Masts and stacking considerations

[a. kerner]

At 11:58 AM 6/14/96 -0400, you wrote:
>In a message dated 96-06-14 10:19:15 EDT, you write:
>>I've seen a lot of talk concerning the 3 inch Rohn thrust bearing .. does
>>this mean many of you are actually using larger than 2 inch masts out
>>there?  Do most beam's boom-to-mast brackets and commonly used
>>rotators handle up to 3 inch diameter masts? 
>
>
>     Boom-to-mast bracketery needs to be increased to 3 inch but that's
>pretty simple.  
>I guess the question you have to ask yourself is "if i need a 3 inch mast, is
>the antenna windload excessive?".  Another complication is that many rotators
>(i.e. Hy-Gain) are designed for 2 inch masts only and will turn eccentrically
>if any other size is used.  
>>

When I used to use 3" mast, I reduced it to 2" from below the  thrust
bearing (mondo!) to the
rotator.  This can be done by looking at pipe tables for id and od's.  No
bending strength
is needed below the thrust bearing, so the 2" is OK.

>  Correct me if I'm wrong, but going
>from 2 inch to 3 inch isn't necessarily going to save you any money because
>it doesn't significantly increase the strength of the mast.  Besides, the
>forces remain the same.
>>

Ah, not so, amigo.  I just plugged the numbers into my mast spreadsheet and came
up with the following:

Allowable bending moment for 2" mast, 1/4" wall, 115 ksi steel is 61,743.
Allowable bending moment for 3" mast, 1/4" wall, 115 ksi steel is 157,826  !!!

Run another way, if we wanted the same bending moment as the first mast,
but wanted to use a lower grade steel, 3" mast, 1/4" wall will give us an
allowable resisting moment of 61,758 with a yield strength of only 45 ksi.

Bottom line, increasing diameter of mast is most effective way of achieving
higher strength.  Unfortunately, also a pain for all of the compatibility
reasons.

>>ELNEC indicated that the tribander's pattern will be adversely affected by
>>the 40M beam nearby, so I twisted the beams 90 deg. relative  .. muuuch
>>better, almost as if the 40M beam disappeared.  I haven't seen any
>>installations done this way.  Am I missing something; other than having to
>>contend with the 90 deg. offset when pointing the beams?
>
>     For some reason, the Cushcraft 402CD doesn't cause the pattern problem
>that you describe where bigger antennas do.  Many hams with this
>configuration do turn them 90 degrees (K7RI and WY7I locally for example).
>

actually, RI's 40 is not outta phase w/ KT34XA.  Doesn't seem to be a prob.

>73,  Steve  K7LXC
>

Ciao, Adam  AA7FT
>