Cobwebb 1 "Building The Antenna"
Picture Gallery Of Building Cobwebb 1
As you will see from the information on this Site I enjoy experimenting and building simple antennas. Don't know how I first came across the Cobwebb Antenna but somehow managed to find my way to the Site of G6PJE Dave.
At the time Dave and others were experimenting with this type of antenna based on the principle that the 5 elements were in fact a set of folded dipoles connected to a common feed point. Having sparked my interest I set off to build my first cobwebb but with a slight twist in the tail, I was sure I could construct the antenna with foldable arms (spreaders), thus the chance to take a set of full size dipoles out and about portable. See my Isle Of Arran page to see how this was put to good use.
Basic components :-
4 x 3m fibreglass fishing poles, EBay (spratreader)
2 x 10" sqr aluminium plates, scrounged via a friend
Some odd lengths of 1 1/4" aluminium box section salvaged from an old 10m beam
100m roll of 6A twin power lead, Maplins
A strip of 30 amp choc block connectors, Screwfix
Small rigid plastic box, formerly held sea fishing hooks.
M6 SS bolts (55mm), washers and nuts, Screwfix
Loads of cable ties, Screwfix
5 x hinges, B&Q, probably most expensive item, robbing sods.
PL259 chassis socket.
As you can see I didn't break the bank putting this lot together and in total it probably cost me between £30 and £40 to build. In addition there was of course, time, patience and a few basic tools. I also use an MFJ259 antenna analyser which if your doing this sort of work is invaluable.
Construction :-
The construction followed the basic steps shown in the pictures you can see if you follow the link above or click here. I built the main body of the antenna first before moving on to the feed point and then the elements. The elements are the bits that seem to confuse but as noted these are basically single pieces of twin lead, shorted at each end and with one core cut in the middle to form the feed point.
Once ready to put together I used cable ties to hold the elements to the spreaders. I wanted to be able to adjust the elements but soon found that cable ties alone tended to slip. I then added small hose clamps to hold the elements more securely but still be able to move them if needed, this worked ok as long as you didn't over tighten and crush the fishing rods. I found the fishing rods easy to use but a little on the flimsy side when you've got all the elements on. As you can also see in the pictures the hinges worked ok so it wasn't long until the antenna was ready for tuning, now the funs begins !!!!
The initial readings were taken using a MFJ 259 antenna analyser connected via 20M of RG 58 with a 6 turn X 6” dia air core balun. The antenna was mounted on a short pole 4’ from the ground.
Tuning note convention to readings ( frequency / radiation resistance / reactance / SWR )
To raise the resonant frequency removed 4” from both ends of the element. = (14.240 / 49 / 44 / 2.3), SWR 1.0 @ 14.03
Checked the other bands and noted no real change on 10/12/15 but SWR lower on 17M
Removed a further 1.5” from both ends = (14.240/45/26/1.7) SWR 1.0 @ 14.123
Made 2 further adjustments removing an additional 1.75” from both ends ( total removed 7.25”), now (14.240/44/8/1.2) with SWR 1.0 at 14.210
This now gave me a resonant point within 4 of the 5 bands with 10M being the most variable so decided to leave this band for now.
Readings on 15M
No real change to 12/17 so question was now how to reduce the RR on 12 and the RR+X on 17 ??
At this point information from Dave’s website (G6PJE) indicated that the correct configuration for the cobwebb was actually a set of 5 dipoles each with their own T match as opposed to a set of folded elements.
T Match principle.
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.
The dimension above make no allowance for fine tuning and additional length should be allowed.
Based on these dimensions I set about trying to get the antenna tuned on all bands. The process was a bit hit and miss, pushing pins through the wire to short at different points. Very difficult to achieve good results and after hours of trying finally gave up and went back to the original folded dipole principle. I was under a bit of time pressure to get the antenna ready for the forthcoming trip to Arran or maybe would have persevered longer with the tuning.
PLEASE NOTE IF NOT CLEAR FROM ABOVE I DID NOT USE THE T MATCH ARRANGEMENT FOR THIS FIRST ATTEMPT
Without the t Match I found that the biggest changes could be made by altering the ends of the elements in terms of the length folded back and/or if the elements were shorted or not.
A final match, with an SWR close to 1:1, was achieved on all bands except 10M. This was done by varying the element length and shorting / opening the element ends. The following shows my final configuration of which elements were shorted / open.
10M = Open
12M = Short
15M = Open
17M =- Short
20M = Open
I found major interaction between the bands in particular trying to make any adjustments on 10M seemed to completely destroy the match on all other bands. As such I decided that 4 / 5 wasn’t bad and left 10M for a later day.
See Isle Of Arran Trip for details of the this antenna on the air
See Cobwebb 2 for details of version 2 built from scratch on the T match principle.