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#1
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Dan Jacobson wrote:
"I will put my wire to work to pull it in even better." Fitst, you need a line-of-sight path to the desired station. Then, you need an obstruction in the response to unwanted signals. You need enough desired signal edge to capture the FM detector and to suppress noise the undesired signals might add to the mix. An axial mode helical antenna uses a lot of wire wound into a coil which is pointed at the desired station, directly or indirectly. Diameter and coil pitch are optimized for axial gain and directivity. If a helical coil is small in terms of wavelength, its field is maximum perpendicular to the coil axis. Such a self-resonant coil makes an omnidirectional antenna in a plane perpendicular to the coil axis. We have a rubber ducky as an example of an omnidirectional helix which is inefficient, limited in bandwidth, but convenient for handie-talkies. Larger omnidirectional (normal-mode) helices are used by some VHF/UHF broadcasters to provide circular polarization, but Kraus says on page 173 of his 1950 edition of "Antennas": "For normal mode the dimensions of the helix must be small compared to the wavelength, so that from band width and efficiency considerations this mode is not readily applicable in practice." There are handie-talkie suppliers and broadcasters who don`t heed Kraus. Best regards, Richard Harrison, KB5WZI |
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#2
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Quote below
REALITY CHECK: a form of a normal-mode, helical radiator is commonly used for FM broadcast transmitting antennas. At their design frequencies, these elements produce the same _measured_ field strength in their directions of peak gain as produced by a matched, linear dipole in its directions of peak gain (over the same path). Some of these FM helical designs have _measured_ input SWR less than 1.15:1 over 15% RF bandwidth, which is most of the 88-108MHz FM band. For the purpose, this performance is far from being inefficient and/or narrow band. NEC2 analysis of an FM normal-mode helix shows this, of course. Examples of it are found in the slide show (item 10) at http://rfry.org . Reading and heeding Kraus has to take the application into account. Generalizations lead to misunderstanding. RF ___________________ "Richard Harrison" wrote: Larger omnidirectional (normal-mode) helices are used by some VHF/UHF broadcasters to provide circular polarization, but Kraus says on page 173 of his 1950 edition of "Antennas": "For normal mode the dimensions of the helix must be small compared to the wavelength, so that from band width and efficiency considerations this mode is not readily applicable in practice." There are handie-talkie suppliers and broadcasters who don`t heed Kraus. |
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