Learning Unit 19

HF Propagation Considerations

Objectives:
Upon completion of this unit, you should be familiar with:

Student Preparation Required:
Review Amateur Radio's HF frequency assignments if you are not familiar with them.

Information:

HF Propagation:
Most short to medium-range HF emcomm operations take place on 40 or 80 meters, depending on the time of day and band conditions. The 40 meter band is generally better during the daylight hours, and 80 meters is best approximately from dusk to a few hours after dawn. Long-range or wide area nets may operate on 10, 15, or 20 meters depending on where the propagation is best at that moment.

When Good Signals Go Bad
One of the toughest challenges for HF emcomm operation is the unpredictability of propagation. One moment 80 meters will be fine, and the next it will "go long". Noise levels can fluctuate, sunspot activity can wipe out all but the strongest signals, and distant stations can rise up out of nowhere in the middle of a busy net. The NCS operator and the Net Manager need to be familiar with these and other problems, and be ready to make changes when conditions change. Casual hobby operating can always be placed on "hold", but emergency nets must be able to function even when conditions are very poor. This phenomenon happens often, and a good net control operator will recognize the problem...AND know what to do next.

The summer season typically presents variable (and sometimes non-existent) band conditions. In the middle of a state or regional net on 40 or 80 meters, the band can suddenly "go long". This happens when the refraction ("F layer") point in the ionosphere suddenly moves further from the earth, or other changes occur as a result of variations in the level of solar activity. Stations in the 0 to 300 mile range may seem to have "disappeared", but stations located at a greater distance are able to maintain at least a fair contact with the net control station.

One option is to ask a distant station to become the NCS, and stations that are close to each other can use the NCS and other distant stations to relay messages. Nets such as the Hurricane Watch Net specifically recruit members from other regions of the country to fill this need.

Another option is to change frequency, usually to a lower band. (e.g. - move from 40 meters to 80 meters). Members of regular nets often know the pre-designated alternate frequency, and after several minutes of futile attempts should try the alternate. The NCS may also ask one or more distant stations to announce that the net is moving to a different frequency. If reception is still possible but poor, you can use a roll call to be sure everyone is aware of the change.

A pre-determined alternate frequency is always a good idea for every net. When 80 meters "goes long" at night, and the interference is unbearable, many times 160M is a good option if stations are equipped with the right antennas. SSB emcomm nets should stay above 1950 KHz to avoid interfering with CW, AM or stations working in a "DX window".

The Operator's Skill Becomes Critical
While skill, courtesy, and discipline are always important for net operations, these qualities become even more critical when band conditions deteriorate. Effective emcomm operators must have a good knowledge of propagation characteristics of the Amateur bands, be able to pick weak signals out of the noise, fine tune antennas, and use their equipment to its fullest potential. Features like DSP and narrower filters can make a huge difference -- so can knowing which antenna to use, and when to use it. If your radio has these features, you should learn to use them well in advance of any emergency.

Antennas For Wide Area Nets
Many wide area net NCS stations use two or more antennas and a switch. A vertical antenna is used for most operations, since it works equally well in all directions. If a signal is weak, the NCS operator can switch to a directional antenna pointed at the station to improve reception for that contact, switching back to the vertical for general net communications.

Net member stations can do the same thing, or simply use a single directional antenna pointed at the station they need to contact. When they are monitoring the net, the antenna is pointed at the NCS. If traffic needs to be passed to or from another station, it is rotated in that direction and then re-pointed at the NCS when done.

NVIS
Any experienced DXer knows how to get his signal to Europe or Asia, but what about getting it just over the next hill? This is the challenge faced by HF emcomm stations. With the exception of wide area hurricane nets, most emcomm nets only need to communicate within a few hundred miles at most, and often much less. For a low-angle DX antenna, this is in the infamous "skip zone".

In some cases, you can reach each other using "groundwave" propagation, but when hills block the signals or the distance is too great, this will not work. The sole advantage to groundwave communication is that vertical mobile antennas can be used for short distances.

The solution is the Near Vertical Incident Skywave (NVIS) antenna, sometimes known as a "cloud warmer" to DXers. A NVIS (pronounced "nevis") antenna is simply one that radiates much of its signal straight up. The signals bounces off the ionosphere straight back down, resulting in an approximately 400-mile circle of coverage.

Various military services around the world have used NVIS techniques for battlefield HF communication for years, although the US military has only recently begun to use it widely. Amateur emcomm operators have used this technique for many years, but it has only recently become widely known.

The technique is simple - hang a dipole at less than ¼ wavelength above the ground. According to US Army studies, 1/8th wavelength is the optimum height. For 80 meters, this is about 30', and for 40 meters it is about 15'. That said, the height is not super-critical, as long as it is below ¼ wavelength. Antennas have been laid on the ground or in the bushes with usable results. The Army even has antennas that are buried two feet under the sand! However, antennas that are very close to (on in) the ground will be less efficient than ones hung at 1/8th wavelength.

You will probably need a good antenna tuner. Antennas that are resonant at ¼ wavelength above the ground will have a higher SWR (standing wave ratio) when hung close to the ground. If ground conductivity is poor (dry sand, for instance), try running a counterpoise wire on the ground directly below the antenna. The counterpoise is actually a good idea with any NVIS installation, since it improves the system's overall efficiency. The counterpoise should be slightly longer than the antenna. Unbalanced antennas should have a balun installed at the feedpoint to prevent the coaxial cable from radiating.

NVIS works best at lower frequencies, such as the 40, 80, and 160-meter Amateur bands. Always use the highest frequency possible, staying just below the NVIS "maximum usable frequency" (MUF), the frequency above which signals will pass through or be absorbed by the ionosphere. This is also known as the "critical frequency". The easiest way to calculate the MUF is with readily available propagation software. Enter the "solar flux index" (a measure of solar activity), your location, and the location of any station near you. The solar flux index (SFI) can be obtained from WWV at 18 minutes past the hour and on many Internet sites including www.qsl.net. With this information, the software then calculates the NVIS-MUF for that day. As a rough rule of thumb, the lower the index is, the lower the MUF will be. If the SFI is below 100, 80 meters may be the best choice. If it is above 100, 40 meters may be best.

One potential problem with NVIS operation is "groundwave interference". If two stations are close enough to hear each other's groundwave signal, multipath interference can cause significant distortion. To reduce this effect, both stations should keep their antennas as low as possible, and point the ends of the antennas at each other to minimize groundwave radiation in that direction.

(Editor's note: As of this writing, the Amateur Radio Service has been granted five 60 meter "channels". The 5 megahertz band is generally very stable for NVIS operations at all hours, and has long been a favorite of commercial and government users. While it may be ideal for emcomm operations, it will take close coordination to utilize such a limited resource effectively.)

NVIS Antennas That Work
The horizontal dipole, inverted V, and V are the mainstays of NVIS systems. At NVIS heights, a V or inverted V will perform the same as a dipole at a slightly lower height. Not only do these antennas perform well, they are easy to carry and simple to erect. If ground-wave interference to nearby stations is likely to be a problem, keep antennas as flat as possible.

You can also use random wire antennas and inverted Ls, but remember to run out a counterpoise wire along the ground to avoid RF burns from your equipment! Place the counterpoise on the ground directly below the radiating element. Avoid stringing long-wire antennas with any significant vertical radiating sections to keep ground-wave propagation to a minimum.

Other proven NVIS antennas include the Shirley dipole and the Patterson Loop (used by the military -- see the Farmer & Fiedler NVIS book listed in Reference Links), but these are more complex to build and install. Some Amateurs have experimented with horizontal fiberglass loaded-whip dipoles (such as those from Hamstick, Iron Horse, and Valor), although they are far less efficient than full-size dipoles. In an electrically noisy environment, they do not have enough gain to work well.

Reference Links:

The ARRL Antenna Book -- 2000-2002 18th Edition -- American Radio Relay League

ARRL Antenna Compendium, Volume 7 -- 2002, American Radio Relay League

Simple and Fun Antennas for Hams -- Hutchinson and Straw -- 2002, ARRL

NVIS Communication: Theory, Techniques and Validation -- (Farmer & Fiedler) 1996 WorldRadio Books

NVIS Antenna Fundamentals -- Maj. Edward J. Farmer, Army Communicator Magazine Fall 1994

The New Propagation Handbook -- Jacobs, CQ Communications1997 (available from ARRL)

MiniProp and W6ELProp are available for free downloading on many Internet sites.

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