Marconi,
Here's your theoretical answer. Now I realize that at 27 MHZ is is not quite as critical as on............say, 3.9 MHZ, but it still is valid.
There are 3 resistances in an antenna system that makes up the system's total system impedance: feedpoint, coil resistance, and radiation resistance. There's not much you can do about radiation resistance, but you CAN tinker with the other factors to get a better match. Let's say the antenna shows a natural impedance of 30 ohms total. The SWR is likely to be a tad high--even when in resonance (The antenna loads but shows a consistant 1.7 : 1 no matter what you do). Actually, that value isn't all THAT bad, but it may be even higher. If you add another 8 ohms of resistance and another 2 ohms at the feedpoint, add it up. It = 50 ohms! So there are ways to
dicker with the thing to make it right. You can add more turns to the loading coil (which make make it more "in tune", but not necessarily for the reason you thot), and change the whip length, OR you can add impedance at the feedpoint. Instead of adding turns to the coil, you are better off to change it at the feedpoint with a capacitor, or little coil, or toroidal device because the radio actually prefers to "see" 50 ohms at the feedpoint rather than up in the shaft where it "sees" this as another value of resistance and adds this value to the RADIATION resistance. Suffice it so say that the impedance is BETTER adjusted at the feedpoint rather than up in the antenna itself.
It isn't nearly as critical, as I said, on 27 MHZ, and indeed, such adjustments may not even be needed as the natural impedance of an HF antenna will tend to increase as frequency rises----most noticible above around 8-10 MHZ until, at 27 MHZ, it may not be needed at all. However, sometimes impedance DOES become an issue with...........say, certain types of antennas. Now, as to the huge coil antennas, as effficiency increases, the impedance *may* decrease. Builders KNOW this, so they may add coil turns and shorten the whip to make an impressive-looking antenna. This not only looks good to the CBer/trucker, but it looks like it will REALLY get out!
And it MAY! It also fills the need for a shorter antenna for the trucker. But the multiple coils are a trade-off. Overall, the 102" whip is the better antenna with drawbacks. It is long, floppy, and may present impedance issues of its own! But ALL mobile HF antennas are compromises and you simply have to choose the one that suits ya best! It just helps to know what the factors are that influence your choice!
73
CWM
