Latest Update: 9/28/16
This is CONCEPTUAL ONLY . . . at this point.
There are many times when an NVIS antenna is quite a bit larger than you would really like. For example an antenna in support of emergency operations, yet needing better performance than you typically get from a pair of mobile whips, yet temporary. With that in mind I have been looking around for a version that would be useful.
The version that I am going to try, but have not been able to allocate the time for, is sometimes called a tri-fold antenna. It is made from 450 Ohm ladder line, and fed with a four to one balun. You cut the ladder line to about 2/3 rds the length of a "normal" leg of a half wave dipole. You strip off enough to connect to the balun. The other end has the two conductors commoned so you have the "normal" folded dipole. BUT you then punch a hole in the center of each of the ladder spreaders, and weave a (I like the idea of insulated) length of #16 wire from an inch or two away from the "commoned" end until you get to the open end, and solder it to the "bottom" leg.
This creates three conductors that maintain equal spacing, and thus do not detune the antenna in the wind. I plan on it being suspended from some of the UV resistant antenna guy line you can get from HRO with the balun tied in a half hitch to keep it from moving and then use some electrical tape to hold the elements to the support line.
One last thought is to use a little bit of fishing mono filament to tie to the lower element and hang some small split shot, once again to keep it from wrapping on itself in the wind.
Should anyone get the time to try this antenna, I would appreciate an E-mail with your results (to be noted in this page). My intent is to build a pair of equal length elements, hook them up, and see what the resonant frequency is. At that point I would make a second pair of a different length (by 10%?) and see what the resonant frequency is for the second one. To me, that would give you a good indication of what lengths you should run, for the frequency you want to operate at.