"Loading" is a way to lower the (primary) resonant frequency of an antenna radiator. This technique is typically used in antennas that have radiating elements that are too short for the desired resonance frequency. There are several standard ways to load a radiator (ref. 1, 2, 3):
- Inductive loading: placing a loading coil (inductance) somewhere between the feedpoint of the antenna and the tip of the radiator. This compensates for the capacitive feedpoint reactance of the short radiator.
- End-hat loading with a "capacitive hat", typically installed at the tip of the radiating element(s). This counteracts the tapering off of the current distribution between the feedpoint and the tip of the radiator. It also raises the radiation resistance of the antenna, i.e., makes the antenna a more effective radiator.
- Linear loading, by folding a long radiator wire in a zig-zag form onto itself. The result is a radiating element that is three or four times shorter than the overall wire length. The folded wires are parallel and closely spaced. The interaction between the parallel wires is complex, and introduces sub-band resonances (ref. 3C).
- Helical loading, by winding the radiator into the form of a linear spiral.
Obviously these loading methods can be combined. E.g., a loading coil and end-hat loading. Or: a loading coil combined with linear loading.
The "Cobra" is a dipole antenna with linear-loading: the wires are folded such that it is configured as three or four parallel wires. Its name refers to the way the antenna wire "snakes" around in an zigzag manner. 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 a certain Italian manufacturer.
The folded wires can be kept separate and parallel with spreaders, such as used in cage dipoles. It is more convenient to use multi-conductor wire (not to be confused with single-conductor multi-strand wire). In this case, the wires are very close together: they are only separated by the wire insulation. An example of this is ribbon cable, and flat antenna rotor control cable (which is what I used).
Just like any dipole, it can be operated as a multi-band antenna - if it is fed with a twin-lead cable or ladder-line and an antenna tuner/coupler. It is also possible to get multi-band operation with a 4:1 balun at the feedpoint (I use an Elecraft BL1 with a "pig-nose" (2-hole) ferrite core, plus a W2DU-type 1:1 current choke balun) and coax to the transceiver. But you probably still need an antenna tuner at the rig to keep the transmitter happy.
The standard full-size Cobra for 160-10 mtrs has a span of 140 ft (≈43 m). The "shorty", "junior", or "half-size" Cobra for 80-10 mtrs spans only half that: 70 ft (≈21 m). The span is about 1/4 λ in the lowest band. I.e., half the span of a full-size dipole.
MY 40-10 COBRA
At my QTH, I can fit neither a full-size, nor a half-size Cobra. So I decided to try a 1/4-size Cobra for the 40-10 mtr bands. My first version (2008) spanned 2x18 ft (2x5.5 m). This span is about 0.5 λ in the 20 mtr band.
I used 3-conductor rotor control flat-cable (ref. 4). The conductors are AWG #22 (0.64 mm Ø) multi-strand. Adjacent conductors are spaced by 0.1" (2.5 mm). One of the conductors is tinned, which is not good for RF applications. But anyway...
Flat 3-conductor antenna rotor control cable
Center-insulator and end-insulators are the same as for any old dipole antenna. I usually make them out of chunks of polypropylene kitchen cutting board.
In my experience, linear loading of the Cobra makes the antenna appear about 10-20% than its span. Obviously nowhere nearly as long as the total wire length. Conversely, it means that a span can be used that is 10-20% smaller than a regular wire dipole for the same primary resonance frequency.
I found the antenna easier to "tune" (load & match) without a 4:1 balun than with such a balun at the feedpoint of the dipole.
Below is a plot of my miniVNA antenna analyzer:
Sweep from 1.5 to 29 MHz with my miniVNA antenna analyzer
(configuration: antenna + 300 Ω twin-lead + 4:1 balun + 1:1 choke balun)
In 2011, I rebuilt it with the span increased to 2x21 ft (2x6.4 m), in order to move a resonance dip down to 7 MHz:
SWR plot for my Cobra extended to 2x21 ft (2x6.4 m)
In 2011 I acquired an EDX-2 external automatic antenna coupler for my Alinco DX-70TH transceiver. The coupler is located very close to the feedpoint of my dipoles. I no longer use a balun. I did some more experiments with the dipole length. In the mean time, I had reduced the span back to 2x20 ft (2x 6m) - don't ask me why (I just don't remember how or why that happened). Then I extended the end of the Cobra wires by 5 ft (1.5 m). Again, don't ask me why, hihi. The SWR plot below was measured at the end of the short 300 Ω twin-lead to the automatic tuner/coupler:
SWR plot of 2x20 ft (2x6 m) Cobra, extended with 5 ft (1.5 m) wires
Compare that to the SWR plot of a regular wire dipole with the same span, installed at exactly the same place:
SWR plot of a regular wire dipole with the same span, installed at the same place
- Ref. 1: antenna loading
- Ref. 1A: "Loading of short antennas", by Doug Flory (WB6BCN) in "antenneX Online", Issue No. 80, December 2003 [pfd]
- Ref. 1B: "Shortened Dipole Study for Conditions On BVARC’s Rag Chew Net", Larry Jacobson (K5LJ), Rick Hiller (W5RH), expanded from same-title article in "Newsletter of the Brazos Valley Radio Club", September 2009
- Ref. 1C: "Element Loading to Achieve Dipole Resonance", part 3 of "Half-Length Dipoles for 40 Meters", L. B. Cebik (W4RNL, SK)
- Ref. 1D: "Signal/noise-ratio performance of loaded wire antennas", P.A. Ramsdale, Proc. IEE, Vol. 124, No. 10, October 1977, pp. 840-844 [pdf]
- Ref. 1E: "Designing a Shortened Antenna", Luiz Duarte Lopes (CT1EOJ), QST Magazine, October 2003, pp. 28-32 [pdf]
- Ref. 1F: "Practical antennas for the low bands", Rudy Severns (N6LF), presented at the 2007 Seapac hamfest, Seaside/OR, 116 slides. [pdf]
- Ref. 2: "Vertical antennas", by Ulli Weiss (DJ2YA), Chapter 9 in "Low-Band-DXing" by John Devoldere (ON4UN), edt.
- Ref. 3: linear-loaded antennas
- Ref. 3A: "The "Cobra" antenna", by Ray Cook (W4JOH), in "73 Magazine", June 1997, pp. 40-41 [pdf]
- Ref. 3B: "Build the "Cobra" antenna", by Raymond Cook (W4JOH) [pdf]
- Ref. 3C: "The mysterious Cobra", by Rick Littlefield (K1BQT) [pdf]
- Ref. 3D: "NEC model of the Cobra antenna", by Owen Duffy (VK1OD) [pdf]
- Ref. 3E: "The K4VX linear loaded dipole for 7 MHz" [an other form of linearly loaded dipole], by Lew Gordon (K4VX), in "QST", July 2002, pp. 41-42 [pdf]
- Ref. 3F: "The Watson 80Plus2 Antenna", by Julian Moss (G4ILO, SK) [pdf]
- Ref. 3G: "How & why I made an aerial for the 14 MHz, 7MHz & 3.5MHz", by Mark Wooton (M5MKW) [pdf]
- Ref. 4: "100-ft. Rotator Control Cable", Radio Shack catalog nr. 1501150
External links last checked: October 2015
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