To: | jsb@digistar.com, towertalk@contesting.com |
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Subject: | Re: [TowerTalk] CobWebb antennas - success stories? |
From: | Jim Lux <jimlux@earthlink.net> |
Date: | Wed, 22 Sep 2004 15:45:20 -0700 |
List-post: | <mailto:towertalk@contesting.com> |
At 05:58 PM 9/22/2004 -0400, jsb@digistar.com wrote:Anyone have any success stories regarding the CobWebb antenna? 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 _______________________________________________ See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA. _______________________________________________ TowerTalk mailing list TowerTalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk |
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