Cobwebb 2    "Building The Antenna"

Picture Gallery Of Building Cobwebb 2

Back

For my second attempt I followed the details as captured in a pdf file created by Steve Roberts (M0SJR). This file is an excellent source of information containing instructions and measurements for the cobwebb, click HERE to view the file or right click and select "save target as" to download.

Components :-

                           4 x 3m fibreglass fishing poles, EBay (spratreader)
                           2 x 10" sqr aluminium plates, again scrounged via a friend
                           Approx 6m of 1/4" aluminium box section, local stockist
                           The rest of the 100m roll of 6A twin power lead, Maplins
                           A strip of 30 amp choc block connectors, Screwfix
                           Plastic box, Maplins.
                           M6 SS bolts (55mm), washers and nuts, Screwfix
                           Loads of cable ties, Screwfix
                           20 off hose clamps, Halfords
                           PL259 chassis socket.

In comparison to Cobwebb 1 this cost about the same to build as some of the items were left over from the first antenna. Again it was probably less than £50 in total, where else can you get this sort of antenna for such a small outlay.

Construction

The construction followed the basic steps as outlined in the pictures you can see if you follow the link above or click here. This time I concentrated first on building the elements and feed point rather than the mechanical construction.

The T match induces inductance in the antenna which at resonance gives the antenna resistance plus inductive reactance. The reactance is tuned out by shortening the antenna to exhibit some capacative reactance. (resonance is achieved when Xl is equal to but opposite Xc). Through changing the position of the tapping point the correct value of radiation resistance can be achieved at the resonant point. In most cases this is required to be around 50 ohms to present a good match to the impedance of the coax feeder.

The input impedance of the antenna increases as the tapping point distance from the centre is increased up to a maximum that occurs at about 40 – 60% of the element length, after this point the impedance decreases.

I haven’t yet fully understood the relationship between the position of the tapping point, antenna length and final radiation resistance / reactance ( i.e. total impedance) but intend to conduct a few experiments in the future. I’m sure there’s lots of people out there who understand this better than me and if they can explain in layman’s terms I’d be very grateful.

Noted that the dimensions for the 17M elements seemed long. On assembly found that the elements when fixed to the spreaders at the dimensions above were in fact too long and crossed over each other. Adjusted the spacing to accommodate the full element length before tuning ( see comments ).

Note that the element lengths above DO NOT include the loop back, this needs to added.

Initial Readings

Before tuning  I undertook a series of experiments to understand the impact of changing the configuration of the element ends.

Variables :-

1)       Changing the length of the element whilst maintaining the loop back length

2)       Changing the length of the loop back.

3)       Changing the gap between the 2 element ends.

4)       How tight the loop was folded back at the end of the element.

Results :-

Each change made seemed to have an impact on the resonant frequency with little or no effect on the R/X at resonance. The largest change was seen when I changed the length of the element itself.

Reducing the length of the loop back increased the resonant frequency slightly, i.e. the same effect as shortening the element a bit.

Reducing the gap between the element ends moved the resonant frequency lower, i.e. the same effect as lengthening the element.

Tightening the loop at the end of the elements also increased the resonant frequency.

With the above knowledge I could now fine tune the resonant frequency to match that desired.

Tuning

The initial readings and tuning were done using a MFJ 259 antenna analyser connected via 20M of RG58 coax. A 6 turn X 4” dia air core balun is incorporated at the feedpoint. The antenna was mounted on a short pole 5’ from the ground.

A combination of adjustments were made utilising the information noted above to control the resonant frequency. Minor adjustments to element lengths and spacings were required and these are captured in the tables below. As suspected the elements on 17M were found to be significantly longer than required and 410mm was removed from each end to bring the element to resonance within the band.

Unlike Cobwebb 1 the tuning on a particular band could be done independently with changes on one having very little or no overall effect on the other bands.

Final Readings At 5 Foot

Final Dimensions

Distance From Centre Of Antenna To Element

 Spacing And Loopback Length

Element Lengths

The above dimensions are of course based on my experience and shouldn’t be taken as definitive. Slight changes in the way you construct the feed point and balun could changes these. The most significant change worth noting is that seen on 17M, where I believe the original dimension for the element length to be incorrect.

Back