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Preparing for Portable NVIS Operation
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Emergency communication groups should create and test an “NVIS kit” which contains a sturdy NVIS antenna, feedline, tuner, and sundry tools, hardware, and accessories. The radio should be a small, “all-band” rig like the Icom IC-706, powered by a deep-cycle battery. Hardware should include a 20-30 foot telescopic pole, #18 nylon line, stakes, throwing weights, hammer and nails, extra feedline and connectors, etc. Tools should include the usual electronics hand tools, including a small butane soldering torch and extra butane fuel. Accessories should include a folding table and chair, a rain tarp with it’s lines, and an ice chest with food and drink. Another piece of hardware worth having is the notebook computer, with appropriate software and cables, that may be used to provide radio teletype traffic. The station should also include a 2-meter transceiver and antenna, and a scanner.
The entire station may be packed in a medium-sized ice chest, using custom-cut foam rubber for the sensitive parts. The serious portable operator will also have a tent and various other camping equipment and supplies. Some clubs even purchase and equip a small travel trailer for this purpose. This is the best solution, since the trailer will contain all the needed equipment and supplies, at the ready, and will also provide a measure of security and protection from the elements.
Don’t have just one NVIS antenna. Have one at home (a dual-dipole, or multiband nonresonant, or a loop), and have another for fast portable deployment. The portable antenna and its feedline may be rolled up on an extension cord reel. You never know when you may be needed to quickly deploy a portable station. The goal should be to prepare to provide reliable regional tactical communications services without power mains, in the midst of large-scale emergency events. It’s also a good idea to have the radio “clipped” so that it may be operated outside the ham bands by emergency officials who are authorized to do so. The station will usually need to be located at the incident command post -- however, it is very important to make prior arrangements with the authorities.
Tuners: The best tuner for barefoot NVIS is probably something like the MFJ 949E. It has a wide tuning range, internal balun with balanced output, three-position antenna switch, internal dummy load, and a large cross-needle meter. Of course, full power tuners must be used with linear amplifiers. Autocouplers by SGC and others work very well at the feedpoint, provided the impedance isn’t too low. The internal autotuners in most radios usually do not have sufficient range to match low antennas on 160. The 75/40 dual-dipole described above does not need a tuner, as the elements may easily be adjusted to resonance. If the SWR at resonance is still too high, raise the antenna a few feet, because the feedpoint radiation resistance is probably too low. Modeling over average ground shows a feedpoint resistance of 50 ohms at around 41 feet high. Short stubs with alligator clips may be clipped onto the elements at various places to provide multiple resonant points, and if bare wire is used for the antenna elements, these may be moved around to match the antenna (don’t burn your fingers).
Power Supplies: The portable station should use a deep-cycle marine battery and a portable generator. Small “camping” generators in the 900 to 1800-watt range, having both 13.8 VDC and 120 VAC outputs, are the most preferable. Connections to batteries should be made using ring lugs soldered to the wire, attached to the battery with stainless steel bolts, washers, and wing nuts. All connections should be greased. The battery should be connected to a power distribution box, of the type with several sets of 5-way binding posts.