[TowerTalk] Tuning Fan Dipoles?

[Jim Lux]

On 5/4/14, 1:46 PM, Tony wrote:
> All:
>
> I decided to forgo the ladder line doublet and try a coax-fed fan dipole
> for 80, 60, 40 and 30 meters. I have two trees to work with (30 and 50
> feet tall) so I can't spread the wires out in different directions. I
> figured I'd use PVC or some other lightweight material to spread the wires.
>
Yes.. pieces of pvc pipe work well as spacers.  If you're not looking 
for 20 year life, then just spray paint pile and call it done. 
Everything hangs off the longest (80m) wires.  you can easily drill a 
hole through the pipe for the other elements. A nylon zip tie holds the 
wire from sliding through the hole.
Spray paint when you're all done, so the ziptie gets protected too.


> I suspect that the biggest challenge is trying to bring each dipole into
> resonance so any suggestions to would be much appreciated. My guess is
> that you would start with the 80 meter dipole cut for the lowest portion
> of the band and then trim to resonance as you add each dipole.
>


They interact a lot. Adding incrementally does not work: when you add 
the new element, it detunes the other ones.  If you want a 50 ohm feed 
point Z, you need a way to raise and lower it easily, because you're 
going to be doing a lot of crumpling and uncrumpling of the ends of the 
wires.

I don't know that there's a "good" way to systematically tune it. What 
I've done is tune each element to approximate resonance in sequence 
ignoring the interactions (that is, if you tune the short element first, 
by the time you finish with the long element, the short one is 
inevitably detuned).

Then, it's sort of an iterative process, and after doing a few changes, 
you'll get the idea of what shortening or lengthening a given element 
does to the others.

with 3 wires (the most I've done: 10,20,40(also 15).. the Alpha Delta 
DX-CC), it took about a dozen up and downs to get them all tuned on all 
bands.




> I predict the process may take some time so it's probably best if I can
> borrow an antenna analyzer.

Yes indeed.. that helps to know if you need to go longer or shorter, and 
how far to go.

If you've got one that can plot the curves, you can see whether the 
other bands went up or down all at once.


>
> Thanks
>
> Tony
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