Homebrew Cobra doublet antenna

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Robert Tadlock:
I have a 100 foot roll of 300 ohm HD twinlead and a 100 foot roll of 3 conductor rotator cable with the intention of experimenting with a home brew a 10-80 meter Cobra doublet .  The original dimentions state 73 feet for the antenna itself ,  however I dont understand why .   Since I will use a tuner anyway and its basically a shortened doublet why cut the 3 conductor wire down , why not just leave it 100 feet and use it that way .  I have the room .  Its non resonent anyway and the 73 footer doesnt seem to resonate near a wanted frequency . With a 100 foot length I MAY be able to work 160 in a pinch . Even if it doesnt why chop it off ? 

Thanks

N7BDY  Bob 

BILL MERRITT:
At heart your question can be rephrased "Why should any antenna be built to a formula based on physics?".

The Cobra is a linear loaded antenna.  Offhand I'm not  aware of a simple formula like that used for ordinary dipoles, which gives the dimensions and operating range.

However, this antenna has been discussed by knowledgeable hams may times, and I'm sure someone can turn up an article or discussion on the design principles.

Of course, the quickest and most effective way for you to learn about it would be to model it in software and find what dimensions best suit your requirements.

b.

BILL MERRITT:
http://forums.qrz.com/archive/index.php/t-277382.html

Here is an old discussion answered by three reliable engineers W8JI, VK1OD, and W9GB.

http://www.eham.net/ehamforum/smf/index.php?action=printpage;topic=81281.0
another on folded dipoles including the Cobra.

BILL MERRITT:
https://web.archive.org/web/20120824211821/http://vk1od.net/antenna/cobra/index.htm

Owen (ex-VK1OD), and engineer, via the WayBack Machine, analyses the Cobra Lite Jr. While the diagrams are missing, his text stands.

Quote

(VK1OD) Conclusions

-System losses are relatively high, and in some frequency bands are so high as to be unacceptable for most applications.
  
-The minimum usable frequency is around 3.5MHz, though system loss does not quite meet the acceptable loss criteria.

-System losses are acceptable in HF amateur bands from 40m to 10m with the exception of the 17m band where losses are extremely high. Note that losses in the 80m band and 60m band exceed the the acceptable loss criteria.

-Model losses adjacent to the 80m, 30m and 15m bands are very high, and there is a risk that with environmental detuning, performance in some of those bands might be much worse than indicated by the model.

-This analysis does not support the proposition that the "loss of the extra wire" in linear loading is "essentially negligible" as stated in the ARRL Antenna Handbook (18th ed).

BILL MERRITT:
Quote

why not just leave it 100 feet and use it that way .  I have the room .  Its non resonent anyway and the 73 footer doesnt seem to resonate near a wanted frequency . With a 100 foot length I MAY be able to work 160 in a pinch . Even if it doesnt why chop it off ?  

Further thought...The difference between lengths may be related to extreme impedances which are hard to match and/or high voltages which may cause tuner arcing or antenna wire insulation breakdown, rather than "it won't work on those frequencies".

Don't be afraid to give it a try.  I would start with low power.  bill

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