----- Original Message ----- From: "Michael Tope" <[email protected]> To: <[email protected]> Sent: Tuesday, April 26, 2011 2:33 AM Subject: Re: Topband: 160 metre vertical with 'top loading'
> On 4/25/2011 7:08 PM, [email protected] 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 _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK
