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Author Topic: LMR400 and a rotator?  (Read 1213 times)
W4HIJ
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Posts: 367




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« on: January 18, 2013, 01:31:32 PM »

 I have a small Cushcraft MA-5 beam on top of an Rohn push up mast. It is turned by an old Gemini OR-360 TV rotator.  It is currently fed by RG-8 coax but I've been off the air for about a year and the antenna is exhibiting problems with high SWR and generally just doesn't want to take power on any of  the bands it's designed for. I have several ideas as to what's up but the main thing I suspect is the feedline as the coax I used in the original installation was pretty old to start with. I have a length of JEFA LMR-400 that was taken down off a satellite antenna installation I had a few years ago that I'd like to use instead. It has been kept indoors since being taken down and is in much better shape. The problem is that I worry about the flexibility of the LMR-400 when it comes to making a slack loop to allow the antenna to turn. I know I could use a short length of a more flexible cable running from under the rotator to the antenna but I hate to introduce more couplers into the line. Besides that, there is no local source for the coupler or flexible cable in my area and I'm trying to get this thing up and running again this weekend. Is there any feasible way to use the LMR-400 direct to the antenna without it breaking the center conductor from the repeated flexing of antenna rotation? I have enough cable that I could put a larger amount of slack up there. Maybe several loops would do the trick? I'd really like to use what I have on hand without investing further in cable and connectors plus there is the fact that I want to get done this weekend and have no local sources for material. Thoughts and suggestions please.....
Tnx and 73,
Michael, W4HIJ
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KB1GMX
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Posts: 714




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« Reply #1 on: January 18, 2013, 02:04:57 PM »

LMR400 is a bit too stiff for that.  Use a large rotor loop if you must.

I'd use LMR400UF as it was meant to be flexed.

However before I'd swap out the coax (unless its really easy) I'd check it out
with an antenna analyzer from both ends. 

It may well be the antenna.   I may have insects in the traps or the match
network might have gotten wet.


Allison
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KB4QAA
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Posts: 2275




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« Reply #2 on: January 18, 2013, 02:06:43 PM »

No sense guessing.  Either test the old cable or install the new LMR type.

If you install the LMR type, you need to use a short section of something flexible. [edit] It would still be a good idea to add a more flexible sections if you continue to use the RG-8 feeder. 

Typical UHF connectors have negligible loss from HF up into high UHF bands if installed with reasonable skill.  Period.  UHF Female-Female barrel connectors have even lower loss than the male PL-259.

Stop agonizing, Get crackin'!    Bill
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WB2WIK
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« Reply #3 on: January 18, 2013, 03:16:46 PM »

You can "rotate" LMR400 if you do it right.  It wastes a bit of cable and takes up some space.

I make two 360 degree loops of the LMR around the rotating joint, each loop about 24" in diameter, and secure it above and below the joint.  It just gently unwinds and then rewinds without stressing the cable.

If you do this be sure the loops can't hang down low enough to "snag" on any hardware, such as rotator mast clamps, guy wire clamps or ends, etc.  If you have someone else use the rotator controller while you stand outside and watch what happens, and you are assured nothing "snags" and there's no possibility of that occurring, it's safe.
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W4HIJ
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Posts: 367




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« Reply #4 on: January 18, 2013, 03:22:15 PM »

I should have mentioned the fact that the LMR-400 has factory installed type N connectors on each end. I actually have a female N connector that I could replace the SO-239 in the antenna matchbox with. It's a five minute job so that would work at that end and I have a way to couple to the N connector at the shack end as well but then I have the issue with breaking the LMR-400 from the flexing of the rotation.  The issue with adding a piece of Flex cable at the antenna end is with the splice at that transition. I have enough  RG-213 to make a "flex" cable and I suppose I could use some PL-259's and barrel connector that I have to do it but I'm not sure that it's even possible to put a PL-259 on the LMR-400 or if I know how to do it if it is. I don't have a way to make the transition from the existing N connector on the LMR-400 to a PL-259 I would put on the RG-213. The only other possibility I've thought of is making a direct hard wire splice from the LMR-400 to the RG-213. I could do that pretty easily if there's not a possibility of introducing an impedance bump or any other issues. I'd actually be the most comfortable doing that if it will work.  I'm going to investigate other issues with the antenna such as the traps and I have the matchbox here on my desk open (it appears to be fine) but I had wanted to use the LMR-400 with this antenna before and now is as good a time as any.
Michael, W4HIJ
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W4HIJ
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Posts: 367




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« Reply #5 on: January 18, 2013, 03:29:23 PM »

You can "rotate" LMR400 if you do it right.  It wastes a bit of cable and takes up some space.

I make two 360 degree loops of the LMR around the rotating joint, each loop about 24" in diameter, and secure it above and below the joint.  It just gently unwinds and then rewinds without stressing the cable.

If you do this be sure the loops can't hang down low enough to "snag" on any hardware, such as rotator mast clamps, guy wire clamps or ends, etc.  If you have someone else use the rotator controller while you stand outside and watch what happens, and you are assured nothing "snags" and there's no possibility of that occurring, it's safe.
Must have been posting as you posted. I'm not opposed to doing as you suggest. I have enough cable length but if adding a piece of more flexible cable is a better option then I will do that.  As stated in my last post, I'm just trying to work out the best way to accomplish that with parts on hand. I like the idea of a hard wired splice but I need to be sure that's feasible and not some type of no no....I've done it between like types of cable before with no apparent issues but this would be between two different coax types.
Michael, W4HIJ
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W4HIJ
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Posts: 367




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« Reply #6 on: January 18, 2013, 05:09:46 PM »

After some further reading on soldering PL-259's to LMR 400, I think I'm inclined to go with WB2WIK's suggestion on how to configure the LMR-400 for rotating. From what I understand it's pretty easy to mess up LMR 400 trying to solder PL-259's, especially the cheapie Cell phone shack...err Radio Shack brand that I have immediate access to right now. The higher quality "silver" type would be better and make the job easier but I would have to order those which rules out getting back up and running this weekend. I'm also inclined to believe that trying to make a hard wire splice between LMR-400 and a short section of RG-213  for rotating purposes would have issues due to the same reasons that soldering connectors to the LMR-400 would. Thanks for the help and advice everyone and if anyone has any further ideas, please let me know.
Michael, W4HIJ
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W4HIJ
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Posts: 367




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« Reply #7 on: January 22, 2013, 02:07:27 PM »

Just an update.... I ended up using a length of RG-213 I had to replace the coax on the antenna. The antenna is now taking power as it should. I decided against using  the LMR-400 because I'm going to put something up for six meters and opted to keep it for that project.
73,
Michael, W4HIJ
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N6AJR
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Posts: 9891




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« Reply #8 on: January 23, 2013, 10:52:37 AM »

as a second thought, I always keep an eye open for "deals" on stuff like coax, and connectors and such.  A while back I picked up 2300 feet of lmr 400 for 600 bucks, delivered. I also got a spool of rg 58 ( 500 feet) a partial spool of rg 59, a couple hundred feet of rg 6 and I think I have three 500 foot rolls of rg8x that I got for 80 bucks for 500 feet delivered.  and I always pick up odds and ends of coax, connectors and stuff at hamfest so I have stuff on hand. I also have enough so I can give a chunk of coax, and some connectors to a new ham to help out. Yea, I got lots of stuff around the garage, but its nice to have what you need. I'm just sayin
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W4HIJ
Member

Posts: 367




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« Reply #9 on: January 24, 2013, 03:38:06 PM »

as a second thought, I always keep an eye open for "deals" on stuff like coax, and connectors and such.  A while back I picked up 2300 feet of lmr 400 for 600 bucks, delivered. I also got a spool of rg 58 ( 500 feet) a partial spool of rg 59, a couple hundred feet of rg 6 and I think I have three 500 foot rolls of rg8x that I got for 80 bucks for 500 feet delivered.  and I always pick up odds and ends of coax, connectors and stuff at hamfest so I have stuff on hand. I also have enough so I can give a chunk of coax, and some connectors to a new ham to help out. Yea, I got lots of stuff around the garage, but its nice to have what you need. I'm just sayin
I'm sure it's nice if you can do that but I'm lucky to be able to muster up 20 bucks at a time towards hobbies these days.....just sayin Cheesy
73,
Michael, W4HIJ
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