Amateur Radio (G3TXQ) - Cobweb Antenna
During recent range testing of a hexbeam I needed a horizontally-polarised resonant antenna covering 20m thru 10m which could be assembled and disassembled quickly and easily. The cobweb antenna meets the requirement nicely - it comprises 5 parallel dipoles bent into a square shape and mounted on a support structure made of fibreglass poles. The turn radius is a very compact 6.25ft. Computer modelling of this arrangement shows that one result of bending the dipoles is that the antenna is close to being omnidirectional in azimuth, with a Free Space eccentricity of about 4dB; a less attractive result is that the feedpoint impedance drops to around 12 ohms at resonance.
The commercial CobWebb antenna marketed by Steve Webb (G3TPW) - and most of the home-made derivitives - overcome this problem by using folded-dipoles constructed from Figure-8 twin lead as the elements. This raises the feedpoint impedance by a factor of 4 and provides a good match to 50 ohm feedline; the feedline is usually coiled to form an air-cored common-mode choke close to the feedpoint.
However, the folded dipole approach to matching has some disadvantages:
- The use of twin lead adds to the weight of the antenna
- A shorting link has to be placed at a position along the folded dipole, determined by the differential-mode velocity-factor of the cable. Variations in cable insulation can mean that the dimensions which work for one constructor may not work for another.
- Ideally, the shorting link position should be adjusted as part of the initial antenna tuning - something which is not easy to do. The folded dipole page on this web site looks in depth at the postioning of the shorting link.
- The "scramble-wound" air-cored balun often installed within the terminal box may not provide much choking impedance unless the dimensions and winding style are carefully controlled
- The use of twin lead makes it a little more difficult to assemble and disassemble the antenna if, as I do, you prefer to store the wires separately
The alternative arrangement I used was to combine the impedance transformation and balun functions into a 1:4 Guanella current balun mounted in the terminal box. This allows single-wire elements to be used, which simplifies assembly and tuning. The 1:4 balun comprises two 1:1 baluns wound with 8 turns of coax on a FT 140-61 ferrite toroid; the two baluns are then connected in series at the 50 ohm feedline end, and in parallel at the antenna end. The coax needs to have a characteristic impedance of 25 ohms - something which is not easily available; so each balun is actually wound with two 50 ohm coaxes which are connected in parallel at each end - inner to inner, braid to braid. Click on the photos on the right for enlarged pictures.
I measured the loss of the Guanella balun between 14MHz and 30MHz - it varied between 0.1dB and 0.2dB. When terminated with a 12.5Ω load the input VSWR varied from 1.06:1 at 14MHz to 1.1:1 at 28MHz.
The support structure is constructed from fibreglass tubing, and the wires are held in place by threading them through a tie-wrap loop which is itself tie-wrapped to the tubing; a stainless sheet metal screw driven into the tubing stops any tendency for the loops to slide inwards. The wires are terminated in solder tags which fit over 4mm bolts screwed through the terminal box. The ends of the dipole elements are connected to spacer cords (approx 6" long) using the brass inserts from plastic connector blocks; this arrangement allows any slack to be taken up easily by releasing the srews and sliding the wires through the inserts. The outer 66" of each fibreglass spreader slots into larger diameter tubing forming the central hub - the wire tension keeps them in place; this arrangement allows the antenna to be assembled and broken down very quickly. The following photos show the general idea:
The following table lists the wire dimensions that were used each side of the feedpoint, the distance from the centre of the structure to the wire corner, the minimum VSWR, the 2:1 VSWR bandwidth, and the 3:1 VSWR bandwidth, for each of the five bands. Please note that the wire used was PVC insulated with an outside diameter of 2mm and a conductor diameter of 1mm; dimensions will vary slightly if you use different wire.
| 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 |