Re: [TowerTalk] CobWebb antennas - success stories?

[Jim Lux <jimlux@earthlink.net>]

At 05:58 PM 9/22/2004 -0400, jsb@digistar.com wrote:

> Anyone have any success stories regarding the CobWebb antenna?
>
> http://www.g3tpw.ukgateway.net/
>
>
> I'd like to get one but it's about $500 to get one here after the exchange
> rate and shipping.  Just wondering if anyone had tried one of these before
> and had any opinions to share...  or if there's a better list to move the
> thread to i'd be happy to take it there.
>
>
> 73 Jason N1SU

Did a quick model of a dipole folded into a loop, like the Cobweb..

at 14.2 MHz, 2mm diameter elements (i.e. AWG 10 wire), 2.67 m long sides, 
with a 10cm gap on the side opposite the feed.

The feedpoint impedance is 10 -1j ohms (A bit of fiddling would get the 
reactive component down to zero)

In free space, with no structure loss, the antenna has 1.26 dBi gain 
(compare to 2.15 dBi for a dipole), down about -4 dB off the sides.  In the 
vertical direction, the gain is down about 0.9 dB.

If I change the elements to lossy ones (Aluminum 3.7E7 conductivity), the 
feed point impedance changes to 11+.12j ohms, and the efficiency is about 
90%. (so the gain drops by a fraction of a dB to 0.81 dBi)

It's pretty narrow band, by the way.. 2:1 VSWR bandwidth is about 150 kHz 
for the 20m version.  Compare to a aluminum wire dipole cut for the same 
frequency, which has a 2:1 bandwidth of 1.2 MHz (fed from 67 ohms) and an 
efficiency of 99.5%

So... overall, not a bad design, especially if you're looking for something 
that will fit where a multiband dipole won't, or if you want 
omnidirectional radiation.

here's the model..
CM Horizontal Loop bent into a square
CM generically like the G3TPW Cobweb
CM Parameters
CM LS = Side length
CM GAP = gap length opposite feed point
CM HT = Height above ground
CM WR = Wire Radius
CE
SY ls = 2.6664
SY gap=.1
SY ht=50/3.28
SY wr=0.001
GW      101     20      -LS/2   -LS/2   HT      -LS/2   -0.10   HT      WR
GW  102  20  -LS/2   LS/2   HT   -LS/2   0.10 HT   WR
GW  109  3   -LS/2  -0.10   HT   -LS/2   0.10 HT WR
GW  201  20  -LS/2    -LS/2  HT    LS/2 -LS/2 HT   WR
GW  301  20  -LS/2   LS/2  HT   LS/2 LS/2 HT   WR
GW  401  20   LS/2    LS/2  HT   LS/2 GAP/2  HT   WR
GW  402  20   LS/2    -LS/2   HT  LS/2 -GAP/2 HT   WR

GE  0
LD      5       0       0       0       3.7E7
GN      -1
FR      0       1       0       0       14.2    0.050000
EX  0   109   2   0  1.00E+00  0.00E+00  0.00E+00  0.00E+00 0.00E+00  0.00E+00
RP  0   73   61 1001 -90       0         2.5       6
EN

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