Topband: 160 metre vertical with 'top loading'
Michael Tope
W4EF at dellroy.com
Mon Apr 25 23:33:20 PDT 2011
On 4/25/2011 7:08 PM, k3bu at optimum.net wrote:
> I do not want to start the argument all over again. One would expect people to read the material or argument carefully, trying to understand it and comment aproprietly.
>
> Just to set the record straight:
> It is not my famous picture but Barry's, W9UCW. He set up real life situation and I was pleased to find it as a confirmation to what I found to be happening (RF current at ends of the loading coil on a 1/4 wave resonant, standing wave antenna, is different).
>
> Again, to simplify and illustrate the situation, I think I presented my view, experiences and real life measurements to illustrate what is really happening. Want to prove us wrong setup the experiment and see what is happening.
> Obfuscating the case with toroids or whatever is not proving anything.
I really don't understand why this is obfuscating, Yuri. For instance, I
just modeled a base-loaded 60ft vertical in EZNEC at an operating
frequency of 1825 KHz. According to the model, I would need about ~480
ohms of inductive reactance at the base to "resonate" this ~1/8 wave
vertical. I can create the required inductive reactance using a toroid
core inductor (Ferrrite Products #61 material would work well) or I
could use an air-wound inductor. Either way I would expect the inductor
to cancel the capacitive reactance of the 60ft radiator if the inductive
reactance were XL=+j480 ohms . If the claim that this base XL=+j480 ohm
inductance "eats" the first 73 feet of the 1/4 wave current distribution
is unequivocally true, then both the ferrite inductor and the air wound
inductor should have an equivalent percentage current taper that matches
the percentage taper that occurs on the first 73 feet of a 1/4 wave
vertical radiator.
> We are dealing with resonant and RF circuits and not DC current and circuit. If the RF current can vary along the solid piece of antenna wire (or is that denied too?) why is it so hard to admit that it can vary when that wire is coiled or folded into hairpin (inductance)?
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.
In my example above, a 4 to 6 inch tall airwound inductor can replace 73
feet of straight wire. You might argue that with the air wound inductor
the 73 ft of wire it replaces is just coiled up so the time delay for
the EM wave to get from one end of the coil to the other is the same as
the time delay for the EM wave to traverse the 73ft of straight
radiator. I wondered the same thing, so I did some calculations to see
how long the wire would be in the XL = +j480 ohm inductor from my
example above. As it turns, out it takes ~25 feet of wire to create the
air wound inductor which replaces 73ft of radiator, so either the EM
waves move slower along the coiled wire (i.e. the air wound inductor),
or the time delay through the air wound inductor is smaller than the
time needed for the EM waves to traverse the 73 foot straight section of
radiator that the inductor replaces. If I use a ferrite core inductor
instead of an air core inductor, the length of wire needed for the
XL=+j480 ohm inductor will be much smaller than for the air wound
inductor case (offhand I am guessing just a few for my example case
inductor).
If the percentage current taper across a given length of radiator is
proportional to the time needed for an EM wave to traverse that length
of radiator (whether that radiator be compose of coiled wire or straight
wire) and if the current taper along the length of an inductor is always
equal to the percentage taper that would occur along the length of the
straight element replaced by that inductor, then velocity of the EM
waves traversing the inductor must depend on something more than just
the length of the wire used to form the inductor. Otherwise how could
the ferrite core inductor composed of just a few feet of wire have the
same EM wave propagation delay as an air core inductor composed of a
much longer length of wire (~25ft) or worse yet the length of the
straight section replaced (73ft)?
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. I think the degree of
taper depends on the velocity of EM wave propagation through the coil
and to some extent on the amplitude and phase of the displacement
current from the inductor to ground (It wouldn't surprise me if there is
an interdependence between the EM wave velocity and the magnitude and/or
phase of the displacement current). These two quantities are probably a
function of the length and form factor of the inductor. That said, it
would not surprise me in the least that for inductors that are
physically small relative to the overall radiator length, the amount of
current taper across the length of the inductor is negligible (i.e.
current at the top and current at the bottom are for all practical
purposes the same). I could be wrong, of course. That I will readily
concede.
In any case, this is a very thought provoking topic, Yuri. Good exercise
for the brain. I look forward to hearing what the gurus at Tree's
workplace come up with.
73, Mike W4EF........................
More information about the Topband
mailing list