Antenne d'EMERGENZA
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A BAD ANTENNA IS BETTER THAN NO ANTENNA
"Una CATTIVA antenna è meglio di NESSUNA antenna"
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Antenne per VHF-UHF
Dipolo semplice
Misure per 430MHz: 17 cm x braccio
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Un modo rapido per realizzare un dipolo verticale
utilizzando cavo coassiale, ad es. RG58
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Verticale d'EMERGENZA in MARE
utilizzando cavo coassiale 50 Ohm
https://ea5dy.ure.es/index_12.htm
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Antenna militare USA AS-2259/GR per HF
da usare con l'accordatore
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... A MALI ESTREMI...
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Riferimenti:
• SGC Stealth antenna kit manual
• Radioutilitario
• Antentop.org
• QST
• 73 Magazine
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http://ari.b3net.it/wp-content/uploads/2011/07/EMG_ANT_30062012.pdf
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Field-Expedient Antennas
Poor or erratic radio communications may be the result of excessive distances between stations, unfavorable terrain or weather, or defective antenna equipment. All fire support personnel must understand the application of field-expedient antennas for maintaining or enhancing communications and for electronic counter-countermeasures (ECCM). Regardless of the type of antenna used, proper maintenance must be performed to get optimum performance from the equipment. The field-expedient antennas discussed below are relatively simple, easy to construct from available materials, and highly effective.
Replacement Whip Antenna
In a static position, a broken whip antenna may be replaced by using WD-1 communications wire and an overhead branch or some support assembly. Cut a 10-foot piece of wire, attach an insulator to one end, and use a rope attached to the insulator to elevate the antenna. Strip about 1 inch of insulation from the end to be attached to the radio. Loosen the antenna base on the radio, place the bare wire between the antenna base and the antenna support receptacle, and retighten the antenna base. Ensure the improvised antenna is vertical.
Horizontal Long-Wire Antenna
This is probably the simplest, yet most effective, antenna for communicating over long distances. Maximum radiation is off the ends of this antenna; thus, it is highly directional. It not only increases the range of transmission and reception, but it also tends to reject or reduce signals from other directions. This makes it an excellent antijamming device.
WD-1 is ideal for making this antenna. The wire should be 100 to 150 feet long. Tightly twist the first section of the WD-1, and connect the end between the antenna base and the antenna support receptacle on the radio. The wire must be adequately insulated to prevent accidental grounding. The antenna should be erected at least 7 to 9 feet high at the radio and 15 to 20 feet above ground at the other end. Connect the other end to a pole or a tree in the direction in which communication is required. This ground clearance is necessary to prevent accidents or injuries involving personnel or vehicle traffic.
To make this a one-way (unidirectional) antenna, add a resistor at the end toward the distant station. A dead flashlight battery BA-30 makes an ideal resistor for low-power radios. Attach a nail or screw to each end of the battery, ensuring they don't touch, and connect the wire to each.
Center-Fed Doublet Antenna
The center-fed doublet is an effective two-way (bidirectional) antenna. It is particularly efficient in jungle environments and for ECCM if both the sending and receiving stations are using the same type of antenna. Unlike the whip and many other antennas discussed in this appendix, this antenna is electronically horizontal and will not communicate with those that are electronically vertical.
The length of each element is critical and depends on the operating frequency. This length must be one-quarter wavelength. To determine the length of each element in feet, divide 468 by the frequency in megahertz (MHz) which gives you one-half wavelength. Then divide this result by 2 to get one-quarter wavelength in feet. An example using the operating frequency of 46.80 MHz follows:
468 ÷ 46.80 = 10; 10 ÷ 2 = 5 feet; so each element is 5 feet long.
After determining the length of each element, construct the antenna by measuring off slightly more than the required length of wire and tie a figure-eight knot at that point. Separate the wire into the elements, and attach insulators at each end. Ensure the elements are the exact length required. Tightly twist the remaining wire going to the radio to make a transmission cable, and strip each end of the wire. Put one wire into the center of the antenna cable connector, and attach the other wire to the metal case of the radio. Attach the insulators to the rope to permit erecting the antenna between two trees or other support assemblies. Raise the antenna 20 to 30 feet, and ensure the broadside is directed toward the receiving station(s).
Vertical Half-Rhombic Antenna
The vertical half-rhombic antenna consists of 100 to 150 feet of WD-1 on a 30-to 45-foot-high support. The support should be centered with approximately half of the wire on each side. Attach insulators to the ends, and fasten rope to these insulators. This permits the ends to be tied down to stakes and the antenna element to be insulated from a ground. Make a transmission cable by tightly twisting the section of WD-1 coming from the radio end of the antenna element. Strip the ends of the cable approximately 1 inch, and connect these leads between the antenna base and the antenna support receptacle on the radio. The antenna in this configuration is a two-way (bidirectional) antenna.
To make this a one-way (unidirectional) antenna, add a resistor at the end toward the distant station. A dead flashlight battery BA-30 makes an ideal resistor for low-power radios. Attach a nail or screw to each end of the battery, ensuring they don't touch, and connect the wire to each.
Resistors and Field-Expedient Resistors
Resistors are used to draw the signal in the desired direction of transmission.
Resistors used to construct the long-wire and half-rhombic antennas are readily available through supply channels and local radio repair shops. These resistors must have a resistance of 500 to 600 ohms and be at least half the wattage of the transmitter power output. For example, a 600-ohm, 2-watt resistor works with the AN/PRC-77. Typical power outputs for combat net radios are as follows:
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AN/VRC-12-series (-46, -47, and so forth):
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High power = 35 watts (minimum).
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Low power = 0.5 to 8 watts.
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AN/PRC-77 = 4 watts.
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SINCGARS = 50 watts (maximum).
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NOTE: SINCGARS radios do not perform frequency hopping very well with field-expedient antennas, but any antenna is better than none at all. The use of a field-expedient antenna may degrade the SINCGARS to a single-channel operation, but it will permit communication. |
Field-expedient resistors should be of the same values as those listed above, approximately 500 to 600 ohms at about half the wattage output. A dead BA-30 with nails driven into each end will approximate 500 to 600 ohms at 1 to 3 watts. An earplug container with holes drilled in the case opposite each other and filled with sand and a few drops of crankcase oil will work much like the battery.
Field-Expedient Insulators
Insulators keep the signal from going in an unwanted direction. Almost anything that will not conduct electricity but has some strength can be an insulator. The very best insulators are glass, plastic, and rubber. Less effective but still usable are cloth, wood, and rope; however, these are not good when wet.
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NOTE: The US Army Signal School is in the process of replacing the terms FM (frequency modulated) and AM or AM/SSB (amplitude modulated and single sideband) in most radio net titles with terms more closely denoting range. The following frequency range designations will be used:
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