[TowerTalk] On the phase shift in current between ends of a coil

[Jim Lux]

An admirable construction approach.  Your limited size elements are also not
so much shorter than full sized elements that the mutual coupling between
them is pretty much the same as the modeling tool assumed.

I suspect that if your requirement was to have elements no longer than 20
feet (i.e. dramatically shorter), you might not have gotten the performance
you'd expect.

On the other hand, if you had done a model with 20 foot elements, and great
big honkin loading coils, and simulated them as lumped elements in, e.g.
NEC, you'd probably be pretty much in the ballpark, and your strategy of
trimming the elements to the same resonant frequency would probably work,
fairly well.  You might get bitten because the Q of the loaded element will
be a lot higher than the Q of a unloaded element at the same frequency, so
the phase of the current in the element will change faster vs frequency,
which might screw up the phasing relationship among elements.



----- Original Message -----
From: "Mark Beckwith" <mark at concertart.com>
To: <towertalk at contesting.com>
Sent: Sunday, November 23, 2003 6:01 AM
Subject: Re: [TowerTalk] On the phase shift in current between ends of a
coil


> Phase Shift?  This thread would have made me cross-eyed if I had studied
it
> so I have not been following it.  Nevertheless this excerpt caught my eye:
>
> >A lumped model serves as a decent design
> >tool to determine a starting point, which
> >will have to be further iterated empirically.
>
> I like to use AO/YO to design yagis, but I needed my 40M antennas to have
> elements no longer than 60 feet because of the guy wires above where
they're
> mounted.  So I knew I had to load up the elements a little bit.  As much
> faith as I have in AO/YO for plain-ol' monobanders, my confidence goes out
> the window when you start loading them up.
>
> So I used the empirical approach to build these 40s.  I modeled a
full-sized
> antenna which gave me the positions of the elements on the booms, worked
> backwards to determine the resonant frequencies of the elements, then by
> experimentation adjusted each element until it was resonant at the right
> frequency, then put the antennas together and put them up.
>
> They work well.  Anyway, 2 cents worth of practical experience from the
> woodwork.
>
> Mark, N5OT