LP AM VERTICAL ANTENNA FOR AM88
Figure of Vert Antenna

The above figure shows the basic construction of a vertical antenna. A radiator of typically 3 meters or 10 feet is mounted on a suitable post in the ground. This may be a length of 1/2 inch copper water pipe, a CB whip antenna, aluminum tubing, or a length of #12 or #10 copper wire enclosed in a piece of PVC pipe for mechanical support. A loading coil is connected between the radiator and the lead to the transmitter RF output. Note that the loading coil and base of the antenna must be well insulated from the wood post and should be within 1-2 feet of the soil level. Use ceramic or plastic standoff insulators. Several wire radials (six shown) are laid on the ground, and can be buried a few centimeters in the ground to avoid tripping or damage from the lawn mower. Soil can be slit with a small shovel or edger and the wire placed in the slit. It does not have to be insulated, any wire 20 gauge or larger, copper or aluminum can be used. Electric fence wire is usable and is very cheap. Alternatively, a grounding mat made of chicken wire or hardware cloth can be used. Two lengths of 25 feet (8 meters) in a crossed configuration will often work well. Ground rods of 2.5 to 3 meters or longer, or a well casing, may be used as a ground instead, but may prove relatively ineffective. The radials should be at least 8 meters (25 feet) long or longer, with least six, but as many as possible used for best results. Radials and any other grounding material must be bonded together at the base of the mounting post and a short length of wire used to connect the ground system to the shield of the coaxial cable feeder (RG58 or RG59 coax will do) from the transmitter. With shorter radials (less than 15 meters or 50 ft ) it helps to use as many as possible. The object is to provide as good a ground as possible, with as much metal under the antenna as you can. A vertical can also be mounted on a roof if the roof is metal or radials are used. It is a good idea to install a short (5 foot) ground rod at the end of each radial. Exact lengths are not critical, use as long lengths as you can

Since a 10 foot antenna is electrically very short at 1650 kHz, a loading coil must be placed in series with the base of the antenna. A suitable loading coil is shown below, made from commonly available hardware items. The coil winding is approximately 440 microhenries (uHy) inductance. This will be sufficient in most cases for radiators made from 12 gauge wire or those made from CB whips. Larger diameter radiators will require fewer turns than small diameter ones. A typical coil can be made from a 6 inch length of 2 inch (5cm) PVC plastic pipe. The winding consists of 121 turns of #22 enamelled wire closewound on the pipe. A 30 meter length or 100 foot spool will be enough to make this coil. It is a good idea to add 5% extra turns to allow for adjustment of inductance. Clear lacquer should be used to hold the turns in place. This pipe has approximately 2.375 inches (6 cm) outside diameter. Acrylic plastic or fiberglass, phenolic, or other low loss tubing can also be used. Place the tubing in your microwave oven for a minute and to see if it is low loss. After one minute it does not get hot, it is OK to use. The exact number of turns and inductance of the coil will depend on external factors and surroundings to some degree, as well as the mechanical dimensions of the radiator, and must be adjusted after installation. The center conductor of the coax cable is connected to the lower end of the coil and the radiator to the upper end. The shield is connected to the ground system. It is a good idea to enclose the coil and coaxial cable connectionsin an enclosure to weatherproof it. A plastic freezer container from your local supermarket works well for this.

Loading Coil Diagram
Adjustment consists of setting the transmitter on the frequency to be used (1650 kHz, etc) and adjusting the loading coil inductance for maximum radiated signal. An assistant with a receiver at 1000 feet from the antenna, and a pair of handie talkies or cell phones, is very handy for this. At first, try spreading a few turns away from the rest of the coil to see if more or less inductance is needed. Now get away from antenna. If the signal improves at 1000 feet, slowly spread and remove turns one at a time and repeat this test until no further improvement in signal level occurs. If signal gets worse, lengthen radiator (No more than 3 meters) or add more turns and try again. Use clear lacquer to cement turns in place. Do not shorten radiator, remove coil turns instead. Be sure to stay well away from antenna when making signal level comparisons as your body can detune the antenna. This procedure takes time and patience, but will greatly improve range when properly done.
 
 
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