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Author Topic: Cobweb antenna  (Read 5846 times)
2E1CLS
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Posts: 23




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« on: December 17, 2011, 02:42:44 AM »

Hi, I am about to start building a cobweb once I get the rest of the bits together, I don't have a antenna analyser so i'm wondering if anyone with experience with them can give me any tips? Such as spacing and how far out the junction box should be on the leg? That kind of thing.

Thanks Carl.
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G3TXQ
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Posts: 1532




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« Reply #1 on: December 17, 2011, 04:45:24 AM »

Carl,

One useful indicator to the position of the junction box is that the 17m elements usually end up completely square (or very close to it).

I have some information on my web site about a design that uses single wires for the elements instead of the folded dipoles used on the original CobWebb:
http://www.karinya.net/g3txq/cobweb/

Click on the top right photo for an enlarged version, and you'll see what I mean about the 17m elements being square.

73,
Steve G3TXQ
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2E1CLS
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« Reply #2 on: December 17, 2011, 05:59:54 AM »

A very interesting read there Steve thanks. On the original cobweb I see some break into the twin wire halfway down each leg and twist and solder it together but others on the web simply join the ends of each leg together, Is there any reason for this change?
The Idea of a single wire does look good though.

Thanks Carl.
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G3TXQ
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Posts: 1532




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« Reply #3 on: December 17, 2011, 06:16:33 AM »

Carl,

When you bend a half-wave dipole nearly into a square its feedpoint impedance drops dramatically: typically to around 12 Ohms. To overcome that problem, the original CobWebb employed 2-wire folded dipole elements that multiplied the impedance by 4 - giving a good match to 50 Ohm coax.

When you analyse a 2-wire folded dipole you find that it doesn't just comprise an extra wire - you have additionally created two short-circuit stubs in series with each other and in parallel with the feedpoint. Often that's not an issue, but when you construct the antenna with typical "figure-8" twin wire, the combination of low velocity factor and low characteristic impedance places quite a low value shunt impedance across the feedpoint. I have a web page dedicated to the details:
http://www.karinya.net/g3txq/folded_dipole/

The solution is to place a shorting link inboard from the antenna ends at the point where these stubs are exactly an electrical quarter-wavelength long. Depending on the type of twin wire you use and its transmission-line characteristics, sometimes you can "get away" with leaving the shorting links at the ends, other times you will need them much close to the centre of the dipole. The reason you may find so many dimension variations on the web is that folk are using different types of wire and the optimum position will be in different places.

This uncertainty was why I developed the alternative design which uses a single wire and employs a 1:4 balun to transform the 12 Ohm impedance close to 50 Ohms. The advantage is easier tuning; the penalty is having to wind a 1:4 balun!

73,
Steve G3TXQ
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WB6BYU
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« Reply #4 on: December 17, 2011, 08:09:25 AM »

Quote from: G3TXQ

...The reason you may find so many dimension variations on the web is that folk are using different types of wire and the optimum position will be in different places.



And also because a lot of hams build antennas that aren't actually optimum because
they don't understand how their design is different from what someone else did. 
They get OK performance from it, and post it on the Web as the "right" way to build
it.

There is no "one right way" to build most antennas - you have to understand the
theory behind the design you are using to know if something needs to be changed due
to a difference in the materials, etc.

In this particular case, you probably wouldn't need the short if you were using open
wire line because the velocity factor is close to 1.0.  It is a good idea with 300 ohm
ribbon cable that has a velocity factor around 0.8, but you probably could find some
sort of length adjustment that would get you close enough without it.
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G3TXQ
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Posts: 1532




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« Reply #5 on: December 17, 2011, 08:39:00 AM »

In this particular case, you probably wouldn't need the short if you were using open
wire line because the velocity factor is close to 1.0.  It is a good idea with 300 ohm
ribbon cable that has a velocity factor around 0.8, but you probably could find some
sort of length adjustment that would get you close enough without it.
Dale,

That's right, but when you build it with Figure-8 twin with Vf around 0.7 and Zo around 100 - as many people over here do - you really do need the shorting links inboard.

73,
Steve G3TXQ
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2E1CLS
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Posts: 23




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« Reply #6 on: December 17, 2011, 09:05:40 AM »

Ok I ordered 50m of Figure-8 from ebay already but I am now convinced (especially without a analyser) that the 4.1 balun and single wire is the way forward! So looks like I will be buying extra cable lol. The main reason is Because The cable I bought is 42 strand with 0.15mm each but the pdf I am going off says 42 strand 0.20mm so clearly there will be a difference between the shorting distance due to the cable size difference.

Thanks guys Carl M6CLS.
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KD2CJJ
Member

Posts: 369




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« Reply #7 on: August 15, 2012, 06:00:35 PM »

How did the antenna turn out?


Ok I ordered 50m of Figure-8 from ebay already but I am now convinced (especially without a analyser) that the 4.1 balun and single wire is the way forward! So looks like I will be buying extra cable lol. The main reason is Because The cable I bought is 42 strand with 0.15mm each but the pdf I am going off says 42 strand 0.20mm so clearly there will be a difference between the shorting distance due to the cable size difference.

Thanks guys Carl M6CLS.
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73

Mike
KD2CJJ
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