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Collapsible Beam for 6 m

Collapsible Beam for 6 m. Steve Kavanagh VE3SMA OVHFA “Do”, Sept. 2009. Idea for Foldable 6 m Beam. Foldable dipole for 6 m used in my VHF contest rover operations for a few years takes just a few seconds to unfold or fold up again for transport/storage

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Collapsible Beam for 6 m

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  1. Collapsible Beam for 6 m Steve Kavanagh VE3SMA OVHFA “Do”, Sept. 2009

  2. Idea for Foldable 6 m Beam • Foldable dipole for 6 m used in my VHF contest rover operations for a few years • takes just a few seconds to unfold or fold up again for transport/storage • Could use similar construction to make a folding framework to support a wire beam • Similar antennas used by others, with bent parasitic elements made from wire • Penn. State 40 m beam [1] • DJ4SA Spiderbeam (multiband HF antenna) [2]

  3. Spiderbeam

  4. Geometry of 4NEC2 Model

  5. Predicted VSWR (@ 25 W)

  6. Predicted Gain

  7. Predicted F/B,F/R Ratios

  8. Modeling Observations • Could optimize element lengths for different combinations of gain and bandwidth • More gain = less bandwidth = lower input impedance • As expected from regular Yagi behaviour

  9. End Loop Compensation • With wire elements held with string, you need loops at ends to tie string to • These loops add capacitance to the element ends • To maintain correct resonant frequency need to shorten elements to compensate • I made some 4NEC2 models to predict the required shortening • not quite sure if correct but included in the actual antenna

  10. Feeding the Antenna • Spiderbeam has 50 ohm input impedance • I chose a higher gain design with lower input impedance as only a narrow slice of 6m needs to be covered • How to feed a balanced antenna with ~25 ohm impedance using 50 ohm unbalanced cable ?

  11. Feed Network • Balun • Two large ferrite beads are slipped over the coax to make a “choke” or “W2DU-type” balun • Matching network • can be thought of as two quarter-wave transformers in series

  12. Feed Network, cont’d • BUT…you can’t get 70.7 ohm cable ! • By adjusting lengths of the two transformer sections, • can perfectly match a range of impedances • and can use standard 75 ohm cable

  13. Here it is !

  14. On-the-Air Results • My 1st double hop Es QSO to the west from home – DM09 • With about 10 Watts • Using antenna tuner to match before building matching network • 1st QSO on Field Day was NP4A on 6m • With 5 Watts • Much better at 12’ than vertical on car on VE3UBL beacon and on a VE9 on sporadic E • Good F/B ratio observed • Used for CW WW VHF Contest from FN05 (Multi-Op) • Satisfactory results – 41 grids worked with 100 Watts • Balun withstood this power • Significantly better than dipole (much better than vertical on car) on VE3UBL beacon

  15. References • [1] Nathan A. Miller, NW3Z, and James L. Breakall, WA3FET, “A 40m V-Yagi”, in International Antenna Collection, RSGB/ARRL, 2003, (and in QST,May 1988) • [2] http://www.spiderbeam.com

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