[TowerTalk] Re: SteppIR SmallIR vertical review

[Jim Lux]

For all the "cool factor" of elements that change length, it's interesting 
to contemplate how is this different than, say, using a fixed length 
radiator and changing a low loss loading element.  Consider a long radiator 
(so it's inductive at all frequencies of interest) and a very low loss 
vacuum variable that's motor driven.

Either way, you're moving something that neatly gets rid of the reactive 
component as seen by the feedline (or, in the case of a multi-element 
parasitic array, changes the amount of reactive component in the mutual 
coupling).

I suspect, but haven't rigorously analyzed it, that the "change the element 
length" approach might be lower loss over all, because of the lack of 
circulating currents between loading component and the element's reactive 
component.  However, is the actual IR loss in the element all that 
significant, except in an egregiously reactive case (the compact loop, for 
instance).  Consider a 20 meter dipole, approximately 10 meters long, made 
of copper AWG#12 wire.Using a crude NEC model.. at resonance (14.4 MHz), 
the structure loss is .363W (out of 100W incident.. <1%)

At 14 MHz, the reactive component is -j32 ohms (61.4-j32). If I tune that 
out with a lossless inductor, the structure loss is now .3781 ohms... a 
huge increase of about 15 milliwatts), mostly due to the slightly higher 
current.


Let's go to a frequency where the structure is seriously inductive (21 
MHz)... now the impedance is 317+j621... Oddly, the loss is less (because 
the current is less at that high feedpoint impedance..).. 0.2207W...

Of course, you'd have some loss in a practical 300:50 ohm matching transformer.

The point is that the IR losses in the antenna are quite low (compared to 
losses in, say, the ground under the antenna, which no fancy antenna tuning 
scheme, lumped or distributed, is really going to change).

So, the real advantage of the SteppIR variable element length thing is that 
it not only allows you to tune out the reactance (which you could do with 
any manner of adjustable reactive components, at quite low loss), but it 
also keeps the feedpoint impedance close to 50 ohms.

Some sort of antenna mounted low loss antenna tuner could probably do the 
same thing.  It would probably need to be something like an L network with 
decent low loss adjustable L and some low loss vacuum variables, the 
combination of which would be bulkier and more expensive than stepper 
motors and BeCu ribbons.



At 04:06 PM 11/4/2003 -0800, Rick Karlquist wrote:
>Rob Atkinson, K5UJ said:
> > other.  I would like to know  what had been done to prevent the metal
> > tape from bumping up against that  ridge and getting hung up on it when
>
>There is a plastic "bullet" at the end of the tape that
>keeps it from hanging up.
>
>Rick N6RK
>
>
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