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13-6 Construction details
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are dangerous to use) Also, be aware that residential air conditioner liquid lines look like copper cold-water pipes in some cases Don t use them 29 deals with grounding The lightning arrester is a safety precaution, and it must be used! Its purpose is to supply an alternative path to ground in the event of a lightning strike Although at least one text calls the arrester optional, it is not Besides the obvious safety reasons (which are reason enough), there are also legal and economic reasons for using the arrester Your local government building and/or fire codes might require a lightning arrester for outdoor antennas Also, your insurance company might not honor your homeowner s policy if the lightning arrester (required by local code) is not used The antenna lightning arrester is not optional, so use it WARNING! DO NOT EVER ATTEMPT TO INSTALL AN ANTENNA BY CROSSING A POWER LINE! EVER! NO MATTER WHAT YOU BELIEVE OR WHAT YOUR FRIENDS TELL YOU, IT S NEVER SAFE AND IT COULD KILL YOU The rest of this discussion touches on antennas, other than the receiver longwire Because construction details are similar, we will not repeat them You are, by the way, encouraged to also read Chap 28 to glean more details on antenna construction methods
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The flattop antenna is shown in Fig 13-7 This antenna is a relative of the longwire, with the exception that the downline is at the approximate center of the antenna section The flattop should be at least a half-wavelength (492/FMHz) at the lowest frequency of operation The advantage of the flattop is that it allows the maximum use of space in the configuration shown
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It is also possible to build shortwave listener antennas in vertical polarization Figure 13-8 shows one version of this type of antenna The support (which could
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280 Antennas for shortwave reception
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Insulator Insulator
Rope
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13-7 Flattop SWL antenna
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13-8 Wire vertical SWL antenna works for hams too
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Wire directional antenna 281 be a tree or building) should have enough height to be at least a quarterwavelength on the lowest frequency of operation The antenna is fed at the base with coaxial cable The center conductor of the coax is connected to the antenna element, and the shield is connected to the ground rod at the base of the structure You are encouraged to see Chap 10, which deals with limited space and hidden antennas, for a different version of this antenna It is possible to install the wire (or multiple wires of different lengths) inside of a length of PVC plumbing pipe The pipe serves as the support structure, and the conductors are placed inside Different lengths of conductor are required for different bands of operation You can calculate the length (in feet) required for quarter-wavelength vertical antennas from Lft=246/FMHz Figure 13-9A shows how several bands are accommodated from the same feedline on the same support structure In this particular case eight different antenna elements are supported from the same tee bar Be sure to insulate them from each other, and from the support structure Again, PVC piping can be used for the support Another method for accommodating several different bands is to tie the upper ends of the wires to a sloping rope (as in Fig 13-9B)
Wire directional antenna
A directional antenna has the ability to enhance reception of desired signals, while rejecting undesired signals arriving from slightly different directions Although directivity normally means a beam antenna, or at least a rotatable dipole, there are certain types of antenna that allow fixed antennas to be both directive and variable See Chap 7 for fixed but variable directional antennas and Chap 11 for fixed and non-variable directive arrays Those antennas are transmitting antennas, but they work equally well for reception This section shows a crude, but often effective, directional antenna that allows one to select the direction of reception with pin plugs or switches Consider Fig 13-10 In this case, a number of quarter-wavelength radiators are fanned out from a common feedpoint at various angles from the building At the near end of each element is a female banana jack A pair of balanced feedlines from the receiver (300- twin lead, or similar) are brought to the area where the antenna elements terminate Each wire in the twin lead has a banana plug attached By selecting which banana jack is plugged into which banana plug, you can select the directional pattern of the antenna If the receiver is equipped with a balanced antenna input, then simply connect the other end of the twin lead direction to the receiver Otherwise, use one of the couplers shown in Fig 13-11 Figure 13-11A shows a balanced antenna coupler that is tuned to the frequency of reception The coil is tuned to resonance by the interaction of the inductor and the capacitor Antenna impedance is matched by selecting the taps on the inductor to which the feedline is attached A simple RF broadband coupler is shown in Fig 13-11B This transformer is wound over a ferrite core, and consists of 12 to 24 turns of no 26 enameled wire, with more turns being used for lower frequencies, and fewer for higher frequencies Experiment with the number of turns in order to determine the correct value Alternatively, use a 1:1 balun transformer instead of Fig 13-11B; the type intended for amateur radio antennas is overkill powerwise, but it will work nicely
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