Антенны




Медная многодиапазонная укв антенна "кактус" - часть 3


The second method is easier to tune than the first method, but does require placing 1/4 or 1/2 wavelength matching sections for all the bands of operation. If you are building a dual band antenna, the use of a T-Connector simplifies the project. Please bear in mind that you cannot use a 1/4 wavelength matching section on one band and a 1/2 wavelength matching section on another band, plus each band of operation requires the use of a matching section, including your lowest band of operation. Unfortunately, the connections will be inside the vertical section, a feat not easy to accomplish, but it does make tune-up much faster and easier than the shorting strap method shown above. The center connector of the coax matching sections is affixed to the tuning stub and the shield to the vertical section, keeping the center conductor length lead as short as possible.

The third method requires no antenna re-tuning from the specifications given on the plans and random length pieces of coax may be used. However, a relay switching assembly must be constructed inside a weathertight enclosure or the use of a duplexer for dual-band operation or tri-plexer for tri-band operation can be utilized. As above, the center conductor of the coax goes to the tuning stub and the shield to the vertical. I will note that I have used random length coax, without *-plexers or relay assemblies, however, this method worked on only three of five duplicate antennas using the same random length pieces of coax on each. Each band showed an SWR of less than 1.025 to 1 until connected together, then two of the antennas showed an SWR of over 3 to 1 and three antennas were less than 1.8 to 1 across all bands without re-tuning. A little re-tuning brought the SWR down to below 1.2 to 1 on two of the antennas, but we could not acheive anything lower than the original 1.8 to 1 on the third. So if you use random length coax and no relays or *-plexers, good luck.

NOTE: The connect distance above the horizontal member on each band is selected for an impedance of around 50 ohms, moving the connect point up or down from this set point can and will increase the impedance as high as 650 ohms within a distance of 1/2 inch either side of the established proper connect point.




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