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Amateur Radio

Antenna project

Portable Cobweb

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I built a portable version of a Cobweb antenna, it is based on a design by Stephen Webb G3TPW and my adaptation of the idea is to make it so that it can be collapsed easily and when required re-assembled and mounted hopefully on a telescopic pole.

its was a project I complete over the course of a weekend or two. I got it finished before the August 2010 bank holiday weekend and then I gave it a good bit of testing.

Cobweb Antenna opened up (without wire) with radials still not extended

Folded position (without wire)

  Basic layout on mounting plate         Close up of mounting plate

The loops are terminated ready for the next stage of assembly

The Co-axial choke balun was wound on a 4” former and hot melt glued to retain shape

The balun was soldered to the jumper pins, note they are not shorting to the link below.

The balun fit’s comfortably in the case prior to testing and final assembly.

The wires are going on now, the connection box is mounted on some 40 mm wastepipe and is free to slide up and down the support to find it’s own position.

Mounted on some 40 mm waste the connection box will be fixed in position after all the final measurements are done.

Well this was the way the cobweb ended up after my Saturday session but there are a few adjustments to do before its ready to test on the antenna analyser.

Here it is all folded up ready for putting away in my shed until Sunday when with a bit of luck I will be able to finish it off, I have noticed that the 20M loop is a bit long and I will double check the positioning of the loop corners before they are permanently attached to the poles. all in all it has been a very constructive day.

Sunday I started to check out the SWR after temporally pinning the tapping points,the readings where quite variable and I think I need to get it mounted a few metres off the ground before making any permanent adjustment. looks like it will be next week before its finished but it will be worth it. I managed one QSO with AO8GLH a lighthouse station on Gomera Island, there is a noticeable drop in background noise with this antenna so I can’t wait until its finally up and working properly.

Well it’s bank holiday weekend and I have decided to review the measurements for the tapping points so I removed all the cable ties and measured all the tapping points from the junction box and to the original points that I pinned they were pretty close, at the most about one cm out. I stripped and twisted the two wires and soldered them to make the connections electrically sound, after they were all done I completed each joint with self amalgamating tape.

I took a different approach to positioning the wires on the supports, the first thing I did was to make sure the gap between the ends of the individual elements was fixed at7.5cm as this appears to be the key to getting the SWR correct. I then repositioned the wires on the supports to get the correct location for fixing in a permanent position, taking up the slack in the wires by spacing them out across the supports.

After the wires were mounted to my satisfaction I took some SWR readings with my analyser and found a big improvement in the results.

Cobweb antenna in an improvised test position so that I can get some better SWR readings, it’s still looking a bit untidy until the final positions are achieved.The mounting clamp is fitted using wing nuts to make the pole assembly easier.

The table below shows how the antenna is performing at present Freq                    RΩ                 ReactanceX          SWR

10m 28.360mhz     70Ω                     34                1.93

12m 24.950mhz   151Ω                      0                 3.02

15m 21.285mhz     46Ω                      0                 1.06

17m 18.130mhz     33Ω                    50                 3.38

20m 14.285mhz     47Ω                      9                 1.21

Obviously the 12m and 17m bands need some further investigation but I am happy with my first attempt at this home-brew project and with a bit of patience I should be able to finish it very soon, needless to say I have had a few QSO’s on this antenna already with surprising results

Well The project has finally come to fruition and I have to say that I am really proud of the achievement, I have successfully produced a fully portable cobweb antenna. The whole project has only cost £60 in parts.

Most of the Items were sourced from EBay, Maplin and local DIY stores and the cobweb now stands at 4.87m and is performing excellently standing next to it in the photograph is John 2W0OSG who has helped me in the final stages of assembly with the tripod and mast.

I took the Cobweb up our nearest high point which is Minera Mountain to see how well it performed in the field and both John 2W0OSG and myself were really pleased with the performance.

It was a good test of the stability of the assembly and of the operational performance of the antenna, we found that we could hear stations well beyond the capabilities of our home based antenna’s. the only thing we were lacking was output power from the Yaesu FT-817 which at 5 watts was not really suitable.

This antenna can be assembled and dismantled in approximately 5 min's and is small enough to be transported in my Peugeot 307 for Dx work, the next time we get up the mountain we will use my FT-857D and what results we get then.