docCobweb Antenna - Bay-Net
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Cobweb Antenna - Bay-Net
Cobweb Antenna John Best, KJ6k Bay Net Meeting January 24, 2015 Ideal HF Antenna Efficient Transmitter power gets converted to RF, not to heating the ground, coax, or matching network components Omnidirectional Well,β¦.. directional is better, but only if it is big, high and rotatable, which most of ours arenβt Insensitive to suburban noise Low angle of radiation DX on HF is magic; nice if it also has high angle radiation at low frequencies Not too big and ugly Chimneys are weak Ugly is in the eye of the beholder Portable is fun Requirements Efficiency β β’ Short is bad π π β (π)2 β’ Ground mounted verticals require low resistance ground (lots of radials, or seawater) β’ Low SWR reduces coax and matching loss β resonant and matched is good Insensitive to suburban noise β’ If it is coming from bad LED or compact flourescent swtiching power supplies in your own home, you are screwed. β’ If it is generated and/or radiated from power lines more than a block away, it is ground wave, and therefore vertically polarized. ο Horizontally polarized antenna is better Low angle of radiation β’ Efficient vertical (really hard to get) 1 β’ High (β³ π) horizontally polarized 2 1 1 For low band (80/40) short to medium distance, 8 β 4 π is good From ARRL Antenna Book Antenna Patterns Horizontal: Ground reflections help, even with poor ground Vertical: Ground absorption is bad, even when elevated Choices Horizontal polarization π π Near-ground, mobile antenna dipole Inefficient because short; high radiation angle β no ionospheric reflection at high frequency Ground mounted delta-loop πipole Hard to beat; but needs two high supports for DX; directional Inverted V Horizontally polarized, easy to set up for portable operation, some gain relative to dipole, radiation angle high Good single support compromise to dipole Vertical polarization Ground mounted vertical Great at the beach; otherwise needs lots of radials, and only so-so even then Roof mounted vertical Easy to mount, reasonable efficiency multi-band commercial antennas available Better than ground mounted, but still not great βHaloβ ο Cobweb Bend a dipole almost into a circle Not a loop β’ Horizontally polarized β’ Single mast mount β’ Mostly omnidirectional 1964 ARRL Handbook http://www.karinya.net/g3txq/cobweb/ Popular for 2m & 6m mobile before FM. Base station single Yagiβs are easier to mount as horizontally polarized G3TXQ Cobweb β’ 5 band: 20, 17, 15, 12, 10 β’ 12 Ohm impedance matched with 4:1 balun Band Wire length Centre-to-corner length Minimum VSWR 2:1 bandwidth 3:1 bandwidth 20m 201" 72.25" 1:1 208kHz 362kHz 17m 157.5" 56.75" 1:1 170kHz 299kHz 15 135" 48.25" 1:1 167kHz 293kHz 12m 114" 40.25" 1:1 156kHz 271kHz 10m 100.5" 34.75" 1.2:1 184kHz 337kHz Construction Details Construction Details after partial tuning of 20, 17, 15 12 & 10 still 6β long too close Late Night Testing Un-tuned initial assembly Band G3TXQ length diagonal length SWR Freq 3:1 - 3:1 + 20 201 72.25 210 1.1 13.535 13.308 13.746 17 157.5 56.75 165 1.2 17.401 17.216 17.587 15 135 48.25 145 1.15 19.950 19.818 20.095 12 114 40.25 120 1.2 24.437 24.348 24.522 10 100.5 34.75 110 1.45 26.365 26.243 26.506 It looks like it works! Will tune it and clean it up for field day Parts List Davis RF PS-18Polystealth 18g (2) ¾β fiberglass RT-34-8G (4) ½β fiberglass RT-12-8 (6) ¾β SS U-bolts (2) 1½β SS U-bolts 6x12x1/8β Al 6x6x1/8β Al (10) 8-32x3/4β brass screws (20) 8-32 brass washers (10) 8-32 brass nuts (10) 8-32 brass knurled nuts (4) 4-40x1/2β brass screws (4) 4-40 nuts 1 Oz Cu sticky sheet (2) FT-140-61 cores (8β) RG/316 coax (1) UHF socket (50) Zip ties (6β) 3/16β heat shrink tubing Crimp ferrules Bud PN1324 6.73x4.76x2.17β box $59.95/250β AES $9 ea mgs4u.com $6.50 ea mgs4u.com $3.14 ea McMaster $4.16 ea McMaster $6.88 McMaster $4.38 McMaster Ace Hardware Ace Hardware Ace Hardware Ace Hardware Ace Hardware Ace Hardware Anchor Electronics $3.75 ea Amidon $18.95/50β jefatech eBay HRO anywhere Orchard Supply KJ6VU $18.80 Newark
