It depends whether you mean the original definition of a "fan dipole", or the
more recent imposter that consists of multiple half wave dipoles on a common
feedpoint.
There is no problem combining dipoles for any of the pre-WARC bands (160, 80,
40, 20, 15 and 10m) on a common feedline. 30m doesn't seem to be a problem,
either. The closer together you bundle the wires, the more they will interact,
and it will take a bit more cut and try to get them tuned, but I've carried such
a set of dipoles in my backpack and strung them up in a number of, um,
creative installations, and used them without any further tuning or
other adjustment (no antenna tuner, either.)
The 20m, 15m, and 10m bands are a half octave apart. (10m is twice the
frequency of 20m, and 15m is about in the middle.) When you start trying to
combine bands that are closer together than that, the unused elements may
not have a high enough impedance on other bands that it can be ignored. So
the main issue will be 20 / 17 / 15 / 12 / 10m.
I was able to model multiple dipoles for 20m / 17m / 15m / 10m with reasonable
results, but once I added wires for 12m everything changed. The SWR dips for
20m, 15m and 10m are still visible on the plot, though sometimes shifted around
by up to 1 MHz, but the 17, and 12m resonances are not at all as one might
expect.
So if there is one band that causes problems it probably is 12m when used along
with 15m and/or 10m. Putting 17m on with 20m and/or 15m seems to have a
better chance of success, but still some possible quirks.
With a
true fan dipole, however the situation is very different: it can cover
all bands from 40m through 10m (as well as frequencies between the bands) with
an SWR of 3 : 1 or better. Covering 20, 17, 15, 12 and 10m is trivial, and it can
be extended to add 30m and 40m if space is available.
I may seem to be a bit of a stick-in-the-mud about the names, but the original
fan dipole seems to be forgotten, and it really is a useful antenna, as it allows
wideband operation with a built-in antenna tuner that matches up to 3 : 1.
It can work over a 3 : 1 or 4 : 1 frequency range with no more effort to build
than multiple tuned dipoles. The design has been around since the 1930s, and
is covered in Kraus'
Antennas, Laporte's
Radio Antenna Engineering and Jasik's
Antenna Engineering Handbook. Commercial and military
versions are available:
http://ascsignal.com/files/hf/omin_directional/1765-2012.pdfHere is an example: cut 10 wires each 25' long. Connect 5 of the wires to each
side of a 6 : 1 balun (as are often sold for OCFD antennas). Spread the wires on
each side out into a fan about 20' wide at the furthest corners. Run coax to the
rig.
And that is all it needs! The SWR should be below 3 : 1 from 10 to 30 MHz, allowing
use with a limited-range autotuner (or the adjustable output stage in a tube rig)
across the entire range. The design can be scaled up or down as needed: actually,
except for some small blips near 47, 66 and 85 MHz, my model shows low SWR to
beyond 100 MHz, though the patterns get rather quirky. But a 40m through 10m
version should be practical. A huge advantage in many applications is that there
is no adjustment of lengths required because the antenna isn't tuned to a specific
frequency.
If you don't have a 6 : 1 balun you can use a 4 : 1 and the SWR might rise to 4 : 1
instead of 3 : 1, but it is still easy to match and coax losses are reasonable. You
can try using 75 ohm coax in place of 50 ohm in that case if you have some handy.
You can experiment with different numbers of wires or varying the angle between
them and see what works best. Installing it as an inverted vee may make a better
match with a 4 : 1 balun.
So that's why the answer depends on what version of
fan dipole you are building.
