[TowerTalk] Re: K factor. How derived?

[K7LXC@aol.com]

In a message dated 99-02-28 19:05:07 EST, alsopb@gloryroad.net writes:

> I'm trying to figure out how this K factor relates to anything physical.
>  Obviously it is a starting torque limit.  However, it is a torque that
>  appears to be computed based upon an "arm" of the turning radius and the
>  entire antenna weight (boom and all!) being at that point.  Actual
>  antennas have their elements weights at the end of 1/2 the boom lenght
>  and the middle elements of 3,5, odd element arrays contribute
>  significantly less to the required torque. There doesn't seem to be any
>  overt inclusion of the wind resistance one needs to overcome to start
>  the thing turning  (except that the K factor seems really non-physical
>  and probably severly limits the size of the antenna you can turn).  This
>  wind resistance would be proportional to the antenna surface area.
>  
>  Can anybody out there explain the derivation of the K factor?

      Brian, I think you're putting too much thought into this. The Effective
Moment/K Factor is an ESTIMATE by the manufacturer of what size of antenna
system can be turned reliably by a particular rotator. There is no test or
scientific formula that will give you the information. And it doesn't have
anything to do with wind resistance or number of elements - it's turning
radius times weight.

      It's pretty well accepted that a simple reference to square footage can
be pretty skewed. Are the rotator/torque requirements for a 4L 20M beam with a
20 foot boom the same as for a long boom 10M antenna with a 40 foot boom even
though they might have the same square footage? They are not. 
>  
>  IMHO, what we really need is a rotor foot-pound limit and a quickie
>  little program to calculate the starting torque of our antenna
>  (accounting for wind in some way) given the element lengths, weight
>  distribution, boom locations etc.  
>  
      Sounds reasonable. And valuable. Feel free to come up with something.

>  I think the starting (or stalling?) torques of many popular rotors were
>  published in the last couple years in CQ or QST.  It would be
>  interesting to compare these numbers to starting torques needed for
>  popular antennas.

      Of course you're ignoring any ambient windspeed. 

     A list of rotating torque and brake torque has appeared in my CQ Contest
"Up The Tower" column (reprint's a buck) and also Dave Leeson, W6NL's book
"Physical Design Of Yagi Antennas". These are places to start but I think that
the topic is still somewhat subjective. You'll never go wrong by using a
rotator that is a little larger than what you think you need. 

      A rotator is the weakest link in the whole tower/antenna system and they
DO fail regularly. 

Cheers,   Steve    K7LXC

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