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2 years ago in Electrical Engineering , Small Antenna Design By Rani
In a Yagi-Uda antenna, how does using hollow aluminum tubes (vs. solid rods) of different diameters affect element resonance and operating bandwidth?
I'm building a high-power Yagi for 144 MHz and plan to use aluminum tubes for strength and weight. I know element diameter influences bandwidth, but I'm unsure about the specific effect of hollowness. For a given outer diameter, does a hollow tube have a different effective electrical length than a solid rod? How does this affect the scaling factor needed when cutting elements to length? Also, does the tube's wall thickness (and thus inner diameter) have a significant impact on the element's Q and the array's overall bandwidth?
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By Jyotsnamayee Nayak Answered 1 year ago
For HF/VHF Yagis, hollowness has negligible direct electrical effect due to the skin effect; current flows on the outer surface. Therefore, a hollow tube and a solid rod of the same outer diameter (OD) are electrically identical. The key parameter is the OD itself. A larger OD increases bandwidth (lowers Q) and shortens the required resonant length. You'll need a length correction factor (typically 0.95 to 0.98 of λ/2) based on the length-to-diameter ratio. Wall thickness matters mechanically but not electrically, unless it's so thin it deforms. For your 144 MHz Yagi, select an OD for mechanical stability (e.g., 12-20mm), calculate the corrected lengths using a Yagi design tool (which accounts for OD), and build with confidence. The mutual coupling and overall bandwidth will be determined by the OD and spacing.
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