Cobweb Antenna

John Best, KJ6k

Bay Net Meeting

January 24, 2015

Ideal HF Antenna

EfficientTransmitter power gets converted to RF,not to heating the ground, coax, or matching network components

OmnidirectionalWell,….. directional is better, but only if it is big, high and rotatable, which most of ours aren’t

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 uglyChimneys are weakUgly is in the eye of the beholderPortable is fun

RequirementsEfficiency

• Short is bad 𝑅𝑅 ∝ (ℓ

𝜆)2

• Ground mounted verticals require low resistance ground (lots of radials, or seawater)

• Low SWR reduces coax and matching loss – resonant and matched is good

Insensitive to suburban noise• If it is coming from bad LED or compact flourescent swtiching power

supplies in your own home, you are screwed.• If it is generated and/or radiated from power lines more than a block

away, it is ground wave, and therefore vertically polarized. Horizontally polarized antenna is better

Low angle of radiation• Efficient vertical (really hard to get)

• High (≳1

2𝜆) horizontally polarized

For low band (80/40) short to medium distance, 1

8−

1

4𝜆 is good

Antenna Patterns

Horizontal:Ground reflections help, even with poor ground

Vertical:Ground absorption is bad, even when elevated

From ARRL Antenna Book

Choices

Near-ground, mobile antenna dipoleInefficient because short; high radiation angle – no ionospheric reflection at high frequency

Ground mounted delta-loopHorizontally polarized, easy to set up for portable operation, some gain relative to dipole, radiation angle high

Ground mounted verticalGreat at the beach; otherwise needs lots of radials, and only so-so even then

Roof mounted verticalEasy to mount, reasonable efficiency multi-band commercial antennas availableBetter than ground mounted, but still not great

𝝀

𝟐𝐝ipole

Hard to beat; but needs two high supports for DX; directional

Inverted VGood single support compromise to dipole

Horizontal polarization

Vertical polarization

“Halo” Cobweb

• Horizontally polarized• Single mast mount• Mostly omnidirectional

Bend a dipole almost into a circleNot a loop

Popular for 2m & 6m mobile before FM. Base station single Yagi’s are easier to mount as horizontally polarized

http://www.karinya.net/g3txq/cobweb/

1964 ARRL Handbook

G3TXQ Cobweb• 5 band: 20, 17, 15, 12, 10• 12 Ohm impedance

matched with 4:1 balun

Band Wire lengthCentre-to-corner

lengthMinimum

VSWR2:1

bandwidth3:1

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

too close

after partial tuning of 20, 17, 1512 & 10 still 6” long

Late Night Testing

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

Un-tuned initial assembly

It looks like it works!Will tune it and clean it up for field day

Parts ListDavis RF PS-18Polystealth 18g $59.95/250’ AES(2) ¾” fiberglass RT-34-8G $9 ea mgs4u.com(4) ½” fiberglass RT-12-8 $6.50 ea mgs4u.com(6) ¾” SS U-bolts $3.14 ea McMaster(2) 1½” SS U-bolts $4.16 ea McMaster6x12x1/8” Al $6.88 McMaster6x6x1/8” Al $4.38 McMaster(10) 8-32x3/4” brass screws Ace Hardware(20) 8-32 brass washers Ace Hardware(10) 8-32 brass nuts Ace Hardware(10) 8-32 brass knurled nuts Ace Hardware(4) 4-40x1/2” brass screws Ace Hardware(4) 4-40 nuts Ace Hardware1 Oz Cu sticky sheet Anchor Electronics(2) FT-140-61 cores $3.75 ea Amidon(8’) RG/316 coax $18.95/50’ jefatech eBay(1) UHF socket HRO(50) Zip ties anywhere(6”) 3/16” heat shrink tubing Orchard SupplyCrimp ferrules KJ6VUBud PN1324 6.73x4.76x2.17” box $18.80 Newark