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Wednesday, February 4, 2009

Building a Low-Cost, Low-Profile Shortwave Dipole Antenna, by Jerry the Generator Guy

First, you decided to get your own shortwave receiver. You wanted to be able to listen to unfiltered worldwide news. Applause, and a pat on the back, for taking a positive step. However, an unexpected problem may soon surface. Any internal ferrite or wand/rod antenna, like what the radio came with, will only effectively receive strong signals. Unfortunately, it can’t do a good job on weak signals.

The obvious solution is to add an external antenna. But it may be spotted by the neighborhood or local “whiners” may complain that your obnoxious visible antenna is interfering with their television or radio reception. The fact that you are only receiving won’t stop their perception that it’s your fault. A second issue is that the “typical" outdoor antenna may not survive severe weather. It may fail in high wind/snow/ice.

Another negative is that any antenna wire in the wind will pick up static charges when dust hits the antenna. This dust hitting the antenna is what causes the “pop” sound in the audio during a storm. This electrostatic discharge (ESD) travels down the lead in wire and may weaken or damage the front end [electronic section] of the receiver. If you have an outside antenna a good antenna discharge unit is strongly recommended.

Is there a satisfactory solution for these problems? Yes! First determine what lengths of wire would be needed for a tuned dipole antenna to receive each desired frequency. Many Ham or Shortwave books either tell you how to calculate the desired dipole wire length or provide a suggested length data table for you. If you are fortunate the manual with the receiver may provide these parameters.

My low cost recommended solution follows:
I bought some 4 -conductor telephone cable and some 50 ohm TV coax cable at the local Home Depot. The 50 ohm cable is routed from the receiver to the center of the antenna. Cut the telephone wire at the center of the total length. Strip the insulation back slightly on all of the center wires. Solder [using electrical - not plumber type solder] the center conductor to one of the wire groups. Solder the coax shield to the other set of wires. Measure the desired distance from the center to the desired endpoint for a specific dipole. Carefully slit the outer cover of the phone cable at that location. Cut and remove the balance of an individual colored wire. Cutting the dipole for the lowest frequency first [ longest length ] will make removing the extra wire lengths easier. Measure, cut and repeat the same steps at the other side of the antenna. Note: Some books will suggest reducing the length of the antenna wire elements by 5%. This reduction is to compensate for the “close” distance to the other dipoles. Precise tune lengths are needed for transmitting but may not be necessary if the antenna is used for an entire shortwave band. The generic “rule of thumb” for most receiving antennas is the more wire available for signal pickup the better. Repeat this process for the other three wires. Cover the soldered connections with electrical tape. Fasten the antenna in a straight line along the cornice or eave of the house. Paint or stain to match the color nearby and it looks like it has been there forever.

If four tuned lengths aren’t enough - then the same approach could be done with 8-conductor unshielded computer network cable.

You now have a good antenna to pick up those weaker signals. In addition, the house now protects the antenna from any severe weather effects. If a nosey “snooper” comes by all that they will see is a “telephone” wire.


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