[TowerTalk] Fan Dipole

[Jim Brown]

On 2/17/2020 11:12 AM, Jeff Widen via TowerTalk wrote:
> I want to add a 20 meter wire to my 30 meter dipole and am looking for suggestions for spreaders to separate the  elements. Half inch PVC pipe seems to bulky and heavy, any suggestions for something lighter that will hold up to the sun? Also what is the best separation distance?

Hi Jeff,

Over the years, I've made good use of fan dipoles. 1/2-in PVC conduit 
cut into short lengths is what I've always used. I've built fans for 80 
and 40, for 20, 15 and 10, and for 40, 30 and 17. Now that I have good 
Al in the air for 20-6M, my only fan is for 80 and 40. I use a spacing 
of roughly 9-12 inches, with holes drilled into the spacers for wires to 
run through. For the longest element, I use #9 bare copper, with a wire 
jumper soldered around the spacer to hold it in place. The shorter 
element is #12 THHN, and can be held in place at each end by cable 
clamps or split bolts. Spacers are roughly 6 ft apart. That 80/40 fan 
has been up 120 ft or so suspended between redwoods for at least 10 
years. The spacers aren't as pretty as they were when I built it, but 
they haven't broken. They get plenty of UV exposure here in NorCal.

The #9 bare copper was obtained by buying a spool of #8 bare copper from 
the big box store and stretching it by tying one end of a 200 ft length 
to the base of a tree and the other end to the trailer hitch of W6GJB's 
pickup and pulling VERY slowly until it breaks. Glen and I have done 
this several times after I first learned it from WA6NMF about 15 years 
ago. What you end up with is close to hard drawn copper, roughly 20% 
longer than what you started with.

The only trouble I've had with fans is when something broke in a major 
storm, dropping one end, and the fan got tangled in a big madrone. 
Madrones are beautiful trees, but they're death on wire antennas. :)

In a fan, the SWR bandwidth of the shorter element will be reduced by 
about half, while the SWR bandwidth of the longer element is that of a 
single wire dipole.

Two of my fan dipoles are shown on pages 25 and 26 of
http://k9yc.com/LimitedSpaceAntennasPPT.pdf

73, Jim K9YC