Fw: antennas for Susan
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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:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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> How is an antenna that is shorter than 1/2 wavelength called a half
> wavelength antenna? Marketing department. Does adding loading coils really turn it back into a > 1/2 > wavelength antenna? Only in the marketing department. > 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. With reasonable size and the correct loading, tip loading with a large capacitance hat, it can almost be the same. What gets me are the trap antennas advertised as "no traps" or "no loading coils", or having "lossless linear loading" when the system is usually lossier than what they are proposing to eliminate. Marketing departments convinced us traps are bad so they could sell us things worse than traps by telling us they have "no lossy traps"...which is claimed even if the antenna has traps by simply calling the trap something else. Most of the time traps would have less loss than the systems we are convinced are better. 73 Tom ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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In reply to this post by Steve Ellington
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 <[hidden email]> 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:[hidden email] > > 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:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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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.>> If we have a given height Marconi antenna, say 20 feet on 40 meters, the radiation resistance would be about 9 ohms. If we feed that same length antenna in a way to make it have a "halfwave" current pattern, radiation resistance is now 7.5 ohms. The loading reactance at least doubles, so for the same Q loading system we have about half the bandwidth and twice the loss. Loss is NOT proportional to bandwidth, they really are mostly independent characteristics when the system is changed. This reminds me of CB antennas, where some crafty marketing person wound 3/4 wave of wire in a six-foot space and told CBers it was a "3/4 wave helical" vertical with gain over a regular six-foot long helically wound vertical. I hope we are not reverting to that level, but it seems we are! 73 Tom ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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