The main advantages of the CobWebb over other 5 band 14 to 28 MHz antennas are that it is small, lightweight, strong (made from fibre glass), only requires a single support, needs no rotator, gives full size dipole performance on all 5 HF bands (without the end on nulls that straight dipoles suffer from), is fed by a single 50 ohm co-ax cable (via an in built air core choke balun), and most important of all, it produces a pure horizontally polarised signal with a confined electric field. This results in much reduced coupling to nearby conductors, so that losses and interference problems are reduced to the absolute minimum possible.
The CobWebb can be easily mounted on a single 20 foot aluminium scaffold pole, fixed to the wall of the house with a couple of stand off wall brackets. The pole can then be pushed upwards to put the CobWebb up at 30 to 35 feet, or lowered so that it is at roof ridge height, to overcome any possible planning problems!
All the other small multi-band commercial antennas that are available are less than optimum. The “broad band” folded dipoles and verticals have lossy transformers or terminating networks in them. These may provide a fairly low SWR over the full 2 to 30 MHz band, but so does a dummy load! Some trap dipoles have lossy traps and baluns, so that a nice 50 ohm match is produced on all bands. Others have no balun at all, the feeder cable will then need to be a particular length to reduce feeder radiation and provide a match. Doublet Antennas may have some gain on the higher frequencies, unfortunately every 3 dB peak has a 20 dB null in other directions! Other small “magic maths” antennas are available, but their performance is so poor that you can’t even hear them on the bands, to measure how far down they are! The many types of vertical antennas are best avoided, unless you have no neighbours, because of all the interference problems.
The inherent losses and poor radiation efficiency of the mini-beams negates any so-called gain. Remember that if a beam has a gain of 4 dB over a dipole in free space, it will be DOWN on a dipole! It will also only “work” over a very narrow frequency range. A dipole or a CobWebb has a gain of about 5dB over a dipole in free space, (due to ground reflection) or 7dB over an isotropic radiator (7dBi) and will work equally well over the entire band. The CobWebb has no lossy components in it, so there are no power limit problems for the QRO operators. The low loss and consequent high radiation efficiency also makes it ideal for QRP!
The CobWebb is available directly from the designer for only £279, plus £15 P&P for delivery to GB and NI. Other countries at cost. (See "How Do I Obtain a CobWebb?" page) It can be paid for by cash, postal orders, or cheque made payable to S. R.Webb. Direct Bank transfers can be arranged for overseas customers.
Steve Webb, G3TPW,
House, The Green, Swinton, MALTON, North Yorkshire, YO17 6SY.
Telephone Malton (01653) 697513 International +44 1653 697513
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