cobweb antenna - bay-net · ideal hf antenna efficient transmitter power gets converted to rf, not...
TRANSCRIPT
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
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