[TowerTalk] Antenna Height

K7LXC at aol.com K7LXC at aol.com
Wed Apr 18 22:15:59 EDT 2007


 
In a message dated 4/18/2007 10:56:00 A.M. Pacific Daylight Time,  
towertalk-request at contesting.com writes:

>  I am planning to replace a cushcraft A3WS that has a failed  trap this 
spring.  It is mounted 10 feet above a 50 foot 25g tower along  with an A3S just 
above the tower top.  As I got to thinking about it,  since I would be doing 
some antenna work, might as well replace the A3S with  something without traps 
and perhaps a bit more punch while there so I don't  have to do another 
replacement in the future.  Well, that then led to the  thought of maybe I should go 
up to 70 feet with an added set of top guys and  the guy bracket in addition 
to the two HBUTVRO brackets which are at about 10  and 30 feet to get a little 
higher off the ground.  The lower house  brackets is bolted to a floor plate, 
the other is through bolted to a wall  with lumber on the inside spanning 
several studs.  

>  So,  the current thinking is a F12 WARC 7 at 8 - 10 feet over the tower 
top with a  Skyhawk just over the tower top at 70 feet.  That's 13.4 sq ft of 
wind  load plus mast.  Since the added set of guys is non-standard for a  
bracketed tower, I see nothing in the Rohn book addressing it.   


The guyed specs show a capacity of 16.2 sq.ft. @  70 MPH with 2 sets of guys. 

>  Seems to me I would have the  equivalent of a 70 foot 25g tower 
"effectively" guyed in three spots with the  two house brackets and a top set of guys.  
I am wondering if this would  withstand the wind loads and twisting or I would 
need to go with something  bigger like 45g?  I am in a 75mph wind zone from 
what I can tell and the  installation is surrounded on all sides by 60 to 70 
foot trees that shelter it  from the wind a bit.



    The maximum distance between guy levels on 25G is  around 32 feet. Your 
proposed guys should be at the top of the tower (you  didn't say which top 
section you've got) and then around 30 feet below at the  40-foot level, that 
would give you 10-feet between the bottom set of guys  and the top housebracket. 
Below that the housebrackets are plenty to support  the tower. The maximum 
stress is at the top of the tower and the top set of  guys so everything below is 
subject to less torque (practically none) so as  far as I'm concerned, you're 
good to go. You don't need the extra  strength 45G.  

>  Secondarily, am I likely to see  much improvement in performance for the 
extra 20 feet of antenna height,  particularly on 20m?
 

And I quote, "Higher is better". You'll  see some improvement but it's a 
small percentage of wavelength which is the  key to take-off and arrival angles so 
it won't be real dramatic - good but not  real dramatic. To see marked 
improvement, you'd have to go to 100' which on  20M is a pretty good sweet-spot 
which is hard to beat. 
 
    I'd recommend going with a big tribander or  SteppIR. Many of them will 
make as much difference as increasing the height.  Check out our tribander 
comparison report (_www.championradio.com_ (http://www.championradio.com) ) before 
you  spend your money. Then you can spend it on the right  antenna.

 Cheers & GL!
Steve     K7LXC
Champion Radio Products
and
TOWER TECH -
Professional tower services for hams
Cell: 206-890-4188



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