HF portable wire antennas Clear Springs Press




Portable Wire Antennas

Backpack Portable Communications

         Backpack portable HF radios have been important to expedition communications, military operations and emergency operations since the invention of radio. This has not changed despite the increasing use of satellite based communications technologies.

         Today, there is an additional interest in using HF communications as a recreational activity. Backpack portable radios are, by necessity, small, lightweight and low power. Effective back country communications often requires the use of the HF bands, especially the lower HF bands. The challenge becomes in designing and building a portable, light weight antenna that is efficient enough to use effectively with the low power radios on the lower frequency HF bands. The lower bands, in general, require large dimensions and higher elevation. While there are a number of antenna kits that are self-supporting, they often come with performance compromises. Shortened dimensions often involve loading coils and lower radiation efficiency.

         An alternative is hanging wire antennas that are full length. Wire antennas are often (but not always) dipoles or some variation of the basic dipole. Some wire antenna designs offer improved gain or multi band performance or both. Wire antennas can be relatively light, compact and easily carried. Even so, deployment is dependent on the availability of supports. Fortunately, NVIS optimized antennas work best when deployed at low heights and NVIS is a preferred mode for many EMCOMM and expedition support functions. Rarely can an antenna be deployed at an optimum DX height of one half wavelength or more on the lower HF bands whether in a fixed or portable location. Performance compromises have to be made. Making the most of what is available includes understanding the tradeoffs and having a variety of antenna choices to apply to a specific situation.

         This presentation minimizes the use of technical language, and concepts. It is intended to be as clear and non-technical as possible while providing sufficient detail to enable anyone to build their own wire antennas.

         We would all like for our portable antennas to be one hundred percent efficient, weigh nothing, be self-supporting and be deployable in seconds.

         A backpack portable station is, by weight requirement, low power. It has to be low power because batteries and the solar panels and other means for charging them are heavy. QRP is generally defined as 5 watts or less. Working with QRP power levels can be very challenging and may not be reliable under some band and traffic conditions. CW and digital modes can be successful at power levels well below that required for SSB phone contacts. Digital modes require more gear which adds more weight, but with modern pads and netbooks, may be practical.

         A number of commercial HF transceivers are small and light enough for portable use. However, pay close attention to the battery drain while in receive and standby mode. The battery packs, chargers, solar panels, etc. add weight. The weight of the power system is the reason that 100 watt transceivers are not carried by many backpackers.

         Military and commercial manpack HF radios generally have a 20 watt transmitter capability. This seems to be the minimum power level that affords generally reliable SSB phone communications.

         Deployment of the antenna is a major consideration with portable operation. The lightest weight antenna with superior performance characteristics is the wire antenna. The challenge is getting it up in the air at an effective height and being able to find suitable deployment sites repeatedly while on the move.

         In this regard, it makes a major difference whether you are interested in DX or regional communications. DX communication requires the maximum height attainable. Regional communications, defined here as 50-500 miles, utilizes NVIS, (Near Vertical Incident Skywave) propagation. The deployment of an antenna for optimum NVIS performance requires a lower height above ground. Heights of 10-15 ft. above ground in the lower HF bands is optimum for a dipole antenna. These heights are much easier to achieve in a variety of terrains on a repeatable basis. Some operators have achieved results with wire antennas suspended only inches or a few feet above ground. There is a tradeoff, however, because close proximity to the ground results in signal loss and lower antenna efficiency, precious commodities with low power transmitters.

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