To add to the info, assuming your 40 dB is correct, that would put a bit over
100 mW (62dBm - 40 dB = 22 dBm) into the receiver. The rx damage point is about
16.7 dBm. I would definitely not trust that setup, and even if there is no
damage, the rx would be completely blocked, precluding SO2R. Filters and
diplexer are needed...
73,
Jack, W6FB
> On Nov 18, 2025, at 2:46 PM, Jim Lux <jim@luxfamily.com> wrote:
>
>
>
>
>
> "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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