Designing an antenna?  Use the following formulas to determine total length of a dipole, or the length of each radiator:

234 ÷ target frequency = each dipole side length in feet for 1/2 wave antenna, or use 468 for total length of a 1/2 wave antenna.  If you have the space, you can use 936 and determine the length of a full wave dipole.  But a warning, an 80 meter full-wave antenna can be over 250 feet long, and if you'd like to try 160 meters, twice that.

Example:  Let's say you wanted to build a dipole to operate on 7.200 MHz.  The math:

• 468 ÷ 7.200 (MHz) = 65 feet total for both sides - 1/2 wave dipole

• 234 ÷ 7.200 (MHz) = 32.5 feet each side - 1/2 wave dipole

• 936 ÷ 7.200 (MHz) = 130 feet for a full-wave dipole

These measurements are approximate.  Generally there is some amount of adjusting necessary to bring an antenna into resonance.  It's best to cut your antenna a little longer, then make adjustments, as necessary.  ARRL publishes an antenna book to give you all of the information you'll need to design and build your own antenna, plus there's a plethora of information available on the Internet. 

Bear in mind that these formulas are for a dipole or long wire type antennas. There are many other designs that could be much longer or way shorter. Designing, building, and experimenting are the basics of ham radio.  If you'd rather just buy off the shelf, that's fine, too.  But there's a whole world of antenna design out there waiting for you to discover.

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