Electric fencing doesn't come cheap, and it's not something I would recommend using on a daily or unsupervised basis as poultry - and other animals - can become entangled in it and electrocuted. Horrible stuff.
You've already built your run so I think your best (and cheapest) option would be to run two VERY TAUTE lengths of tape/polywire around the outside of the chicken run. Use proper insulators and electric tape (preferable, link shows example) or poly wire:
http://www.molevalleyfarmers.com/mvf/store/products/mvf-20mm-tape-200m;jsessionid=52F13B8E23ADFED2F294E9C1E4490FF8Don't let it touch grass or the chicken wire of the run, it will short out. I'd put the first strand about 12"/18" high and the second 18"/2ft high. Connect it to an energiser, either mains or battery fed, and job done.
Radio on to deter fox? Not a chance. Fox has walked right past me when the radio has been on - it only had eyes for dinner.