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Author Topic: Coax down a Rohn 25 Tapered Top to In-tower rotor  (Read 3224 times)
N5TEN
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« on: August 17, 2012, 11:06:24 PM »

I have a 40 ft. Rohn 25 tower going up and my top section is the tapered kind. Planned for the top is a SteppIR 2 element yagi coming in a few weeks. I have a Yaesu 800SA going inside the tower and my mast will be through the tapered top, loosely fitted without cinching the two mast holder bolts.  I haven't decided if I'm putting anything else up over the yagi, but possibly a color night vision long distance web-enabled security cam which will mount so it turns on the mast like the antenna. Or my UHF repeater antenna (vertical). Whatever ends up there, will probably have coax or cable to go down the mast.

Anyway, I'm going over coax/cable logistics here.  Is this a situation where the coax/cable is wound in a large coil around the outside tapered section of the tower, down to the rotor?  Anyone else here have this scenario?  How did you handle the coax from the yagi/antenna on the turning mast, down the outside of the tower's tapered section and then into the tower around the rotor? I googled this question in the images section and found absolutely nothing on the subject in photos.

Also, as a side note, I have tentative plans to tram the antenna up there because I have phillystrand at 35'. I thought about bolting a pully into the mast and running a cable in a loop (like a folded dipole kind of loop) for easy pull tramming. The pully would be installed above where the yagi is to be attached to the mast.  Am I on the right track here or is there a better way?

The other option there is to use my gin pole to haul the yagi up, or just use a come along from the top to crank it up there and then manhandle it up on the mast. I'm not looking forward to that kind of scenario though. The ant is only 37lbs, but hanging from the tower on a lanyard with that over your head isn't fun, I'm sure.

Thanks
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W8JI
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« Reply #1 on: August 18, 2012, 12:54:52 AM »

A rotor loop is a slack area with enough slack to allow around  one turn. It is initially set half way, like with the rotor south if it is a north stop rotor.

Watch that tram idea. A pulley on the mast with a loop increases mast side loading. You have two rope ends pulling on the pulley with tension, it can be a 2:1 multiplier on mast loading. Making things worse, if you are tramming, there is oncreased side load from the weight of the antenna and angle of the "triangle" or ellipse formed by the rope sag. Plus you have rope tension.

You better have a strong mast if you tram out horizontally any distance, or a very lightweight antenna, unless there is a great deal of slack and sag in the tram line. Here are pictures of tramming large antennas. Tension in the line with a 150 pound antenna would have been over 3000 pounds peak if I kept the line taut. The tram line has to have slack, or tension gets large fast.

http://www.w8ji.com/rotating_tower_w8ji.htm

Here is a tension calculator:

http://kristinandjerry.name/cmru/rescue_info/CMRU%20System%20Testing/Highline%20Tension%20Spreadsheet%20Calculator.xls

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K3GM
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« Reply #2 on: August 18, 2012, 06:33:29 AM »

You shouldn't need to tram it up.  With the 2L,  there are 2 methods to getting it on the mast.  The boom is just under 5' which makes it easy haul up.  With a pulley mounted on the mast above the antenna mount point, you can haul it up completely assembled, with a man on tower stabilizing it and easing it thru the guys.  The other method is to mount the boom first without the motors and elements.  Offset the boom slightly and bring up one of the assembled elements with motors attached.  Set the motor on the mount, and install a couple of bolts quickly to secure the assembly to the mount.  After it's attached, slide the boom the other way to access the other motor mount.  This is how I assembled my 3L SteppIR on my tower which is in a clearing with woods surrounding it.  I used a temporary SteppIR fixture which allowed me to slide the boom from one end to the other to reach the motor mounts, leaving the driven element and its motor mount off the boom for last.   It was a bit of a tense process and is probably why they no longer sell the Boom Slide. But you won't need to do that with the short boom of the 2L.

As Tom stated, the rotor loop is merely some slack in the transmission line which allows it to twist gently around the mast.  Again, with my SteppIR, I secured the control cable to the coaxial transmission line using UV resistant zip ties.  Understand, you should have all of this planned out prior to lifting the antenna.   I suppose you could use an uncut run from the antenna to the shack, but I used two pieces, Heliax from the entrance panel up the tower, then traditional "coax" up the mast to the feedpoint.
« Last Edit: August 18, 2012, 06:46:27 AM by K3GM » Logged
N5TEN
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« Reply #3 on: August 18, 2012, 07:36:02 AM »

  So UV resistant zips and tape securing the coax right below the antenna where it attaches, then a loose loop around the needle point and big slacky loop around the rotor to the tower leg for downward installation? Will that look sloppy?

Okay, I wont' tram the antenna. Actually, I could pully it from that point at an angle with tag lines from the ground without much pressure on the mast. Once it's up there, I'd have to climb to install it. Truthfully, I'd like to get that gin pole off the tower as soon as the top section is on. That thing is a monstrosity and I've grown to dislike how difficult it is to work with because of the weight.  I could make a small, light antenna gin pole or just use the pully.

I'd rather not do all that attaching and antenna building on the tower in the air, if I can help it. 

As for the coax run. I was simply going to use the RG213 I have going to my AD up in a tree now, and pull that antenna down permanently. Then just re-direct the coax to the SteppIR. We're talking 40ft up the tower and 30ft from base to shack (if I do a ground run to the base and up), or even shorter if I do the "power line" method which I'm leaning toward. It's a shorter run and easier to do in my situation because I have a septic tank below ground btw the tower and my shack. I'm not really interested in putting a coax pipe run under there if I can help it.  I'm thinking that I'll run a strong rope for the coax to tie wrap onto for the sky run. Any advice on that?

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K3GM
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« Reply #4 on: August 18, 2012, 05:31:48 PM »

I did you one better.  I climbed the tower and leaned back as far as I could to get this picture of the rotator loop and how it's dressed to the tower.  It's shown in the "unwound" position, pointed North. The loop allows 450 degree rotation (the rotator can over rotate and has no hard stops). The installation has been maintenance free for five years.  Note the 3 control cables zip tied to the coax jumper form the Heliax transmission line coming from the shack.  Excess control cable and coax jumpers were looped  and zipped tied.  Also note the additional coax jumper going up the mast to the remains of a 2m K1FO yagi which you can partially see.  It got destroyed last year in a 14" late October snow storm.  The paste like snow brought limbs down within 2 hours of starting and resulted in an 8 day power outtage.  It gets replaced over Labor Day and will require removing the rotator, loosening the clamps on the SteppIR and and sliding the mast down through the tower about 6 feet until I can reach the antenna.
http://i9.photobucket.com/albums/a96/TwoSevenRight/steppir.jpg
« Last Edit: August 18, 2012, 05:49:24 PM by K3GM » Logged
N5TEN
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Posts: 54




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« Reply #5 on: August 18, 2012, 06:38:23 PM »

Very nice... thanks for doing that!!  I see it now. And I also see how low you have the SteppIR from the top of the taper. Interesting. ....
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K3GM
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« Reply #6 on: August 18, 2012, 07:56:26 PM »

No problem.  The reason I mounted it low was to not over stress the mast and to separate the antennas above it as much as possible. I also use a work platform, and where I have it mounted puts me almost at eye level with the boom.  I've found the planning and actual tower and antenna work to be very enjoyable.  Enjoy yours!
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N5TEN
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Posts: 54




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« Reply #7 on: August 18, 2012, 11:08:25 PM »

I found it to be fun, but also stressful, painful a few times, and expensive!  I can't wait for it to be all done. Next is figuring out the guy lengths and actually attaching the philly strand to the dead ends, gin poling the last section, mounting the rotor, mounting the mast, calculating the mast length, running the coax, etc etc etc. I feel like I will never be done.

Good news though...I passed my Extra Class exam today! 86% after a 12 hour cram yesterday and half day today before taking it. I found the formulas and some other stuff to be very hard. But after being a General for 17 years, I thought it was time....especially now with the good dx antenna coming and tower...

de N5TEN/AE
« Last Edit: August 18, 2012, 11:19:19 PM by N5TEN » Logged
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