[TowerTalk] Fan dipole Re: Looking for a HP 80/40 Duplexer

[Jim Brown]

On 11/18/2025 9:44 AM, Jim Lux wrote:
> most fan dipoles (e.g. DX-CC from alpha delta, if you want a commercial unit) use the same element for 40 and 80 with a sort of trap.

NO! They are NOT traps. They are loading coils. I've measured the coils 
of commercial units -- they resonate FAR above where they would function 
as traps. When modeled in NEC, current distribution shows them 
functioning as loaded dipoles.

Fan dipoles connected to a single feedline are seen as independent 
antennas, with power transfer to the one resonant at the frequency being 
excited, BUT with it's feedpoint Z modified by the parallel dipole(s). 
I've used fan dipoles extensively, modeled them in EZNEC, and measured 
their impedances.

Yes, there have been commercial fans with one of their elements being 
loaded dipoles to cover an additional band. I had an 80/40 fan with the 
80M element loaded to cover 160M. W9INN sold dipoles loaded to cover two 
harmonically related bands for years. I've done 40/20 designs that W6GJB 
has built for CQP expeditions both as wire dipoles and with tubing to 
mount on a mast. These 2-band dipoles are just as efficient as single 
band dipoles, but the lower frequency band has narrower SWR bandwidth.

My 80/40 JA killer is a fan at 125 ft. I've been using it (and 
rebuilding after storms) since moving here in 2006. I first learned of 
the concept when I bought a loaded 80/40 to fit between supports on my 
Chicago house and garage that were spaced about 105 ft.

Two feedlines could easily be used to separately feed the elements of a 
2-el fan dipole whose elements built in parallel but were not WIRED in 
parallel. I would expect some coupling between the dipoles.

73, Jim K9YC