|Subject:||Re: [TowerTalk] Re: SteppIR SmallIR vertical review|
|From:||Jim Lux <firstname.lastname@example.org>|
|Date:||Tue, 04 Nov 2003 16:38:53 -0800|
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
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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