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[40-and-up version]     [experimental 80 m expansion]


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

Articles:

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"Build the "Cobra" antenna" by Raymond (W4JOH)

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"The mysterious Cobra" by Rick Littlefield (K1BQT)

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"Detailed Explanation of Cobra Ultralite" by Thomas (K3MOV)

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"NEC model of the Cobra antenna" by Owen (VK1OD)

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"The K4VX linear loaded dipole for 7 MHz" by Lew (K4VX) - an other form of linearly loaded dipole

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"A linear loaded vertical antenna for 80m and 40m" by David Reid (PA3HBB / G0BZF)

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"The Watson 80Plus2 Antenna", by Julian Moss (G4ILO)

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"How & why I made an aerial for the 14 MHz, 7MHz & 3.5MHz", by Mark Wooton (M5MKW)

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"Helically Loaded Magnetic Loop Antenna", by Richard Fusinski (K8NDS)

My 40 m Cobra

I based my Cobra 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 $23 (mid-2009, up from $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 this 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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Update 31 Jan 2010: I have rebuilt and "pruned & tuned" this antenna today, for an antenna system configuration with 40 ft of R/G-58 coax, a 4:1 balun, and a current choke. Purpose was to have an SWR dip close to 7040 kHz (to cover 7030-7050 kHz for Hellschreiber and PSK31 mode). I reduced the size from 662 cm per leg (the maximum that I can fit on my terrace) in five steps to 628 cm per leg. Approximately 10 kHz increase in resonance frequency per cm length reduction (25 kHz/inch). Note that the resonance frequencies shift a bit when only the current choke is used. Impedance values at the resonance dips suggest that I should try a 2:1 balun.

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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 


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!
 

October of 2009: the bungee cord is no longer elastic. Replaced with heavier gauge bungee cord .

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)

Update 31 Jan 2010:

February 2010: 80 m experimental version

The discussion above shows what I was able to do in the space available at my location. I would still like to be able to use this antenna on 80 mtrs. I can receive fine with it, but I cannot get it tuned below SWR 3-4 on 80 mtrs. Recently, I have been successfully experimenting with my short 80 m vertical antenna. It uses a loading coil at the base of the radiator to get resonance on 80 mtrs. My 20 m mini dipole (and derived verticals, and yagi) use center-loading coils. The Cobra antenna is based on linear loading between the 3-conductor wires. So why not expand the Cobra with loading coils? For ease of construction, it is easiest to place the coils at the feed point of the Cobra, or at the end-insulators. See diagrams below.


Configuration 1: loading coils at the feed line


Configuration 2: loading coils after 1/3 of the Cobra-wire


Configuration 3: loading coils after 2/3 of the Cobra-wire


Configuration 4: loading coils and extension wire at the tip of the cobra-wire

There is not much literature about linearly loaded antennas. There is plenty of literature about short dipoles with loading coils (a.k.a. "lengthening coils") placed somewhere between the feed point and the tips of the antenna. The last article referenced above is about a linearly loaded dipole that is expanded to 80 mtrs with a trap coil and extension wire. I have not (yet) come across any literature about "double loading" or "combined linear and center loading". It would be nice if it is a novel approach - and if it works. Right now I'll refer to it as the "N4SPP double-loaded short dipole" (of course, if it doesn't work, I will deny any association with the idea).

I re-used coils that I salvaged from another 80 mtr experiment; they have 32 mm ID, a PVC core, and a little over 100 windings of 0.8 mm CuL.


The experimental loading coils

First experiment was to try Configuration 2, and hook up the antenna analyzer. The intent was to move the lowest resonance dip from 7 MHz to 3.6 MHz. However, with the loading coils, the lowest dip moves up instead of down! The lowest dip is now at around 10 MHz!


Loading coil installed at one of the end-insulators
(coil simply slid over the bungee cord)

The only explanation I can come up with at this point, is that the coils act as traps, rather than loading coils. Or, the coils mess up the capacitive loading between the conductors of the triple wire...

This leaves three more configurations to try!

Experiments in march of 2010 with my 6m short vertical for 80 meters show that Configuration 1 should work, and using the 3-conductor wire in Cobra-configuration has the same effect as increasing single-wire length by 15-20%.

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)


                                   
Another version of the center insulator - I made it out of 16 mm PVC pipe

 


©2008-2010 F. Dörenberg N4SPP

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