Look at priors with https://www.google.com/search?q=can+i+add+an+antenna+to+my+cell+phone&ie=utf-8&oe=utf-8
The size and shape change with the target band. Remember you are subject to fines and liabilities for the modification.
I've attached a link to the shapes in question. I have a T-Mobile phone and live in an area with poor reception. I wish to construct an antenna to which I can attach my phone. Which shape would best enhance antenna gain. Is wire diameter a factor. Each antenna is 6 inches in length. Thank you. The link below displays the shapes.
http://i589.photobucket.com/albums/ss334/trueshooter/Cell%20Reception%20Jpg_zpsrvsmff3v.jpg

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