I use separate phased Beverage (note the capitalization, this is a man's name!) arrays on 160M, 80M and 40M. (hey... why do u refer to 80M as 75M??? 300 divided by 3.75 = 80 !!! Or have ur SS interests caused to to forget about (80M!) freqs below 3900 kHz?) If u are planning to use a simple Beverage antenna, as opposed to phased Beverages, u can certainly use a 580 foot Beverage on both 80 and 160. A 580 foot Beverage antenna works very well on 80M although a 580 foot Beverage antenna (not array) isn't long enuf to work particularly well on 160M... A pair of phased 580 foot Beverages with 270 spacing makes a fabulous 160M receiving antenna however, likewise a pair of 295 or 440 foot Beverages with 135 foot spacing on 80M! N2RM uses a pair of phased Beverages with compromise spacing that work well on both 80 and 160. Here are some factors to keep in mind when selecting Beverage antenna length:
1. As W1WCR points out in his Beverage Antenna Handbook (results easily confirmable with EZNEC), certain preferred lengths produce a very desirable "cone of silence" off the back of the antenna. The preferred cone of silence lengths are (it is NOT necessary to use precision lengths, furthermore, ideal Beverage antenna length is somewhat affected by local ground conditions!)
2. Longer Beverage antennas produce POORER front-to back ratios! Beyond the minimum useful Beverage length (the first length in 1 above), about 3 to 4 dB of front-to-back is lost each time the length of the Beverage is doubled! As a result, the first two "cone of silence" lengths would be preferred if (but only if!) front-to-back were the primary design factor...
3. Short Beverage antennas have a very broad main lobe, poorer response to low angle signals and lower sensitivity... Of the four lengths listed under number 1 above, the shortest length has a 3 dB beamwidth of almost 180 degrees, not very desirable! The second length has a 110 degree beamwidth (better, but still not good...), the third length has an 80 degree beamwidth and the longest length has very desirable 50 degree beamwidth (but only 17-18 dB front-to-back ratio...).
The clear advantage of phased half-wavelength-spaced pairs of Beverages is that you get the front-to-back ratio advantage of short Beverages, the narrow beamwidth and sensitivity of long Beverages, and the fantastic front-to-side ratio resulting from the phased pairs that is unavailable with any single Beverage antenna length!
In summary: On 160M I'd suggest phased 585 or 880 foot Beverages spaced 270 ft. On 80M the preferred phased Beverage lengths are 295 or 440 feet, spaced 135 ft On 40M the preferred phased Beverage lengths are 150 ft or 225 ft spaced 70 ft. N2RM's compromise of two 580 foot Beverages spaced 200 feet is a proven performer on both 80 and 160.