation pattern go from 5 to 15 degrees to basically one huge lobe that is pointing straight up!
Now you know why many hams have so much wire in their trees! Some antennas are for talking around the corner while others are for talking around the world!
Contesting
Contesting is another question that has come up a few times. There are many amateur radio contests every weekend on the radio sponsored by different organizations like CQ and the ARRL. Each contest has a different set of rules and objectives but in general the operator is trying to work as many eligible stations as possible during the contest period. Some of the more popular contests include the CQ World Wide DX Contest, CQ World Wide VHF Contest, ARRL DX Contest, and the ARRL Sweepstakes just to name a few.
TO BE CONTINUED…