I made four attempts to improve the G3TXQ's Cobweb. Here is a short summary of my efforts. First, the antennas views.

G3TXQ's Cobweb

SP3L's Cobweb Mk.1

The real Cobweb Mk.1 I built in summer 2015

SP3L's Cobweb Mk.2

The real Cobweb Mk.2 built by EB5KT in December 2016..

SP3L's Cobweb Mk.3

SP3L's Cobweb Mk.4

And now - the simulation results. Please, pay attention to the SWR values shown in the table in blue frames – these were the results I wanted to improve by changing geometry of the antenna. In the first versions of the modified antenna, the penalty was the gain Gmin measured at the least preferable direction for the Cobweb, that is along the Y axis (see the results in the orange frames). When SWR was improved, Gmin was degraded. I kept trying to improve bandwidth on 15 and 10 m bands without sacrificing too much antenna gain. I think I succeeded, and the Mark 3 and Mark 4 are really worth your attention. 

The Mark 3 seems to be too complicated to gain popularity among the hams. Therefore, I publish only the dimensions of the Mark 4. In this version, I added capacitance hats at the ends of four radiators. Using 2 different types of the cap-hats allowed me to reduce coupling between the neighboring radiators, what very positively influenced antenna bandwidth. One of the cap-hat types is in the form of a letter T, the other one brings to mind a trident. T-type cap-hats can be implemented by adding conducting vertical rods (e.g. brass rods), the trident ones require additionally insulated vertical rods (e.g. nylon rods) and extra non-conducting strings to stretch the wires to the desired shape. The simulated antenna was made of insulated copper wires of radius 1 mm what is roughly 2.5 sq. mm. Find below dimensional drawings. The overall antenna size is 2.7 x 2.7 m (as the original Cobweb). The dimensions with asterisks (*) are to be adjusted during tuning, 

Click here to download these antennas models in the 4nec2 format. The models assume copper wires in insulation.