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Tyying to understand parallel dipoles. Jul 22nd 2017, 14:32 3 1,202 on 24/7/17

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Tyying to understand parallel dipoles. AC2MM on 22/7/17
Hello,
I have looked through all my ARRL books on parallel dipoles for this answer, and can't locate the answer to my question.
I am very impressed with the quality of the Alpha Delta product line, and I must say up front that the people to whom I've asked this at Alpha Delta have really tried to answer my questions, but I still don't understand. (probably age )
They make a multi-frequency product called their DX-EE, which allows you to use 40/20/15/ and 10 M bands, all using parallel wires (with separators) coming to a common feed point at the center. What makes this product different is it's use of coils on the 40M line, reducing the overall total length to a little over 40 ft. This is ideal for those whose space is limited, and this is what this product is marketed for.
So I wrote a letter asking them if they would make me a similar product using all their top-quality materials, but without the coils and making the 40M top line a full 1/2 wave 66 ft long. They replied that although they don't, at this time, make custom versions, that it would not work unless I used the fan dipole method which is also found in various ARRL publications. This is what I don't understand.

Using their Current DX-EE product design, the parallel wires apparently cause no coupling or resonance problems. Yet when I want to remove the coils and go to a 1/2 wave version on these frequencies, I now will have an unworkable situation unless I use the fan dipole techniques. (which I can't do in my new back yard)

The only comments I have been able to find, on EHam I believe, that this type of design narrows your usable range, whereas using the standard 1/2 wave technique broadens the usable range for a low SWR.

Will someone please explain why I can't use two wires connected to a common feed point separated to construct a parallel dipole for at least 40M and 20M bands without using these coils?

Thank you,
Robert

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