> 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.
"Mobile antenna shootouts" are about as much like real antenna
engineering as the World Wrestling Federation is like Olympic
wrestling. Any coil that shows a 60% decrease in current through
it has a significant amount of unexplained loss.
73,
... Joe, W4TV
On 4/26/2011 9:28 PM, k3bu@optimum.net wrote:
> W4EF writes:
>
>
>> I agree. Clearly the current can vary a long the length of an
>> air-wound
>> inductor. If it didn't vary at all, then a helically wound
>> vertical
>> 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
>> radiator
>> 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
>> simple
>> rule that it equals the amount of taper that would occur in the
>> length
>> of straight radiator replaced by the inductor.
>>
>> 73, Mike W4EF........................
>>
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>
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