Wednesday, February 21, 2018

Cobra Antenna Project by W4ZJOH--160 Meter and up Restricted. Post #1410.


Cobra Antenna Project by W4JOH--160 Meter and up Restricted

(http://www.hamuniverse.com/cobraantenna.html).

Author:  Raymond A. Cook (W4OJH)--revised by N4UJW.

Accessed on 22 February 2018, 0147 UTC, Post #1410.

Please enter the title URL into your browser search box to read the full article.

Comment:

Here's a HF antenna that you don't see very often.  The Cobra Antenna, which can be used on Amateur Radio bands from 160 meters through 10 meters, first appeared in the June 1997 edition of "73 Magazine Amateur Radio Today."  In that article, Raymond A Cook (W4OJH) described a type of folded dipole made from standard insulated wire (about 14 gauge) and fed with 450 ohm ladder line into an antenna "tuner."  The antenna's name is an apparent reference to the antenna's "S-Shaped" configuration of its multi-band elements.  The antenna works on its primary and harmonic operating frequencies as a standard ladder line fed dipole.

N4UJW does a nice job of summarizing the main features, required formulas, and construction practices needed to bring this unusual antenna to life.  Although N4UJW encourages you to build your own version of the Cobra Antenna, he does offer a few suggestions:

"Keep in mind that there are actually 3 conductors connected in series per side and folded back on each other..... or another way of saying this is that there is one continuous length of 210 feet per side in the 80 meter version connected to one side of the ladder line and the same on the other half. Because the Cobra antenna is a balanced load, it is recommended to install a 4:1 current-style balun at the station end of the feedline (many external tuners provide a built-in balun). Ladder-line feed may have to be trimmed for lowest SWR, but using about a 100 foot length seems to make for easier tuning on all bands.
 Extra feedline should be suspended in loose coils and not in a tight roll.

A 4:1 balan possibly could be installed at the antenna, then fed with 50 ohm coax to the radio, BUT, it is not known if this would upset any characteristics of the original design.

Experiment!

Raymond is quoted from the article..... 
"Some of our more skeptical, and perhaps knowledgeable, friends have expressed concern about impedance, power rating, wave-cancellation, etc.  All that we can offer as an answer is the slogan used for many years by the Packard Motorcar Company.Ask the man who owns one." ........W4JOH"
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