W7CY 80 Meter Yagi

Cette page était sur le site de W0LMD; mais le lien ayant disparu, j'ai reproduit ici l'article que j'avais conservé. (F5AD)

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A NEW CONCEPT IN BIG YAGIS
W7CY - "CapYagi ^TM

Three dimension capacitive loaded design shrinks the size without shrinking the performance. The *CapYagi* ^TM minimizes tower, rotor and real estate requirements. Shortened elements allows greater front to back than a full size yagi with virtually none of the losses associated with use of lumped constants or "magic" linear loading. An article about the remarkable qualities of the W7CY cap yagi appears in the ARRL publication "Low Band DXing" by ON4UN.
A 75 meter yagi consists of an 80' cross boom (left to right) with two 36' end spreaders. A spreader 12' above the boom supports the center of the active elements, Wire elements stretch from the top spreader to the tips of end spreaders. The tapered aluminum end spreaders are insulated at the center and act as capacitance hats for the elements. The antenna is configured as a driven element and reflector. The driven element is physically 111' 9' +/- long and the reflector is 116' +/' long. The difference is in the length of the insulated section at the center of the end spreaders. This is adjustable for tuning. Effective antenna height is gained by raising the center of the elements.
The antenna is designed to withstand 120 MPH winds. It weighs about 250 pounds and has only 12 sq. foot wind loading. Mounted on a U.S Tower HDX 589. It turns easily with an Orion rotator. A trussed structure is used for maximum strength at minimum cross sectional area. Structural design was done by Kurt Andress - K7NV - using his commercially available YS software for the end spreaders and Finite Element Analysis software in conjunction with a custom beam/column buckling software for the guyed cross boom structure.
The front to back ratio is in excess of 20 db between 3770 to 3805. Computer models show gain at 4.1 dBd in free space. Feed point impedance is 50 ohms at resonance. The antenna is tuned to 3789 with a 2:1 SWR from 3750 to 4000 (The effects of the reflector diminish above about 3850 with gain dropping off to about 2 dbd with 5 db front to back at 4mHz). The antenna is fed directly with 50 ohm coax through a 1:1 balun. Electrical design was done by Rod Mack - W7CY - using custom software for phase analysis, AO and NEC wires for pattern and tuning parameters. Installed resonance was 3780 - just 9 kHz below computer predictions. Virtually no tuning was required!