"sort of trap" - I wasn't sure, because modeling it as a straight inductor
didn't seem to produce feedpoint Z that matched the measured values. So I just
assumed it's sort of a L with parallel C, but not necessarily high Q.
ballparking, the parasitic C is "some pF" and the L is some tens of uH, so the
resonant frequency is well above 40 meters.
I'd expect substantial coupling between the fans, though.
Ballparking, let's say they're 6" apart, and it's 10 meters long on each side.
That's about 60 pF. At 7 MHz that's about 2.4k. So, looking at it as a
voltage divider, it's 5k feeding 50 ohms or 1:100, which is only 40 dB.
On Tue, 18 Nov 2025 11:12:22 -0800, Jim Brown <jim@audiosystemsgroup.com> wrote:
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
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