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If your attic is fairly long and your roof is not metal an attic longwire should be decent. You want around 75' to 150' for good healthy performance (even 50' will be decent)....If you can go longer by all means do so. The funny thing about longwire antennas is despite AM and SW optimization (based on length) they will work on all bands in some positive form.....Mine was able to better than double the signal to my VHF scanner/weather radio, and on vacation in the remote north woods actually take a cell phone from no signal (with it's own whip) to decent usable signal.
There are formulas for wavelength and guidelines for what fraction of wavelength is optimum for 'tuning' a longwire antenna on the web. There are also broadband longwire antennas with some type of RLC trap spaced every few feet....The traps effectively reduce the length at higher frequency. The higher the freq the shorter ant you want.
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