NVIS and ALE: Some Preliminary Studies

L. B. Cebik, W4RNL (SK)


In 2005, I conducted some preliminary analyses of antennas that are used and some antennas that might be used for combined NVIS-ALE work. Although the studies are far short of definitive--suggestive or indicative might be better terms for them--the papers might be useful to those traveling the same exploratory routes. Therefore, the entries below lead to three items in PDF format (for ease of transfer to the site).

NVIS, of course, stands for Near Vertical Incidence Skywave. Often casually called cloud-burning, the idea is simple: to direct a radio signal upward and have it refracted/reflected downward for relative short-range communications in the upper MF and the lower HF region, where the ionosphere supports such operation. Although amateurs tend to focus on 80 and 40 meters for NVIS efforts, other communications services may use frequencies between 2 and about 10 MHz. The range depends as much on antenna design as it does on propagation. Hence, the antenna beamwidth upward is a major concern. As shown in other notes on NVIS-related subjects, the more gain provided by an array, the narrower the beamwidth becomes. Hence, the ideal NVIS antenna for a given need strikes a balance between appropriate gain and a beamwidth that allows coverage of the target area. For narrow-band antennas, such as those used by amateurs on 80 and 40 meters, any number of practical antennas exist, despite the tendency of many NVIS aficionados to use inadequate government surplus antennas. The antenna design problem becomes truly interesting when we add operating bandwidth to the equation. How can we cover 2 through 10 MHz with adequate NVIS patterns throughout the operating range?

Now let's add in a second operating desire: ALE. ALE stands for Automatic Link Establishment and is the method developed over the last 10 years or so to improve HF communications and make it more usable by untrained operators. It has two main benefits: 1) it automatically selects the best frequency out of many, and 2) it takes advantage of short-term ionospheric conditions that can improve connectivity by as much as 20 dB with respect to conventional HF predictions.

The ALE system begins with "sounding list" or "scan list" for a communications net. A receiver then operates in the scan mode, sequentially listening for a call on a list of perhaps 10 or 20 frequencies. The CPUs are can detect calls made to their particular address, or a general call can be sent to many receivers. If one or more signals are received at a desired station, the computer there performs a link quality analysis (LQA) by computing the relative "score" on each channel, usually bit error rate. The receiving station then answers on the channel with the highest LQA score. The initial station then acknowledges the link-up and begins to send the message. All this is done with push-to-talk techniques as usual, although some protocols are now available for automatic data transmissions. But most of the applications are for SSB Voice.

ALE (originally called "adaptive HF") evolved to a second generation, which is now fielded throughout the military. A most interesting set of developments includes the expansion of ALE techniques to amateur radio operations and to various Homeland Security participants, such as the Civil Air Patrol. The FAA is also interested, especially in locations like Alaska, where downed aircraft needing location and rescue are common.

Third-generation radios have very accurate frequency standards and may thus be synchronized, so the receiving station that may be in a covert situation need not engage in the LQA handshaking. In that case, the receiver simply answers on the best frequency, having performed it's own LQA, and the distant transmitter, having reverted to it's own scanning mode, receives the message, which is usually a short burst. Most of these radios work in the data mode, and send e-mail-like traffic.

ALE places high demands on the radio equipment itself. However, successful ALE operations are equally dependent upon antennas that can support this mode. ALE places a high emphasis on NVIS techniques, since much of the communication is within a radius of 500 miles. Hence, there are needs for special antennas having a broad operating bandwidth and a very high radiation angle. Among the most popular field antennas are terminated folded dipoles and related antennas. Terminated antennas achieve the requisite broad SWR passband via the terminating resistance, but, of course, suffer considerable loss relative to a similar antenna with no termination. Among the unterminated antenna types available commercially are fan dipoles, vertically oriented LPDAs and vertically oriented LP loops. In general, the unterminated antenna types are far more complicated--and expensive--than the simpler terminated antennas.

The combination of NVIS and ALE is presently defined by essentially military needs and resultant communications standards. It is not clear at the moment of writing whether these standards will or should apply to non-military work, such as emergency services. In some links, they make more sense; in others they make less. For example, transfer of a heavy emergency traffic load from a devastated hurricane site to a relatively near unaffected station might make good use of burst transmission technology with high frequency agility. However, if we are trying to communicate with a downed aircraft in the wilderness, a slower-paced form of ALE seems in order. As we reduce the pace of frequency sampling and selection, we gain the ability to perform many mechanical operations between hand-shaking trials, such as switching antennas or components in a matching circuit. The ability to work at a slower pace also opens many possibilities for antennas that may be more effective that the low-gain very-broadband terminated systems in present use.

The following preliminary studies follow both lines of thought: single broadband antennas and collections of tuned antennas requiring switching of one sort or another. The field is wide open beyond these two more obvious paths, so these notes are simply a start along some lines of thought.

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