Kingston Amateur Radio Club Articles
Dave Lawrence, VA3ORP
Updated 25 April 2007, 0200 UTC
At the April 2007 Club Meeting, Dave Lawrence, VA3ORP, gave a presentation on Near Vertical Incident Skywave (NVIS) Propagation. This consisted of a review of skywave fundamentals, an examination of characteristics of antennae suitable for NVIS, a demonstration of three propagation prediction programs suitable for NVIS and a summary of what could reasonably be expected from NVIS.
(Clicking on the Chart thumbnails or the see chart n below will open the chart up in a new window at its fullsize - Webmaster)
NVIS, like any ionospheric refraction mode, depends on the quality of the ionosphere. The significance of this is that the most suitable frequency to use changes with the time of day, season of the year and phase of the eleven year solar cycle. NVIS propagation can be expected as low as the 160 M band and as high as the 20 M band. In all cases, however, the radiation angle will be high – usually greater than 65 degrees above the horizon. (see chart 1)
The simplest antenna for high angle radiation is a dipole mounted no more than one-quarter (1/4) of a wavelength above the ground. In fact, the antenna can be as low as one-twentieth (1/20) of a wavelength and still give excellent results. The only disadvantage of mounting the antenna close to the ground is that the radiation resistance becomes very low (approximately eight ohms for an 80 M dipole mounted at 5 M - see chart 2). In addition, the bandwidth will be narrow (approximately 15 kHz for 2:1 SWR – see chart 3) . Knowing this, a wideband matching transformer can be used at the feed point and the antenna can be trimmed to match the exact frequency in use. As the antenna is mounted close to the ground, these adjustments are made without difficulty.
Predicting NVIS band openings is easily done by using either tables or computer programs. The New Shortwave Propagation Handbook (Jacobs, Cohen & Rose (Hicksville, NY: CQ Communications, 1995) has excellent tables for short-range propagation. A summary of that information for 50-mile propagation during a solar minimum was presented (see chart 4) .
Also reviewed was the freeware program “W6ELProp” (http://www.qsl.net/w6elprop/ ) and an online service from the Australian Government (http://www.ips.gov.au/HF_Systems/7/1 ). While these are more complex to use than the tables, they provide predictions that are specific to any situation that might be encountered. A very practical procedure has been to examine all three sources and select operating times/frequencies where they all agree. This technique has been very successful for special events organized by the Wireless Set No. 19 Group. It is unlikely that one band will remain open for NVIS throughout an entire 24-hour period. Operators should, therefore be prepared to operate on at least two bands in order to maintain any critical links.
Three points summarized the presentation. First is that NVIS propagation demands high angle radiation, which is easily provided by a dipole antenna, mounted less than ¼ wavelength above ground. Second is that NVIS band openings can be reliably predicted using a number of readily available sources. Finally, if it is important to maintain a particular NVIS link over a 24-hour period, operators should anticipate band changes – lower at night, higher during the day.
The briefing was followed by a demonstration of readily available poles, which would be suitable for temporary installation of an NVIS dipole.
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