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Roger wrote:
"Any special considerations?" A dipole in the HF range using aluminum strip 10 inches wide and .006 inches thick has a small length to width ratio and that`s good for bandwidth. The width is sufficient for good surface conduction and that is good for efficiency. Most data are for copper but aluminum is only 1.6X as resistive as copper, so there is not much electrical difference in performance. .006 inch is sufficiently thick to prevent significant RF penetrationat at HF, and if RF did penetrate it would not be detrimental. Bandwidth of an antenna may be determined by acceptable directional character or it may be determined by acceptable drivepoint impedance. Ed Laport in "Radio Antenna Engineering" has made a "Free-Space Dipole Characteristics" table for bandwidth as limited to the region where the antenna`s reactance is less than its radiation resistance at resonance. At 10 MHz, a dipole made of Roger`s aluminum strip would have an equivalent length over diameter (L/D) of about 59. From Laport`s Fig. 3.1 on page 248, a dipole of L/D of 200 would be resonant at an electrical length of 168.3-degrees. Its 3 dB bandwidth is 11.2% and its 1 dB bandwidth is 5.6%. The aluminum strip dipole at 10 MHz has a smaller effective L/D and produces a wider bandwidth than any shown in Laport`s table. Consult "Radio Antenna Engineering" for more information. Best regards, Richard Harrison, KB5WZI |
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