As for plungers. One made in California, one that looks more like a screwdriver made down in S. Carolina, and another made in Ohio, use a fixed coil. The bottom of the coil is electrically connected to a plunger which moves up and down inside the coil, although the one from Ohio is on the outside of the coil. As the plunger moves up, more and more of the coil is short tapped. Short tapping isn't ideal as doing so reduces Q. Whether this is a detriment, depends on a lot of factors. In one case, the top end of the coil is a large chunk of aluminum. Even when the plunger is all the way up, some coil is still active, and caught between the metal plunger and the metal end cap. Under the right circumstances, the coil operates above self resonance, and the losses are almost massive.
In the traditional screwdriver, the coil is swallowed by the mast. At the top of the mast are contacts which circle the coil. The top of the coil is connected to the whip, obviously. The lower end of the coil is inside the mast. It doesn't actually contact the mast, but in some models it does.
Cheaply made screwdrivers often have enough slop that the bottom of the coil that it is possible that it contacts the mast, and when it does the coil's Q and resonant frequency changes. I know of two that do this, and it is very evident when you're traveling down the highway. At first, I didn't know what was causing the problem, but did manage to figure it out in due course.
I've put a lot of thought into this, and I can't see an easy way to measure the current in the unused portion of the coil. About the only thing you can 'see', is when the gap between the coil and the mast gets too large. There can be arcing between the lower sections of the coil, and the interior of the mast. I personally have seen the results of this happening when playing with inexpensive screwdrivers. Based on nothing other than that, one has to draw a conclusion that circulating currents can be significant.
As Belrose points out in many of his articles, the best solution is to have a coil of the exact size needed with no shorted turns. I'll buy that. But the rest of us like the idea of changing bands while under way. The least wind loading is offered by one of the two types mentioned. However, when compared to a fixed coil, both methods are a compromise Q wise, all else being equal. Since so many factors are at play, it's difficult to calculate and/or measure reductions in Q. About the only thing you can do fairly reliably, is measure field strength, and compare that to the same antenna modeled with EZNEC or NEC4. Belrose wrote about this methodology in an article which appeared in the ARRL Compendium #4. I think I've read that article 10 times.
I am not encumbered with massive amounts of test gear, and I don't have the real estate either. This sort of leaves me on the short end, so to speak. In any case, if one really had the time, inclination, and equipment, testing a few models of each over their operating range, would certainly be enlightening.
Alan, KØBG
www.k0bg.com
