An antenna is usually called by its ELECTRICAL properties, and if tophats, loading, etc present current magnitudes and phases associated with a halfwave, it will usually be called a halfwave. Some will refer to a halfwave with a lot of loading as a loaded or shortened halfwave.
The pattern, however, as well as the feed impedances and efficiency will vary as an antenna is physically shortened but loaded by some to maintain electrical length. This is where caveat emptor should apply. There is a good deal of snake-oil dBi specification out there for shortened antennas. When radiating length is shortened, radiation resistance is lowered, and higher current and voltage must be handled properly or performance goes in the tank. So there is a science in making shortened antennas work properly. Conductor joints, conducting surfaces and dielectric materials used for insulation, support, etc, have to be engineered for the application. This last is where you will find all the lossy shortcuts. Lastly, shortened antennas will experience narrower bandwidth than their full size versions. Some snake oil antennas, with extra loss just about everywhere, will display decent bandwidth, but only because it is loaded down by all the extra loss. This bandwidth is sometimes advertised as an "advantage". They are depending on hams' persistent fascination with SWR as an indication of antenna health. And they make money from it, as convenience and simplicity of "explanation" win out over physics and the headache of understanding the more difficult set of rules actually in play. Force 12, happily, is one place where the issues of shortening, and an honest specification of gain, can be had for predictable results that you do not have to "un-fudge" to make a reasoned decision. However you may like or dislike their particular physical construction philosophy and "cell" multiband designs, at least you have a specific, reproduceable and publicized method for annotating gain. 73, Guy On Mon, Aug 2, 2010 at 11:47 AM, Steve Ellington <[email protected]> wrote: > > > > I know it's off topic but I have a question. > > How is an antenna that is shorter than 1/2 wavelength called a half > wavelength antenna? Does adding loading coils really turn it back into a > 1/2 > wavelength antenna? > > Seems to me that the current distribution and resulting radiation pattern > would change if it's short regardless of loading. I keep seeing these > antennas show up on the market and just want to be sure I'm right. > > Steve > N4LQ > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[email protected] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
