About T-bars -- does anyone else remember pictures of Sigma 80 yagis
broken at the junction between the T-bars and the elements? I think I
recall one station where all 4 broke (on a 2-element).
73, Pete N4ZR
The World Contest Station Database, updated daily at www.conteststations.com
The Reverse Beacon Network at http://reversebeacon.net, blog at
reversebeacon.blogspot.com,
spots at telnet.reversebeacon.net, port 7000
On 9/17/2011 7:33 AM, Jim Thomson wrote:
> From: Robert Chudek - K0RC<k0rc@citlink.net>
> Subject: Re: [TowerTalk] Aluminum tubing insert overlap...
>
> Thanks for all the feedback so far...
>
> After I posted my question I dug around in my book library and found
> "Physical Design of Yagi Antennas" by David B. Leeson W6QHS. That was
> ARRL publication #3819 in 1992 but is now out of print.
>
> There are 11 Chapters which cover many aspects of mechanical design,
> including improvements for survivability of 10, 15, and 20 meter yagis
> and the popular Cushcraft 2 element 40m yagi. I have not found an answer
> to my specific "overlap" question, although I haven't read the book
> cover-to-cover yet. It might be in there somewhere, yet to be found.
>
> %%% A bare min of triple the diameter for any overlap..and preferably
> a lot more, like 4-6 times the diam...and a snug fit, not a sloppy fit.
>
>
>
>
>
> The book contains all the math needed to make wind survivability
> calculations for homebrew elements and masts. A quick Google search
> turned up a Lotus WK1 spreadsheet created by the author at:
> http://www.realhamradio.com/Download.htm
>
> %%%% The arrl has free mast software. K7NV and also DX eng
> has ele mech design software. You will get a real eye opener
> after playing around with them for hrs on end.
>
>
> Experimenting with this tool last night was a real eye opener... It
> reveals what you thought would be stronger element (extending the larger
> tubes) is in reality a potential failure point! Longer "whippy ends"
> help create survivability. I also learned that you model an element
> starting at the tip and working toward the boom with the larger
> sections.
>
> %%%% yes, start at the ele tips and work inwards. Ditto with
> booms on yagi`s.
>
>
>
>
> My original plan was to investigate shortening this full sized dipole
> using top hats or linear loading. My goal was to reduce the windload and
> torque exerted on the tower. The failed antenna was over 70 feet long
> and presented a big lever at the top of the tower. My target length was
> 50 feet +/- 10% without using loading coils.
>
> %%%% My F-12 3-el 40m yagi .... called a 340N uses els that are aprx
> 51-53feet long...and use linear loading. The linear loading consists of
> three sixteenths
> solid AL rod. The LL extends OUT towards the tips. It`s a superb design.
> The tips are
> .375 OD....and the very middle is 1.875 x .125 wall. Aprx 24 lbs per
> ele..and good for
> over 100 mph. Minimal wind load too.....like 3.3 sq feet per ele.
>
> %%% capacity hats..or T bars as F-12 calls them also work superb. F-12
> does not put the
> capacity hats out at the extreme ends either. On a 50 foot ele... the T
> bars would not have to
> be very long either..like aprx 16 foot...tip to tip. And aprx 2 thirds
> the way out on each side.
> That`s like adding a 10m ele part way out on each side. A pair of 16 ft
> T bars can easily be
> made to be very light. The simplest conversion would probably be to take
> the stock 70 ft long
> telrex el..and lop 10 ft off each end....then add the 16 ft T bars at
> aprx 15-20 ft on either
> side of the center feedpoint.
>
> later... Jim VE7RF
>
>
>
>
> 73 de Bob - K0RC in MN
> ------------------------------------------------------------------------
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|