Topband: 160 metre vertical with 'top loading'

[ZR]

----- Original Message ----- 
From: "Michael Tope" <W4EF at dellroy.com>
To: <topband at contesting.com>
Sent: Tuesday, April 26, 2011 2:33 AM
Subject: Re: Topband: 160 metre vertical with 'top loading'


> 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........................

With a sufficiently sized air coil the loss is very low, but with a ferrite 
toroid the core loss may be substantial. Loss in any form is additive to the 
ground loss.
If forced into the situation Id use a large powdered iron core before 
ferrite as a loading coil, a FT-240 is the largest size in a 61 material.

The sum of the currents argument has been around for a long time and is 
still valid AFIK.

Carl
KM1H