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Author Topic: Home Made Shims for Tower Bearing?  (Read 1632 times)
KB3MDT
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Posts: 205




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« on: October 07, 2013, 08:06:12 PM »

Hi,
     I'm getting ready to order a 4.5' (i.e. mini) Glenn Martin RT-424 Roof Tower, K4KIO Hex Beam and a Yeasu G-450A rotor.  I'd like to minimize custom metal fabrication, do things right the first time, etc. as this is big bucks to me.   I wish I had room for a tower, or push up mast with guy wires at my QTH but I don't Sad.

     The default base plate for the Hex Beam is designed for a 1.25" mast.  I have a new 5' section of 1.25" steel mast from another project.   The Yaesu rotor manual includes calculations on Antenna Weight, Wind loading, installation recommendations, etc.  Short Answer, the calculations suggest I use a Bearing on the mini tower to handle lateral (i.e. wind) forces.  Antenna and mast weight is not an issue.

     Now for the fun part.   The Glenn Martin tower is pre drilled to work with a Yaesu GS-065 Bearing (per Texas Towers advertisement).  This bearing handles masts 1.5" and larger.  Yeasu also sells a GS-050 Bearing that works with a 1.25" mast, but it looks like the mounting holes on the GS-050 are too close together for the Glenn Martin Tower.  I'm thinking of going with the GS-065 bearing and making my own shims from some flat strips of aluminum.   I have some 1/8" stock and can get other thicknesses if necessary.  Alternately, I could take a 2" piece of aluminum tubing and cut it along the axis into two shim pieces.   Temporarily Tape the shims onto the mast, use the existing bearing centering screws to line everything up, and declare problem solved.

    While I'm checking for advice, is there any problem using RG-213U Coax Cable for the loop needed so the antenna can turn?   RG-213 has a stranded center wire.  Its about 1/4" in diameter but seems to bend easily if the radius is 1 foot or more.  (min bend radius is 5" per spec.).  I'd like to avoid using a 2nd piece of "more flexible" coax and barrel connector just below the antenna.  The connection adds looses, and is just another potential problem point for moisture and coming loose. 

73
KB3MDT
Ken

PS.  Previous Post readers - I am reinforcing the roof for the mini tower.  The Nipper Pliers ($10 Harbor Freight) did a good job on protruding roofing nails.   
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WB6BYU
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Posts: 13481




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« Reply #1 on: October 07, 2013, 08:28:57 PM »

Quote from: KB3MDT

...RG-213 has a stranded center wire.  Its about 1/4" in diameter but seems to bend easily if the radius is 1 foot or more...



RG-213 should be 0.405" diameter, and is commonly called "half inch coax".  It should work fine if you make the
loop large enough.  I like to use about 3 turns (nominal) perhaps 18" in diameter, allowing it to vary from 2.5 turns
to 3.5 turns as the mast rotates, which minimizes the change in radius.  That results in relatively little flexing of the
coax, with no sharp bends or twists.

Splicing in a short piece of more flexible cable generally won't make a measurable difference in loss at HF (at least not with
the equipment commonly available to hams) but shouldn't be needed.
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N7BMW
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Posts: 127




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« Reply #2 on: October 07, 2013, 09:01:26 PM »


     Now for the fun part.   The Glenn Martin tower is pre drilled to work with a Yaesu GS-065 Bearing (per Texas Towers advertisement).  This bearing handles masts 1.5" and larger.  Yeasu also sells a GS-050 Bearing that works with a 1.25" mast, but it looks like the mounting holes on the GS-050 are too close together for the Glenn Martin Tower.  I'm thinking of going with the GS-065 bearing and making my own shims from some flat strips of aluminum.   I have some 1/8" stock and can get other thicknesses if necessary.  Alternately, I could take a 2" piece of aluminum tubing and cut it along the axis into two shim pieces.   Temporarily Tape the shims onto the mast, use the existing bearing centering screws to line everything up, and declare problem solved.


Or you could get a short length of 1-1/2 inch aluminum tube with an 1/8 inch wall.  Slip it over the pipe and be done.  If it is too tight cut one slit.  Either drill and put a screw through or if you slit it you can hold it in place with a hose clamp.

Here is a source for the tube:  http://www.onlinemetals.com/merchant.cfm?pid=1215&step=4&showunits=inches&id=71&top_cat=60
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VE1IDX
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Posts: 154




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« Reply #3 on: October 09, 2013, 02:55:23 PM »

Why not just use the GS-065 bearing and be done with it? The mast clamping bolts are adjustable to fit various size masts and if they are too short simply replace them with longer ones. No messing around with drilling new mounting holes or getting a piece of mast material to use as a sleeve.
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KB3MDT
Member

Posts: 205




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« Reply #4 on: October 11, 2013, 07:48:21 PM »

Hi,
      I decided to go the slightly more expensive, less effort way.  I ordered a 5' long 1.9" OD aluminum mast (Sch 80 Alum Pipe with 0.2" walls) from Online metals, the Yaesu GS-065 Bearing, and the larger Base Plate for the K4KIO Hex Beam.   All parts are compatible with the 1.9" OD mast, problem solved.

     Thanks to everyone.

73
KB3MDT
Ken
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