The G3TPW CobWebb Design

The most important design point about the CobWebb is that it is a completely horizontally polarised, confined electric field antenna, which provides maximum radiation efficiency and the absolute minimum of interference problems. TVI fears have encouraged many people to shy away from the HF bands, in favour of VHF, UHF or 160 metres.

New stations often begin operating on HF using an end fed wire or a vertical antenna, a recipe for disaster, sometimes even on QRP. After starting in this way and getting involved with various EMC problems, it is often found that planning permission for a horizontal antenna is refused. You can’t really blame your neighbour for being concerned. If that small inconspicuous vertical or simple wire antenna causes so much trouble, what would it be like with the proposed mast and special horizontal antenna? The fact that the use of horizontal polarisation, particularly if it is from a confined electric field antenna like the CobWebb, would probably cure the breakthrough problems is very difficult to explain.

During experimental work on a 5 band beam, using full size resonators on each band, it was noticed how well the driven element worked by itself. It also became obvious that a full sized dipole, up in the clear, worked far better than the multi-band minibeams!

The CobWebb is a full size half wave dipole on each of the 5 amateur bands, 14, 18, 21, 24 and 28 MHz, for maximum performance. There are no lossy traps, stubs or loading coils, so there is no reduction of radiation efficiency on any band. Each dipole is bent round to form a horizontal square, to make the antenna omni-directional. Thus no expensive rotator is required. Each resonator looks like the “square halo” that is often used on VHF, for SSB mobile work using horizontal polarisation.

The parallel but anti-phase “sides” of the antenna cancel the radiation that would normally be wasted as high angle radiation from a straight dipole and fill in what would otherwise be the nulls off the straight dipole ends. The resulting omni-directional pattern has many advantages over antennas with directional effects. Unless an antenna with nulls in it’s response can be rotated, it will be found that certain parts of the world will be very difficult to contact.

The five “squares” are made from white PVC covered multi stranded copper twin cable which is supported by a horizontal cross, made from white fibre glass. Each element is folded and tapped for impedance matching, so that the antenna looks like 50 ohms on all 5 bands. The small size ensures minimum windage and the low weight means that TV type brackets and masts may be used for supports. There are no aluminium elements to corrode and cause high resistance joints, or snap off in the wind.

The antenna is fitted with a co-axial choke balun at the common feed point, to prevent any current from flowing down the outside of the co-axial cable feeder. This balun is absolutely vital, to prevent any radiation or pick up of signals by the feeder. EMC problems can be just as bad as with verticals if this balun is not exactly right.

The electric fields of the CobWebb are confined because the high impedance ends of each element are only a few inches apart. This reduces the coupling to nearby objects so the antenna does not need re-tuning for operation at different heights and locations.

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