"Rick (W-A-one-R-K-T)" wrote in message
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I'd like to put up a 20-10 meter beam (probably the Butternut Butterfly),
a 3-element 6-meter beam above that, and a 2M/440 gain vertical (probably
the Diamond X-510MA) above that, all on a 50-foot Rohn 25G tower that's
guyed at 30 feet.

So that I can figure out how long a mast I need, what are the minimum
spacings between the antennas on a single mast?


Here is as good a song and dance as any about the spacing.

http://www.ifwtech.co.uk/g3sek/stacking/stacking2.htm

Usually on small towers you just do the best you can. I just installed a
stack of antennas . Used a 21 foot joint of pipe on a 60 foot 25 G tower.
About 6 feet is in the tower to the inside tower mounted rotator. That
gives me about 15 feet of pipe out the top. At the very top is a 432 mhz
beam on a 15 foot boom , four feet below is a 2 meter antenna on a 15 foot
boom , five feet below that is a 6 meter beam on a 18 foot boom and then a
triband TH3 just over the top of the tower. All fed with some Davis
Buryflex coax.

The vertical on top should not have much effect on the installation if the
ground plane elements are small

The tower is in a yard of cement and guyed at the 30 and 55 foot levels.