A mistaken idea that many Hams get is that a wire antenna has no radiation in the "nulls". For example, a half-wave wire is often thought of as having no energy radiated off of its ends. There is LESS off the ends, but a real-world wire has some radiation in ALL directions as Dave notes. It's just stronger radiation in some directions.
A real long wire (many wavelengths) is easy to match since the longer a wire is, the smaller the impedance excursions across the RF spectrum. The hardest to match are wires a half wavelength (or less) long. However, most compact ATUs are limited in matching range based on simple physics. Their small size cannot tolerate the huge RF currents and voltages frequently encountered even at moderate power levels. There's a good reason why the old time "antenna tuners" (matching networks) were so huge. It's just a matter of basic physics. But today most of us use antennas that offer an feed point impedance limited to the range our ultra-fast, super-smart "automatic antenna tuners" can handle. 73, Ron AC7AC -----Original Message----- From: elecraft-boun...@mailman.qth.net [mailto:elecraft-boun...@mailman.qth.net] On Behalf Of David Gilbert Sent: Thursday, January 11, 2018 11:41 PM To: elecraft@mailman.qth.net Subject: Re: [Elecraft] Long wire antennas MORE There is only a fixed amount of total energy contained in all the lobes of an antenna. You almost definitely did get lots of lobes ... but you also got lots of nulls that exactly offset all those lobes. You just never heard the the hams that were in those nulls and they never heard you. Whatever you gain in one or more directions is sacrificed in one one or more other directions. This is basic physics. More lobes is not necessarily better. In fact, taken to the extreme it is self defeating because a very large number of lobes (assuming they were somehow all of equal strength as you stated) begins to approximate a unidirectional antenna with no azimuth gain in any direction. Just for grins I modeled your 700 foot antenna in EZNEC+ and on 20m it gave a maximum gain of about 9 dbi in a fairly narrow lobe at 16 degree elevation in both directions along the axis of the wire. It also gave a total of 36 other sharply narrow lobes arrayed symmetrically in all other directions, each with a gain of about 6 dbi. Between each lobe was a deep null of around minus 10 dbi. This was all at the same 16 degree elevation angle ... there were literally too many lobes to count on the 3D pattern, with lots of lobes and nulls at every azimuth and elevation angle. A simple dipole at the same 40 foot height would have given similar gain with a much broader lobe (both azimuth and elevation) in the two main directions, but of course without the multiple smaller side lobes. Three poles and two perpendicular dipoles would have given better overall single band results ... the only advantage of the long wire being that it gives a similar pattern along with similarly ugly match on multiple bands. Dave AB7E ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
