[Antennaware] Fan dipole: similar or identical segment lengths? (Kevin Zembower)
Guy Olinger K2AV
k2av.guy at gmail.com
Mon Jul 27 16:56:51 EDT 2020
When the wires converge to a close point, like a feed point, the segments
close to the convergence should be equal and small. LIterally modeling two
inches of connection wire at a feedpoint is a special problem, one far more
easily solved by the transmission line device. But some close quarters are
not center to a collection of dipoles, and have to be dealt with. Depends
on what you are modeling. There are no silver bullet, one size fixes all,
kinds of solutions.
The other thing is to see how many aspects vary, such as R and delta R,
beyond just resonance and a comfortable SWR. You can always trim a dipole
to X=0 and tune to it. A pi network and an 807 did that very efficiently.
But tetrodes and grounded cathodes are fairly out of style these days. That
worked very well, and nary a model in sight. Never had an SWR meter to go
with my 807 either. Dip and load for rated plate and grid current.
With more complex antennas, there comes a time when the models have to be
acknowledged as only an estimate, never a fact for direct implementation.
Further progress requires a physical implementation, real measurements and
QSOs. Measure and trim, measure and trim, and finally measure, no trim
Wire antennas, antenna placements with ground involvement, won't model
reliably, though you CAN get lucky, and will from time to time. Do not take
the luck as a rule. In the end, a model is just a model.
You can get good enough that your model is never very far off, and nail it
often enough. But it is not NEC that is making the magic, it's the
subtleties understood and practiced by the modeler.
What the model does very well, is to identify ideas that can't fly, without
having to construct it, actually put it up in the air, only to discover it
can't fly. Good modeling puts you in the zone, and everything you try that
passes good modeling DOES at least have a chance at conquering reality.
That IS worth an awful lot.
Cebik IS a good source of subtleties and tricks.
73, Guy K2AV
On Mon, Jul 27, 2020 at 1:32 PM Grant Saviers <grants2 at pacbell.net> wrote:
> The "equal length" is one way to say "segments should align" which could
> be different if the fan angles are large. Aligned segments I think is
> what the math does best with. So that takes a bit of fiddling to
> determine the divisor # of segments for each dipole. Of course you need
> to make a good guess at the starting lengths. Close is good enough.
> 3 segments are not enough IMO as the currents near the feedpoint
> junction are high. Another rather essential trick is to have a
> transmission line between the feedpoints. My model feedpoints were
> separated by 2" 50ohm lines and the ends by 20". This is discussed by
> Cebik in one of his modeling tutorials. Implementation between
> feedpoints was a small Cu flat strip, unknown Z, but unimportant for my
> two 80/40/20 fans, especially since 20m is closest to the coax.
> At least that is what I have done and gotten good correlation model to
> built 3 band 80/40/20 fans.
> Grant KZ1W
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> Antennaware at contesting.com
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