What is a CobWebb antenna and what is it’s gain?
It is a very efficient horizontally polarised omni-directional antenna for the 14, 18, 21, 24 and 28 MHz bands, with a gain of 7 dB over an isotropic radiator (7dBi). Note that this is the same as a standard dipole, although a straight dipole will have sharp nulls off each end. A standard straight dipole has a gain of 5 dB over a dipole in free space! I notice that the specification of my standard large 3 element tribander says that it has a gain of 8.5 dB over a dipole in free space. This means that it has a gain of 3.5 dB over a dipole at the same height!
How does it work?
Technically the CobWebb is 5 separate full size dipole antennas, each bent into a square. This makes it very small (only about 8 foot square) but it is still full size! It has 5 separate double gamma tee matches to match each element to a common 50 ohm co-axial feeder, and has a built in air cored co-axial choke balun, to prevent feeder radiation.
What is the secret ingredient. Why does the CobWebb work so much better than other antennas?
Simply because it radiates all the power fed into it, and it also radiates in an omni-directional manner so there are no nulls. There are no lossy ferrites, traps or loading coils to heat up, and each element is full size and made from 84 strands of 0.2 mm diameter pure copper wire with a plastic covering.
The confined electric near field (caused by the high impedance ends of each dipole being close to each other) also ensures that the antenna does not couple to other electrical conductors i.e. telephone wires, power cables, television antennas or even the ground and lossy di-electrics such as trees and buildings. Thus the radiated power is not absorbed by nearby objects, it is all radiated into free space. Breakthrough and noise pick up are also reduced to an absolute minimum and the ground conductivity and height do not affect the antenna tuning.
What is the maximum power that can be fed into it?
Nearly all the power fed into the CobWebb will be radiated, so the antenna will not heat up and so limit the power rating. There are no ferrites used so no intermods are produced. It has been tested with 3 kilowatts of RF, above this level there could be a problem with corona discharge sparking at the ends of the elements as the air is ionised!
How is it constructed?
A single horizontal fibre glass cross supports all 5 elements. The feed-box is on the end of another solid fibre glass rod, such that the feed is in the centre of one of the sides of the squares. Each rod is secured with “U” bolts onto a 0.25 inch thick aluminium plate. Another similar plate allows the antenna to be fixed to a vertical mast of up to 2.25 inches in diameter, with the “V” bolts provided. Plastic covered “figure of 8” section 84 strand copper conductor is used for the elements, which are secured to the fibre cross by the unique G3TPW system to prevent pre-mature breakages!
I’ve not seen any of your adverts for ages. Why don’t you advertise the CobWebb more?
The last time we advertised in a magazine was in 1993! The CobWebb was last reviewed in a magazine in 1996 though. It is interesting to note that the magazines always asked us if they could review the CobWebb, we never had to ask them!! Of course the reviews in the magazines were very good adverts, but they only last for a month or two. We find that the 750+ antennas that we have sold are the best long lasting advertisements, literally being broadcast to the world every time somebody says, “And the antenna here is a CobWebb”.
So are your antennas sold purely by word of mouth?
Most of our sales do come from people who have received personal recommendations from existing users. However, the interest is mainly generated when people hear stations using CobWebbs doing so well on the bands. The recommendations are normally received when the enquirer is trying to find out our address/telephone number!!
This is, of course, a scale model made specially for your lectures. How big is the full size antenna?
No, we only have full size CobWebbs! We don’t need a scale model of the CobWebb because it is so small. I must admit that even I am still amazed when I look at a CobWebb and realise that though it is so small IT IS STILL FULL SIZE!! A 33 foot dipole is a much bigger beast than a square with 8 foot 6 inch sides!
Can two CobWebbs be used as a beam?
Yes, but only by phasing them, i.e. they both have to have separate feeders. Parasitic elements will not work because the CobWebb does not couple to other nearby conductors! The maximum gain obtainable is less than 3 dB over a single CobWebb and of course a beam rotator is needed. The phasing adjustments needed to provide optimum directivity across all the bands are quite complex and the phasing unit becomes expensive and difficult to operate. If you increase the antenna height by 50%, then the DX signals will increase by more than they would by phasing 2 antennas!
I’ve heard that different antennas suite different locations. Will the CobWebb work at my QTH?
YES! The ground conductivity
for many wavelengths around an antenna will affect its performance. Vertical
antennas that use a ground connection can be as much as 20 dB down on a
dipole. Verticals that do not use a ground connection i.e. elevated feed
vertical dipoles or ground planes etc. can still be as much as 8 dB down
on a horizontal, due to reflection losses. The difference between sea water
and very poor ground is up to 9 dB for a vertical antenna, but only 1 dB
for a horizontal
The CobWebb is not affected as much as other antennas by trees, buildings, power lines, TV aerials feeders and telephone wires etc. The poorer or more cluttered that your QTH is, the better the CobWebb will perform, compared with other normal ants, including a straight dipole!
These are the reasons why CobWebb antennas so often out perform other antennas, they are not affected by the nearby environment so THEY WORK AT ALL LOCATIONS.
How do I mount it?
The CobWebb can be mounted on a vertical pole of up to 2.25 inches in diameter. It can be added to existing antenna installations such that the mast goes straight through the CobWebb. A 20 foot scaffold pole makes an ideal mast, this can be fixed to a wall with a couple of stand off brackets.
Do I need planning permission for it?
Most people don’t bother to apply, if they are just mounting a CobWebb on a 20 foot pole. The pole can be slid down through the brackets so that the CobWebb is below the roof ridge height. This meets the standard “not above the roof ridge” planning restriction; it can then be gradually pushed up to the optimum height of 33 feet!
Will it stand the high winds at my QTH?
CobWebbs are in regular use in the Falklands and on Ascension Island. They have stood up to gales that have destroyed many other antennas. Most of the breakages that have occurred have happened when either the mast has come down, or the antenna has been dropped. Even then the antenna can be quickly and cheaply fixed because the short 1 inch diameter fibre glass joining tubes break, and the antenna simply folds up. These joining pieces can be easily changed in a just a few minutes. If the wires do get broken they can be repaired with choc block screw connectors or crimp connectors, or we can supply new wires.
How does the CobWebb minimise the chance of TVI and breakthrough problems?
The pure horizontal polarisation and use of a choke balun reduces the chance of breakthrough to the absolute minimum possible. The confined electric near field (caused by the high impedance ends of each dipole being close to each other due to being bent into squares) also ensures that the antenna does not couple to other electrical conductors i.e. telephone wires, power cables or television antennas.
Are there any complicated adjustments to be made during installation?
None at all! The CobWebb is not detuned by nearby objects, including the ground, because of the confined electric field. Thus the CobWebb can be pre-tuned and matched during production, so that it works at any QTH and any height!
Back to front page