Antenna plans that don't make any sense.

The plan is to construct a portable cobwebby antenna.

Looking over and attempting to construct plans found on the net.


Constructed the hub. Have the fiberglass rods attached, so now to build something.


...Ok this looks feasible.

http://mw0mxt.co.uk/lars/archive/cobweb.pdf

Started by cutting the 20m antenna section. Plans state the "double" wire (folded dipole loop) should at be total of 34 feet long. Shorted at the ends, with exactly a 3 inch gap at the secured ends and one of the two wires at the feed point measured at the 17' mark (half of the total 34 feet) which is where you are supposed to attach the feedline. So I cut all the wire and make the wire exactly as stated. ...Sounds easy right?

The drawing also states the wire position should be mounted 5'9" away from the center of the spreader. So I measure exactly 5'9" and attach clamps on each spreader to accommodate the wire according to the plans.

Well I start hanging all the wire on the clamps at the 5'9" mark, and the wire sag horribly. The whole thing just sags like the antenna wire is cut 6 feet too long, ...but yet it isn't?!

OK, so I take sure measurements, do some math and find they all fit nice at exactly 6.0" instead. ....But why do the drawing plans state the 34' of wire will fit exactly at 5'9" mark then, if they clearly don't? ...Not even close!?

So I start comparing a few other articles and find other anomolties....

Calculations tell me that a 34' folded dipole should resonate around 20m. (which is what SOME but not all of the plans seem to suggest)

Incidentally 34' is 408 inches. The original cobweb design is here:

Another cobweb design (the original) also uses a 2 wire setup for each dipole leg but the dimensions seems radically different though.. http://www.g3tpw.co.uk/

The original design calls for the 20 meter section to be 5139mm or 201.9 inches..?!?! compared to the other design I tried which is 408 inches. So is that supposed to resonate on 20m ??

?!?!

Seems none of the cobweb plan designs correlate in any way with one another. But they are all supposed to work ....right?

Some contain erroneous information about measurements, and they all seem to lack the "correct" information about where to place wires on each of the spreaders exactly. Even though the wires when cut to the suggested dimensions, they don't fit on the spreaders at the point the plans indicate.

Here: http://mw0mxt.co.uk/lars/archive/cobweb.pdf

Other cobweb plan contradictions...

Some plans suggest the ends should be shorted while other ends on the same antenna should be left open...... (How is it a folded dipole if the ends are left open?)
10M = Open
12M = Short
15M = Open
17M =- Short
20M = Open

See it here: http://domain1809176.sites.streamlinedns.co.uk/cobwebb1_build.htm

Some plans suggest finding a center sweet spot point in the wire and shorting it out to obtain a good match and others say, with a folded dipole it isn't necessary to do that.

http://2.bp.blogspot.com/_ZSFuWiwjvpo/SCURg4d3_pI/AAAAAAAABDo/hhxWq26z-a0/s1600-h/Diagram.jpg

Other designs have a folded dipole on one side of the antenna, and just a straight wire on the other side.. etc etc..

...If you're not confused yet, compare these following diagrams with one another for example:

http://2.bp.blogspot.com/_ZSFuWiwjvpo/SCURg4d3_pI/AAAAAAAABDo/hhxWq26z-a0/s1600-h/Diagram.jpg

http://homepage.ntlworld.com/andymuza/G7VOT/Cobweb/Cobweb1.html

http://homepage.ntlworld.com/andymuza/G7VOT/Cobweb/DIAG1.html

??

So will the real Cobbwebb antenna please stand up...

 

Guess I should also mention, the antenna which requires (5139mm) 202 inches for the 20m wire section calls for the spreaders to accommodate this wire length to be 8.5 feet long, (http://www.g3tpw.co.uk/Page3CobWebbSpecification.htm ) This design calls for 9 8 feet (3m) http://domain1809176.sites.streamlinedns.co.uk/cobwebb1_build.htm

While the much longer (34 feet) 408 inch, 20m antenna on this design only requires the spreaders to be just 5'9" long to hold the 20m band wire. (http://homepage.ntlworld.com/andymuza/G7VOT/Cobweb/Cobweb1.html)

So I guess this means the longer the wire you are using, the shorter the spreaders need to be right?


??

 

Charles, I never built a Cobbwebb so can't speak from experience at all; however based on your findings, it sounds to me like a 160 meter version could fit in your pocket.

 

Here try these directions.
http://test.g0mtd.co.uk/CB1.pdf

from this site
http://domain1809176.sites.streamlinedns.co.uk/cobwebb_features.htm

also check
http://www.karinya.net/g3txq/cobweb/

 

(5139mm) 202 inches Thats each leg.

Looks like that 5'9" is off. Even with the feed pointmore toward the center shortening the wire. The corner should be 6' 3 "
All this (shorted, open, shorted at midpoint) is the wire used. Some plans are using speaker wire. Some are using 300 ohm window line
Looks to the most consistent would be the window line. Even though that particular plan seems to have bad dimensions.

Also take a look here goes into the design of the antenna pretty well.
http://www.karinya.net/g3txq/cobweb/

He also uses a 1:4 12.5 to 50 ohm current balun for matching

 

Yes 202" inches seem consistent for each side however if you look at the original G3TPW design "cobwebb revealed .pdf" on page 4 it indicates the length for the 20m wire is 5130mm and that's NOT each leg... but rather the diagram represents the entire antenna size! You will also note one of the wires in that diagram has a break in the center at the feed point in the photo. This break is normal, just not the dimensions.

I agree the 20m wire position on the spreader (5'9" position) is not right as indicated here:





I think the wire position for the 20m band on the spreaders indicated by G3TXQ is dead on (6.02 feet) and is actually consistent with my own findings in the real world when constructing it out of 34' of "double wire" shorted out at the ends in the following fashion:



The 34' folded dipole loop (408") as shown above is electrically longer and shorted at the ends. This might end up too low for the 20m band, but I guess that might depend on the velocity factor of the wire I am using.

You have suggested the total length is 404" in the original G3TPW design if we correct the diagram to represent a single side. However it is not shorted at the ends and requires "shorting" points in the wire. This presents a problem if you are not using the exact same wire they are using in the design. (Speaker wire in England might be different? 42 strands of uncertain wire gage with a PVC covering?)

Incidentally, some are using TV window line and other are using speaker wire. I am making it very lightweight so I am trying antenna rotor ribbon cable. Speaker wire will probably provide more bandwidth, power handling and of course more weight.

Hmmm.

However, I am giving G3TXQ's design some thought. Not sure which design is superior (or accurate) at this point. I am thinking a folded dipole design is twice the size and length of the G3TXQ's single wire design. ( although they are physically around the same size) I wonder how this shorter wire length (1/2 the electrical size compared to a folded dipole) affects performance by comparison to one another. For example a folded dipole design might have better bandwidth because 2 wires are bigger than one?

 

You mean this from page 4


Uploaded with ImageShack.us

 

Yes!! Exactly.

The diagram on page 4 indicates the whole thing should be 5130mm. (202") If you look at the center of the diagram, you will note it shows the correct "break" in the center at the feed point in the photo which suggests this must be the entire 20m antenna length.

What it should indicate is to cut the whole thing 10260mm to avoid confusion. But it doesn't.

5130 x 2 = 10260mm

10260mm = 404"

The folded dipole design I am working with (closed ends and no shorting points) specifies 408" (34')

 

Ok so 5130mm on the left. 0 in the middle. 5130 on the right. So 5130+5130=10260 mm or 404 inches or 33 feet.

 

Gee, 33ft... that's about right for the 40m band isn't it? lol

Ok, so 16.5' x 2 is a halfwave dipole on 20m. Sounds about right then.

468/14.200 = 32.957 or thereabouts

My current design is 408" or 34 feet:

468/13.750 34.03 feet.

13.750 MHz? .. hmm wonder if the velocity factor of the wire I am using will change things and what direction?

I should mention the majority of the designs are specifying 34' Whether they are right or not will be seen when I connect the contraption to the rig tomorrow and report my findings.