▲The Cobra Antenna Is A Folded Dipole
The original Cobra antenna was designed by W4JOH and it’s still being used in the amateur radio cosmopolitan universe. The 80 meter version is considered the standard, simply because it covers nearly all of the bands, and it covers them well.
Pertaining to the uber popular 80 meter band, which is a night time band designated to restless night owls, the Cobra is definitely a winner. The Cobra antenna is long lasting, versatile, requires little maintenance, and is a very consistent performer. You don’t need a half-dozen telephone poles and an instruction booklet to get going.
The Cobra’s weakness is on 160 meters; it’s a bit difficult to tune by nature. A full wave loop would probably fare better on the 160 meter band.
W4JOH used the Romex cable for the elements. Some have successfully tried rotor cable: RadioShack 100-Ft. (30.4) Three-Conductor Antenna Rotator Cable. It’s lighter, less cumbersome, and nearly maintenance free. Awesome!
If you’re running an internal automatic tuner then a balun is probably the best bet: MFJ Enterprises Original MFJ-913 Balun 4:1 Current 300 Watts.
The traditional folded dipole is a half-wave dipole with an additional wire connecting its two ends. The folded dipole has a wider bandwidth than a single dipole. This is one of its strong points.
THE 80 METER COBRA ANTENNA DETAILS
1. Each leg is 210 feet per side, and this is 420 feet total across the top of the antenna.
2. The stub is around 100 feet in length (75 to 120 ft is recommended), using 450 ohm ladder line: 100′ 18 AWG SOLID 450 OHM LADDER LINE ANTENNA WIRE.
3. Width of antenna runs 140 feet total—from end to end. The 40 meter version is half of that.
4. The 3-conductor wire is connected in series per side and is folded back, or stacked on one another.
5. The Cobra antenna uses standard insulated wire of 14 gauge and is fed with 450 ohm ladder line. Any gauge can be used, and even the 20 gauge wire may be preferred because it’s lighter in weight.
6. A 4:1 balun should be placed at the end of the feedline, and one could run 50 ohm coaxial cable into the shack. An experienced ham operator may run the 450 ladder line to the shack and into an external tuner. Some might be hesitant to bring ladder line into the shack because of rf (radio frequency).
(Hint: Some external tuners supply a built-in balun. Palstar is an option.)
Editor’s Note: The Radio Shack 100-Ft. Rotator Control Cable or similar is the way to go if you can find it. Amazon is probably the best option.
THE 40 METER COBRA ANTENNA
1. Cobra antenna 40 meter version is—105 feet per leg—210 feet total—50 feet for the stub.
2. Width of antenna runs 70 feet total—from end to end.
Analysis
1. The Cobra Folded Dipole is 1477/F (estimated).
2. This is 1.5 wavelength (estimated).
3. Original formula — 210 / 3.750 (band center) = 56 (unknown multiplier)
More:
1. The Cobra UltraLite — The Alternative Multi-band Solution
2. RadioShack 100-Ft. (30.4) Three-Conductor Antenna Rotator Cable
3. RCA VH127N Antenna Rotator Cable
4. 4:1 BALUN 160-10 METERS UNADILLA W2AU
5. 4:1 Balun Jetstream
