HF Communications Simplified

Written By Robert Thibault

In the modern world of mobile technology and digital communications, it may seem odd to reference older mediums such as HF 1.6-30MHz (High Frequency) as being reliable. With the technology now coming up on 130 years of continuing development and innovation, the facts are clear, HF communication has stood the test of time. It’s not overly complicated especially with todays tools: ALE radio, frequency projection software tools and good antennas.

Benefits of HF Systems

To this day, HF offers communications ability unavailable with other technologies:

  • These systems are reliable, offering quality communications in remote areas or locations where conventional communications infrastructure has been compromised or doesn’t exist.

  • HF technology is a cost-effective solution more so than other options such as mobile networks, SATCOM, or Microwave systems.

  • High-quality HF systems are designed with interoperability in mind, enabling uses to interconnect them with other HF radio equipment, mobile devices, or telephone networks to provide an all-encompassing communications solution.

“HF is similar in some ways to SATCOM, where a signal is aimed up toward the sky and then bounced back down, HF if done correctly is probably about 85% reliable. Nothing is 100% be it HF, SATCOM, VHF/UHF/ L-Band / Microwave / Cellphones.   

HF sends a signal up toward the sky which then bounces off (hopefully) one of four ‘ion bands” that are like 4 huge mirrors encircling the earth and which are agitated by various things, primarily sunspot activity, that do or don’t reflect the HF signals. The rings are known as D, E, F1 and F2 and come and go somewhat at will. They are relatively predictable with certain tools: VOACAP (Voice of America Coverage Analysis Program or HamCAP (HF propagation prediction software) but not always guaranteed just more probable. The ion bands also go through 11-year cycles; thus, HF gets better or worse over a 11-year period (fun fact to know). All the other frequencies (VHF, UHF, Microwave) are “line of sight” (LOS) meaning they have to “see” each other or a tower and have a relatively short range. HF, like SATCOM, is “beyond line of sight” (BLOS) communication. HF signals are sent up into the ionosphere and reflected down, enabling the signals to reach the other side of the mountain (BLOS).

So, visualize a signal sent upwards 100 miles or so and ricocheting back down that’s HF. Like a stone skipping off a lake, but at a sharper angle and that’s the trick (having the right angle). Aim it correctly and you’ll link. So how do you “aim” HF. 

HF done right requires three things: 

 1.     Selecting the Right Frequency

2.     Correctly programmed radio

3.     Correct Antenna Selection and Installations

In very simple terms there are roughly three distances for HF

 1.     Short (NVIS) – 0-500 miles (2-10MHz)

2.     Medium – 400 – 1200 miles (8-18MHz)

3.     Long – 1000 – 2500+ miles (16-30MHz)

 However, HF doesn’t always obey these suggested areas and sometimes you will link 2500 miles on 4 MHz yes that’s odd, but that’s HF.

Let’s talk about the antennas. You must have the right antenna for the job, and that is critical.  Regardless of the radio, if you have the wrong antenna your connection probably isn’t going to work.

There are two very different types of antennas that are prevalent within government and military agencies (Whips and Dipoles). Both essentially radiate signal perpendicular off their side which means that a straight-up whip sends signal out parallel to the ground (but since the signal is a lobe, there is also signal going slightly up say, above distant trees and slightly down into the earth). The ‘slightly up’ signal can travel many miles down range before it reaches one of those ion bands, then it bounces back down to the earth. The problem here is that after it climbs above the trees (maybe 25 to 30 miles out) there is no signal for maybe 100-200 miles until it finally comes back down creating a “skip zone”. Whips work very well for longer distances, but most HF is done within 10 / 200 / 400 miles i.e., NVIS Dipoles.

NVIS Dipoles are essentially a wire going left and one going right.  A “folded dipole” is a wire going out a distance, doing a “U” turn and coming back to center (balun/resistor point). Pattern off a dipole is again perpendicular, but this time straight up (with some going out at angles off the center, thus creating the NVIS pattern for close (0 to 500 miles) communications. Need additional range? Hoist the balun a bit and make the antenna into an upside down “V” which then aims the signals a bit further outward and the bounce-down gets further downrange.

Need to make a long shot a Whip might do it, or you can “aim a Dipole by putting one end up at the top of your mast and slanting the wire down to the ground, balun in the middle, and aiming the wire toward your target. Signal still comes off perpendicular to the wire, but now the wire is “aimed” and tilted toward the target, giving you that long slice shot directly toward your goal.

Broadband means working 2-30 without a tuner. Wideband is the newest technology that allows more data to be sent. ALE is the radio automatically picking the frequency. NVIS means nearly straight up /down signal. So, study the range and pick the right frequencies for the job, program the radios before going to the field (and crosscheck them hopefully with a dummy load), and select the right antenna (set it up correctly) for the job. Turn your ALE loose and let it seek the right linking frequency and you should be good to go.”

 Authors: Bill Whittington / Kurt Stephens- White Wolf Systems

Robert Thibault