3G help

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Hello everyone. Im new to this site. Can anyone tell me, is there a part on the iphone which helps with getting a better 3G reception. Im on O2 but my 3G is very poor. Im going to try a new simcard first. I had an iphone 5 about 3 months ago which picked up 3G more than the secondhand iphone 5S I have now. That's why im wondering if its the phone. Hope someone can share any ideas with me. Many thanks,Gary.