Cobwebb Calculations

Part #5 of the "Roger Writes" series - August 2023

Background

I recently re-instated by amateur radio license (after 30+ years of non-use!). Thanks to advice from some members of the Newport Amateur Radio Society.
Now that my old Class-B license is recognised as a full license, I'm allowed on the lower frequency bands. I looked around, and got a 2^nd hand Icom IC-706MkII which covers the HF bands, as well as the 50MHz and 144MHz ranges. So I needed an antenna (or two)!

Antenna options

At my house, it's difficult to put antennas externally, so I've been looking at loft based options.

VHF

I started with an old 300ohm ribbon Slim-Jim, but found a cheap X20 dual band (2m/70cm) white stick, so using that for now.

HF

My loft space is 6m x 4.5m at the bottom, 3.5m x 3.5m under the A-frames (avoiding the loft hatch area), so I looked around for options. I found that the 5-band Cobwebb would fit in under the A-frames, but there are a few variants:

Cobwebb variants

There are at least three different antennas that are descibed as Cobweb/Cobwebb antennas.

The original "G3TPW Cobwebb" (with two 'B' on the end)
- By Stephen Webb, Design details and constructions details (Watch out for the typo 4400mm, should be 4040mm - But see later!)
- Uses folded dipoles of low cost speaker cable, and an air-cored choke.
- Has a reasonable bandwidth per band
- It's claimed to have a good omnidirectional radiation pattern, and 5dB gain over a dipole.
The "G3TXQ Cobweb" (with one 'B' on the end)
- By Steve Hunt, Design details
- Uses dipoles on single wire cable, and a matching balun (or two)
- Has a much narrower bandwidth per band
The "300 ohm Cobwebb" (actually a square halo)
- By G7SYW, Design details
- Uses 300ohm ribbon per side, with shorted ends
- Has a reasonable bandwidth per band
- Is actually a folded dipole, but uses the same mechanical construction to the cobweb/cobwebb
- As an alternative to 300 ohm ribbon, these can also be made from the same speaker cable as the Cobwebb.

Note: PA9X has built all three, and compared them.

Cobwebb Design

Although the original Cobwebb antennas are no longer available, I bought a damaged 2^nd hand one.
It's a square folded dipole, with a twists on each side. It uses the open ends to match the impedance to 50ohms. The various loops (one per band) are made from two core speaker cable, the original uses 42-strand, but I have have used 42-strand, 24-stand and a mix, and it doesn't seem to make much difference to the dimensions.

For each band there is a seperate antenna made from 2-core speaker cable. Stretched out they look like this:

Each band is folded into a square, and the feed points are connected together (because the other bands don't resonate they are high impeadance, so no coils or matching is required). (But best to add at least an air cored choke).
The antenna wires are supported on non-conducting diagonal supports (traditionally fibreglass poles, but as I'm loft mounting I have used nylon rope). They are arranged like this:

In the junction box, there are a pair of screwdown terminal blocks (aka "Chocolate blocks"). One wires from each side is connected via a joining wire, the other left and right connections connect to the coax. Coiled up inside the connector box is 6 loops of RG58, as an air-cored choke. This is what mine looks like:

You can see the left to right joining wires between the connector blocks, and the left/right feed lines going from bottom to top (to the coax).

Cobwebb actual dimensions

Based on my (allegedly original) Cobwebb, I measured the following dimensions, but the outer two loops were damaged, so while replacing them I experimented with different lengths and twist points (using sewing pins, through the cores to short them together, and folding the ends over). I found that moving the twist points further out gave better results. Using a VNA you can see the main SWR dips moved by changing the lengths, and smaller dips moved by changing the shorting point (the Smith chart shows the effects of the shorting points more clearly, but with a bit of experimentation, it's easier to estimate how much to move it by from the SWR vs frequency plot).

Mid frequency	Mid wavelength	Half Length	Original Twist length	Better Twist length
14.175MHz	21.15m	511cm/201.2in	274cm/107.9in	332cm/130.7in
18.118MHz	16.55m	398cm/156.7in	224cm/88.2in	260cm/102.2in
21.225MHz	14.12m	347cm/136.6in	211cm/83.1in	222cm/87.3in
24.940MHz	12.02m	294cm/115.7in	180cm/70.9in	189cm/74.3in
28.850MHz	10.39m	256cm/100.8in	158cm/62.2in	163cm/64.2in

With these lengths and using the modified twist points, I measured a per band SWR dip of below 1.5:1 and below 2.5:1 over all bands. It also seems to work well on the 7MHz/40m and 50MHz/6m bands (under 2:1 over both bands).
I've modelled it in 4nec2 , but I've had to move the wires apart in the model, as 4nec2 needs at least 1/1000 wavelength between parallel wires. In the model, the shorting point doesn't seem to make much difference, on the actual antenna you see an SWR null moving with the shorting point, so the model is not perfect, but it's a good starting point for experimentation.

Testing

I used pins to find the best shorting link position:

This is the antenna reassembled, using nylon rope spreaders, strung out for tuning before loft fitting. It's only 75cm above the ground, so I didn't worry about getting it perfect, as things will change when fitted in the loft!

I've used cable ties to hold the antenna wires to the nylon rope supports.

Loft mounted

I've mounted this in the loft, as well as the ground proximity change, the random items around it have significantly changed the tuning, I've tweaked it a bit, and the ATU can pull in all 5 bands, as well as 40m / 6m, so that's good enough for now. The main problem is the local QRM, 40m/20m are S8/S9, but during good conditions, I've worked 1000+ miles on it (with 100W), so it will do for now.

Next?

I've been experimenting with Horizontal Halos and Vs and have built a 40m/20m halo. Based on that, I am looking to re-build the loft based Cobwebb as a hybrid, with 2 or 3 bands using square halo wiring. The square halo uses the same speaker cable loops, but shorts the ends rather than the mid-points, this makes it easier to tune, as there is only the length and gap to adjust. Bandwidth wise I suspect that the Cobwebb is better, so will probably keep the Cobwebb for 10m. The 20m and 17m loops are the most out of tune, so those will be the first ones to change over, in a loft full of random things, easier tuning would be good.

The Roger Writes series

I research / dabble with lots of things, and figured that if I write my notes here, I can quickly reference them, also, sometimes, they are useful to others!
Here is what I have so far:

ATU-100 modifications (RW#12 - February 2024)
Garage door open warning (RW#11 - January 2024)
Horizontal Halos and Vs (RW#10 - December 2023)
Faking Dell PSUs (RW#9 - December 2023)
Designing NEC2 antennas (RW#8 - November 2023)
4NEC2 antenna models (RW#7 - October 2023)
3-element cubical quad for 2m (RW#6 - September 2023)
Cobwebb Antenna dimensions (RW#5 - August 2023)
WS2812 and similar 'Smart' LEDs (RW#4 - January 2023)
Investigation and details of the AFHDS-2A protocol (RW#3 - June 2022)
Changes we've made to our caravan (RW#2 - January 2022)
Basic solar/battery/off-grid calculations (RW#1 - January 2022)
Credit card size walking robot aka Monster-Chan (RW#0 - Originally 2016 - Moved here 2022)
Why you should avoid certain number of drives with ZFS (aka why isn't my ZFS/raidz array the size I expected)
Simple way to connect a Servo to a PC (RS232 port)
Using ZFS with FreeBSD

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This page was lasted updated on Monday, 18-Dec-2023 18:00:33 GMT