When I saw fan dipole listed as the 40/80 antenna I was thinking of a
homebrew dipole with separate wire elements attached to a common center
insulator (and maybe choke). I am not at all familiar with the commercial
type you're referring to. I thought it would be easy to make the fan into
two separate dipoles, a little space could be added between them or once
could be rotated a little minimize interaction. I never suggested not using
filters. I would use bandpass filters with the individual dipoles or with a
dual band dipole and a diplexer. For a number of years I had a homebrew 20m
Yagi with a 40m Yagi on the same boom, each had their own feedline and
filters. I had no interference between the bands except for the 2nd
harmonic and never had receiver damage.
John KK9A
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.
And if you did have a 40/80 fan dipole, the two elements would be so close
together that they might as well be connected at the feedpoint.
If you're putting 1 kW into one of the dipoles, you want around 60 dB
isolation (i.e. 1 mW into the receiver on the other one, and trust that the
receiver can take 0 dBm into the front end).
Just as a note, there's two sort of approaches on this - one is basically a
pair of bandpass filters, one for 40, one for 80. The other approach is a
low pass/high pass combination. There's advantages and disadvantages for
each.
On Tue, 18 Nov 2025 11:47:21 -0500, <john at kk9a.com> wrote:
W8FN's QRZ pages listed his 40m/80m antenna as a fan dipole, not a trap
dipole. I am sure most people on this towertalk list can figure out how to
separate a fan dipole into two antennas with two feedlines. If a diplexer is
a better option than go for it, I was just suggesting another option.
John KK9A
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