Topband: 160 metre vertical with 'top loading'
k3bu at optimum.net
k3bu at optimum.net
Tue Apr 26 18:28:13 PDT 2011
> I agree. Clearly the current can vary a long the length of an
> inductor. If it didn't vary at all, then a helically wound
> antenna element would not have a current taper. I don't think
> that has
> ever been in question. I think the real question that has always
> been at
> the heart of this debate is whether or not the percentage
> current taper
> is significant for physically short inductors, and in particular
> if that
> percentage current taper is exactly (or even approximately)
> equal to the
> taper that would occur along the equivalent straight length of
> that the subject inductor effectively replaces.
No, the original argument was: is the current at the ends of the loading coil (1/4 wave standing wave radiator) equal or is it diminishing at the top.
The idea of "winding the antenna wire into the coil" is to visualize the effect and illustrate the fact RF current can vary along the the straight piece of wire, why can't it vary along the wound wire - coil, hairpin.
The argument that we take the "missing" wire and wind it into a coil or hairpin and get the same result as straight wire - resonance is not the point.
In the coil or hairpin we are dealing with other effects, capacitane between the turns, against the ground, skin effect, dielectric - they affect the physical properties and this is wide open field for scientwists to explore.
If we are given the mast size and tip size and frequency, then the coil (or cap) has to bring the system to resonance. Current along the mast and tip can follow portion of the sine curve that "belongs" to their electrical length. Loading element has to "eat" the rest - causing decrease, step down between the top of the mast and bottom of the tip. It is not insignifiant some 5% but more like 40 - 60% as Barry measured.
I tried to use toroid as loading inductance, but it did not work well.
Modeling programs might have problems incorporating all variables and parameters to properly capture the real life situations. It was initially EZNEC tak took lumped inductor of physical zero size and "proved" that there is no current drop across the inductor. Using hairpin or coil modeled as bunch of defined turns, we get better results now.
Glad to see Barry, W9UCW to get involved, he has lot more detailed info and measurements on the subject.
My main concern was to set the record straight about the current distribution in loading coil, that it is not equal (or some 3% variance) as some claimed, but more like 40 - 60%. Measurements prove that in mobile antenna shootouts.
73 Yuri K3BU.us
> My suspicion is that some taper does occur in air wound loading
> coils at
> HF frequencies, but that the amount of taper doesn't follow the
> rule that it equals the amount of taper that would occur in the
> of straight radiator replaced by the inductor.
> 73, Mike W4EF........................
More information about the Topband