[TowerTalk] Tuning Fan Dipoles?

[Grant Saviers]

Thanks to Dan AC6LA, who recently posted the Cebik article link that 
explains a good way to model fan dipoles.

Cebik article on using "zero" length transmission lines to connect fan elements:
http://www.antennex.com/w4rnl/col0507/amod111.html

In the past, couldn't get sensible results from EZNEC, even Pro4, with 
my fan dipole models. Converting to the transmission line feed method 
between dipoles yielded a model that is very close to an 80-40-20 built 
with cut and try methods.  Hooray!  Now I can move resonances around as 
I need without multi cut and solder iterations.

I use solid grey PVC 1/2" square rods to space the dipole ends and a 
1/4" thick acrylic sheet 6 x 12" to space the center feed points. These 
materials are sun resistant and sturdy.

Grant KZ1W


On 5/4/2014 1:56 PM, Jim Lux wrote:
> 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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