The "Cobra" is a multi-band dipole antenna. The antenna wire is folded, such that it has 3 parallel wires. This makes it a so-called linearly loaded antenna. No, the name has nothing to do with the Cobra brand of CB radio products, nor with Coaxial Beam-Rotating Antennas, nor with science- and nature-defying Cobra EH-antennas from Arno Elettronica. It simply refers to the way the antenna wire "snakes" around in an s-shape..
3-conductor wire
I based mine on descriptions from W4JOH and K1JEK, see Build the "Cobra" antenna. I don't have a large back yard. So, rather than constructing the standard 140 or the "shorty" 73 feet version (43 or 22 meter), I made a 36 feet (11 meter) version. I.e., only ¼ size! Actually it has 3x36=108 feet (33 m) total wire length: the antenna wire is 3-conductor. Click here for a write-up on how this antenna supposedly works, and here for a NEC analysis of a Cobra Junior.
Main components:
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2x 5.5 meters (2x 18 feet) "antenna wire": 3-conductor 20 AWG antenna rot(at)or control cable (100 ft (33 m) for $23 (mid-2008) at Radio Shack; catalog nr.15-1150); 2.5 mm (0.1") distance between the centers of adjacent conductors. |
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9 meters (30 feet) of 300 ohm flat twin-lead (100 ft for $19 (mid-2008) at Radio Shack; catalog nr. 15-1175). I ended up with this length more or less by accident: availability of 300 ohm line and the size of my apartment (in my previous apartment, I operated this antenna indoors). Note that the full-scale original design calls for 70-100 feet of 450 ohm ladder line. Maybe the SWR can be improved for the ham bands, by tinkering with the length of this feed line - similar to the ZS6BKW optimization of the G5RV antenna; feedline length of a whole multiple of quarter-wavelength is to be avoided. |
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center and end insulators. Photos below show the ones made by K1JEK. I made similar ones out of scrap plastic ($0). I have weatherized them with a good coat of DipIt/PlastiDip. Hard plastic kitchen cutting boards, from the supermarket, kitchen supply store, or from IKEA, are a good and inexpensive source of material for insulators. |
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I use a 4:1 ferrite-core balun (BL1 balun from Elecraft for $25 mid-2005; replaced by BL2 for $35 + S&H mid-2008 pricing) plus a W2DU-type ferrite-bead choke balun ($17 for two dozen no. 77 beads; Amidon FB-77-6301). Both baluns are placed at the tuner-end of the feed line. I am contemplating replacing the 4:1 Elecraft balun + 1:1 W2DU balun with two W2DU baluns, configured as a VE2VCV 4:1 balun. Supposedly wide-band with lower loss. |
I have tried the antenna on 40-20-17-10 m:
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Easy to tune, like my Slinky® Coil Dipole, but the power/SWR meter says I get about 20-40 watts more than than the 50+ watt output power that I manage to get with my Slinky® coil antenna. Whatever that may mean for power actually radiated by the antenna... |
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So far, this is my favorite home-built multi-band antenna (I don't use commercially built ones, hi)! I have set my personal distance records with this one (10570 km/ 6568 miles with 50 watts, 211 km=131 miles/W) 7838 km / 4871 miles and 90 watts - up from 6350km=3940mi for 50 W or 127km=87mi/W when the antenna was installed inside my previous apartment - all with SSTV). I participated in a Hellschreiber contest during the last weekend of 2006. Using this antenna indoors and 70 watts, I worked 68 stations from France: 64 in Europe (22 countries), 3 in the US (FLA, VA, MA), and 1 in northern Africa (most on 20m, a couple on 40m) - despite QSB. Can't complain about that! |
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Update 5 January 08: tried it once on 17 meters and right away had a nice RTTY QSO with W2V in NY (~6000 km, 3750 miles). First RTTY QSO and first on 17 mtrs. |
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When it rains (the antenna and most of the feed line are now outside), I do need to change the rotary-switch for the inductance of my tuner by one click (of 11). No surprise and no worries. |
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Just for fun, I have tried to tune the antenna without the 4:1 balun: hard to tune, SWR much higher, forward power 30 W, down from 50 W. Looks like the 4:1 balun ratio is about right, hi! |
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Update 26 June 08: just had my first QSO in the 12 mtr band with this antenna (599 BPSK31 with OK4TO). |
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Update 28 June 08: used my miniVNA antenna analyzer to plot the antenna system characteristics; see below. still interpreting... |
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One of these days, I'll try a ⅛ size Cobra, hi! |
The Cobra
dipole is strung east-west at my
apartment
The Cobra
at its current location
My tuner settings for the Cobra
November
2008: replaced "dacron wire + spring" with a shock-absorber
made of a 45 cm (1½ ft) long loop of multi-strand elastic cord (5 mm Ø, 3/16").
Much better!
Hooked my miniVNA antenna analyzer up and obtained the plots below. Still trying to interpret, and figure out how to improve things (in the given space).
Sweep from
1.5 to 29 MHz with my miniVNA antenna analyzer (antenna + twinlead + 4:1 balun +
1:1 balun)
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Articles:
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Cobra antennas (160-10 and 80-10m) are available commercially from Joe, K1JEK, for close to $100 (ex. balun and shipping) and get good reviews.
Homebrew
end insulator (before weatherizing)
Last update: 1-August-2008
©2008 F. Dörenberg N4SPP
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