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We recently found some good information about random wire lengths that you should and should not use. Jack, VE3EED, hopefully has solved a major headache we all have when we attempt to go thru the trial and error and frustration with getting the random wire to work where WE want it to work.
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The graph below shows lengths to avoid for different collections of bands. The fewer bands, the fewer high impedance regions to avoid. You also want the antenna to be at least 1/4 wavelength long for each band you plan to use.
The table below represents half wave lengths and multiples that you DO NOT WANT TO USE! You have to stay away from a half wavelength on any frequency. Therefore, we came up with the following numbers to avoid (IN FEET): These lengths in the table below are the culprits that cause all of the trouble when using random lengths. Frequency MHz 1/2 ...
Calculate the lengths that should be avoided when making "random wire" antennas. The length of a "random wire" antenna has a direct impact on the impedance present at its end for a given frequency.
When on the trail, at Field Day, or whenever a good antenna is unavailable, a random wire can save the day, but it is important to understand its properties. At multiples of half wavelengths of the transmit frequency the impedance is so high that tuners in most rigs often can't match it.
All you need for a random wire antenna is some wire, your tuner, one or more supports up as high as you can get them to string the wire from the supports to the tuner, at least one or two insulators and a little time.
22 kwi 2022 · The length of the wire needs to avoid any (preferably all) of the combinations of half wave lengths and their harmonics on ALL the desired bands of operation. The frustration here is that (as shown in the table) this great list of lengths to avoid tells you little of which lengths to actually use!
