HF Operations: The Difference with HF
For many newer hams the world of HF operations may seem a strange and mysterious place. There are no comforting predefined voice channels, the audio is often noisy and the signal strength quite variable, sometimes multiple stations overlap in a confusing manner, and there are cryptic sub-bands providing bizarre and varied digital sounds. And that’s just to name a few of the many HF quirks.
If some of those things confuse you and make you shy away from trying HF operations, this article is for you. Let’s unfold some of this HF messiness and simplify that which is different with HF.
No Channels? For the typical Technician Class licensee who has mostly conducted VHF and UHF voice operations on FM repeaters and simplex channels, the absence of defined channels on the HF bands is perhaps one of the most stark differences initially confronted. On an HF band an operator may tune to any carrier frequency that keeps transmitted signals within the band privileges of the operator’s license class, usually down to a tuning resolution of as little as 10 hertz steps. Contiguous tuning, free of restrictive channelization.
This contiguous tuning allowance across a band, combined with multiple possible operational modes, may seem chaotic at first and you may ask, “How do operators find one another and have a QSO via a common operating mode?” There are a couple of different considerations in answer to this question. Let’s start with sub-bands, and then consider the matter of contiguous tuning within sub-bands.
Sub-bands: The HF bands from 10 meters down to 80 meters are divided into sub-bands (with exceptions of 30 meters and 60 meters that each have special rules in lieu of sub-bands). An HF band’s sub-band may be designated for only phone operations or for only digital data operations, including CW. So, within a sub-band operators can begin to match up their desired operating modes.
In an HF phone sub-band the overwhelming mode of choice among hams is single sideband (SSB). Full bandwidth AM is also used occasionally, but SSB tends to dominate for reasons of bandwidth efficiency. With the exception of a segment of the 10 meter band, FM phone is prohibited on the HF bands due to its large consumption of bandwidth.
So, operators seeking voice contacts will tune within the phone sub-band and use either the upper sideband or the lower sideband SSB mode, and there is a standard convention for which to use depending upon the band in use. The higher frequency bands above the 30 meter band (20m, 17m, 15m, 12m, 10m, VHF, and UHF bands) utilize the upper sideband (USB), while the lower frequency bands below 30 meters (40m, 80m, 160m) use the lower sideband (LSB).
Similarly, an operator seeking contacts by CW or a digital mode will tune into the CW or data sub-band, avoiding cross-mode interference with those jawing by phone mode. Each band tends to have even finer subdivisions of spectrum within which the various digital modes will congregate. For instance, CW ops tend to group in the very lowest frequency portion of a data sub-band, while computer-generated digital signalers will group by specific mode in slices of spectrum a little higher in frequency, but still within the data sub-band. So, you’ll find the RTTY signals near one another, the PSK31 users grouped, the FT-8 or JT65 operators in a gang, and so on. Further, each digital mode and CW have unique characteristic sounds that allow receiving operators to identify like kinds.
Contiguous Tuning: Let’s assume you like to talk. With your voice. Using words. And because of that eccentric penchant you seek phone contacts on an HF band. Let’s imagine you have a multimode transceiver that operates on the 15 meter band. What do you do to snag another enthusiast who has the same communication whims regarding real, actual, talking?
First, of course, you identify and tune within the phone sub-band of the 15 meter HF band. Referencing our handy band plan chart compliments of ARRL you can see that the 15 meter phone band extends from 21.275 MHz to 21.450 MHz for a General Class licensee.
Assuming you are a SSB kind of person you also ensure that your transceiver is set to upper sideband mode. Then, with grace and precision you twirl your mighty VFO across the vast contiguous range of frequencies for which you are privileged to operate!
And perhaps you hear other QSOs ongoing. Dialing further you may hear a lonely call of “CQ, CQ, CQ…” To answer the CQ call you simply match up your tuned carrier value with the calling operator’s frequency, judging the tuned value by the quality of the sound received. When you have a nicely received signal you’re ready to respond by simply transmitting your call sign and awaiting acknowledgement by the other station. You may want to use ITU standard phonetics to issue your call sign for weak signal reasons we will explore shortly.
Alternatively, you may wish to make your own CQ call on the band, casting your hook into the RF sea. Before you do, you should take some additional actions before you push-to-talk, as follows.
Selecting a Frequency: Be sure to do the following before you start your CQ call or call to another specific station on an an HF band.
• Keep it in the sub-band – Make sure your entire transmit bandwidth will be within the sub-band for which you have frequency privileges. Remember, a SSB signal consumes about 3 kHz: It will extend 3 kHz above the USB carrier frequency to which you have tuned, or 3 kHz below the LSB carrier frequency. Be sure the entire extent of that USB or LSB signal will be within the phone sub-band for your license class. In our 15 meter band example, you would not want to tune above 21.447 MHz or part of your transmit band may exceed the sub-band’s limits. Similarly, you would not want to tune below 21.275 MHz with a General Class license or you will be transmitting in the Advanced Class and Extra Class exclusive portion of the 15 meter band.
• Listen first – And listen for a good long while. Several dozen seconds is a good idea. Why? Because you want to know if the frequency you have selected is already in use by other operators. If so, you should move on down the dial and try again. And why wait so long? Because you may be able to receive only one side of a QSO, and the unheard station may be transmitting a long description of his recent harrowing antenna-erecting adventure.
• Ask if the frequency is in use – If you hear no other user on the frequency, simply transmit, “Is the frequency in use?” followed by your call sign. Other users will usually respond politely if they are indeed using the frequency.
• CQ! – If you have followed these steps and the frequency is clear, make your CQ call. Usually when calling CQ you will want to elaborate and transmit for several seconds to allow other operators dialing around the band to stumble across your signal. A typical CQ call may go something like this: “CQ CQ CQ. CQ 15 meters. Whiskey Zero Sierra Tango Uniform calling CQ. Whiskey Zero Sierra Tango Uniform calling CQ and standing by.”
Those Messy, Overlapping, Irregular Weak Signals: Another huge difference with HF operation that most VHF/UHF FM hams will immediately note is the quality of the audio. Compared to typical FM phone audio the SSB audio heard on the HF bands can seem substantially degraded. Multiple factors contribute to the reduced fidelity of SSB phone audio.
Bandwidth: The quality of transmitted audio is commensurate with signal bandwidth. A relatively narrow bandwidth 3 kHz SSB signal will not have the same quality and clarity you may be accustomed to on FM repeaters or simplex channels.
QRN: Amplitude modulated signals like SSB are subject to naturally produced noise, such as that from lightning strikes, as well as other electrical noise. FM signals are much less susceptible to such noise sources.
QRM: Interference or noise from other operators can frequently degrade the signal you desire to receive. Given two or more SSB signals that are each about 3 kHz wide, any amount of partial overlap of those signals is feasible. For example, suppose you are tuned to 21.400 MHz conducting a QSO and your USB signal extends up to 21.403 MHz. Another station other than your contact that you can receive is tuned to 21.402 MHz. Your QSO signals will overlap with the other “offending station” signal by 1 kHz, from 21.402 to 21.403. You may hear the offending signal as garbled audio that interferes with your signal reception.
And remember, to that other QSO you are the “offending station.” What do you do? Initially, make sure you have sufficient clearance on the band when selecting a frequency. Once established and a conflict occurs with another QSO or operator, you can either move along the band to find a clear 3+ kHz region or you may politely inform the offending station of his interference and ask if he would please move away a bit. Usually whoever has first established a QSO on a given frequency is yielded to by subsequently transmitting stations, but no operator has sole rights to any frequency. Good amateur practice calls for integrity, politeness, good will, and patience on the air with our fellow hams.
Weak Signals: The frequencies of the HF bands will propagate over the horizon great distances thanks to the ionosphere’s effects. This contrasts with the local RF “line-of-sight” range of UHF and VHF signals. As signals propagate they spread out geometrically, with effective power falling off as the square of the propagation distance (inverse square law). In other words, by the time your 100 watt signal reaches Sydney, Zurich, or Buenos Aires it will be many orders of magnitude weaker than when it left your antenna. It will be very weak, making reception a bit more of a challenge than picking up the megaphone signal from your local repeater, and especially so when much stronger local signals are nearby on the band. This is why high quality receivers with low noise floors and good selectivity are desired by the serious ham.
Irregular Signals: Because HF signals are refracted from the ionosphere, the characteristics of the ionosphere impact the quality of the signal and its propagation. The ionosphere is not a smooth and consistently behaving atmospheric phenomenon. Rather, it is patchy, with higher density of ions in some regions than in others, and changing constantly. As a result of changing atmospheric conditions you may experience irregular signals. A signal that was strong just a moment ago may fade away to nothing in only a few seconds. A station that you could not hear when you began your QSO may seem to suddenly impose itself on your conversation. Fading and irregular signals have their own Q signal, QSB. When QSB happens, refer to that part above about politeness, good will, and patience on the air. It’s just part of the game and part of the magic!
Filtering Signals: Another aspect of radio science that is more common with modes other than FM, and quite frequently employed with SSB or CW on the HF bands, is signal filtering. The whole point of filtering received signals is to avoid nearby interference on the band and to reduce noise (increase signal-to-noise ratio) so that the desired weak signals may be received. Many modern transceivers employ digital signal processing (DSP) that offers great flexibility of filtering options. You may want to establish any of the following types of basic filters on a received signal, and DSP lets you do it with ease:
• Band Pass Filter – Remember that your signal has a specific bandwidth. It is good to match the receiver bandwidth to the bandwidth of the desired signal to obtain the best signal-to-noise ratio. For instance, a SSB signal is about 3 kHz wide. You can establish a 3 kHz receive bandwidth, or you can establish a band pass filter that is somewhat narrower (2.4 kHz is a good selection) in order to filter out interfering signals immediately adjacent to the desired signal on the band. With the adjacent interference filtered away your receiver can better detect the desired signal on which the filter band is positioned.
• Notch Filter – A notch filter is a very narrow filter that can remove an interfering carrier or other narrow bandwidth interference while leaving the bulk of the desired receive signal intact. It is sort of the inverse of the band pass filter in that it eliminates a narrow range of frequencies rather than allowing a specified band to be received.
• Digital Noise Reduction – Various types of noise can be reduced by digitally processing a received band of signals. Most DSP systems provide multiple digital filter types for noise elimination, sometimes using sophisticated algorithms that are available to you with a simple press of a key. Digital noise reduction techniques can improve the intelligibility of another station’s audio under poor conditions.
You can learn more about digital signal processing in our DSP article.
Even More Different: That summarizes some of the most salient differences you’ll find with HF operations , but there’s even more differences to be aware of. Here is a brief list of some additional topics you may want to explore.
• Phonetic Alphabet – Phonetics are used more extensively in HF operations due to the poorer audio characteristics of SSB. Learn and use the ITU standard phonetic alphabet. Hear audio of ITU phonetics use in Technician Learning, Section 2.3.
• DX Contacts (out of country contacts) – Talking to other countries has some unique operational considerations. See Ham Radio School General License Course book, section 1.3, DX and Details.
• Q-Signals – Q-signals tend to be used more on the HF bands. Study up on the Qs in our list in Technician Learning, Section 1.2.
• Split Mode – Operating in split mode is common with HF operation, where transmit and receive frequencies are separated on the band. Reference your transceiver manual for how to establish split mode on your radio, and see how this is typically used in HF operations in our Split Mode Operations article.
• Solar and Atmospheric Conditions – The sun’s behavior and resulting atmospheric effects have enormous impacts on HF signal propagation and you should understand those relationships to better operate with over-the-horizon skip. See HamRadioSchool.com General License Course book chapter 3, Propagation. Also reference our Technician Learning, Section 5.0 and General Learning, Section 3.0.
Getting accustomed to the different characteristics and operations on the HF bands just takes a little time and experience, but don’t be shy! With privileges earned, jump onto the HF bands and start listening to others, and then dive right in. You’ll find many new friends who are happy to help you get up to speed in HF’s strange new world.
-- Stu WØSTU