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Author Topic: Antenna to Tower Spacing  (Read 27980 times)
AF6D
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« on: December 06, 2013, 12:57:51 AM »

We are using a DB224e antenna that is currently side-mounted. It neglects an important area o coverage. We have an opportunity to move to a new tower and I am wondering how far from the pictured tower the DB224e should be mounted to obtain an omni-directional pattern?

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AD4U
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« Reply #1 on: December 06, 2013, 10:57:49 AM »

One of my two meter repeaters uses a Celwave Super Station Master antenna side mounted on a Rohn 65 tower at 350 feet.  I contacted Celwave and gave them as much info as I could.  Their engineering department replied with a tower-to-antenna spacing that would make the tower (almost) invisible to RF.  Having run it like this since 1989 I think Celwave was right.

Your tower has much wider spacing than the Rohn 65 so your situation may be quite different.  I suggest you call DB Products or whoever made your antenna and ask them.

Dick  AD4U
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AF6D
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« Reply #2 on: December 06, 2013, 04:57:56 PM »

Thank you for your reply. I was hoping for more of an immediate answer since we're moving over rather quickly. I think that a pole mounted with X supports in between the two station Masters at the top should place our antenna in good company. I don't want to side-mount and lose coverage anywhere. This site has awesome 360° coverage.
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KS4VT
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« Reply #3 on: December 08, 2013, 09:46:21 AM »

A lot has to do with the frequency of operation, the face of the tower, and the standoff distance from the leg of the tower.  There is no "rough approximation" that anyone can give you.  If you contact db products they will usually do the calculations free of charge, or at least they have done this in the past for me.
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KB4VVE
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« Reply #4 on: December 16, 2013, 04:31:57 PM »

Agree with contacting manufacurer, but all have recommended 18" in the past to me for our 150 commercial antennas.  Seemed to work OK on up to 42" tower faces.  Our 800 MHz side arms were 10' out on a 10' face tower.  Best we could do as manufacturers couldn't figure it over a 10MHz wide freq spread.  Got i bit directional.
Greg
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KB4VVE
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« Reply #5 on: December 16, 2013, 04:34:59 PM »

Additional on DB-224E.  Requires a tob bracket for wind survivability as it is longer than other DB-224 frq speads - more wind area, too due to bigger elements.
Experience - the hard way!
Greg
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AF6D
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« Reply #6 on: December 22, 2013, 05:28:41 AM »

Thank you for the replies. We already have (2) DB224's deployed using the 2 foot stand-offs and the top stand-off. We had to custom make one of them because of the  triangle tower.

But I am very curious about Station Master's and the like. I see them mounted mere inches from the tower when everything that I've read indicates that one should mount 1, 2 or 4 feet from the tower. With that said I have a piece of software called ANTPLOT http://tinyurl.com/antplot that shows that if one mounts an antenna 6 inches from the tower the resulting pattern is almost omni-directional. I don't get it. Is this really the case and why I've seen hundreds of antennas mounted mere inches from the tower? I've seen this on small Rohn 25's; on Sabers, and a host of other towers.

I'd really love a consistent pattern that is as close to omni as possible when side-mounted.

Your advice is appreciated.

Happy Holidays and Merry Christmas!
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HURRICAINE
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« Reply #7 on: December 30, 2013, 10:57:07 AM »

You will always get an image and a shadow off the side of the tower in the direction between the antenna and the tower.

Most hams just place the antenna on the side of the tower that gives the best coverage.

2 wavelengths is probably the suggested spacing - hence 80 inches or 7 feet would be sufficient.

Most two meter antenna's I see on towers here locally are side mounted about 3 feet away from the tower, due to wind and ice loading factors.

I would also place a ice guard near the bottom of the tower if you are placing it in a cold environment.

I see lot's of antenna's mounted incorrectly on towers with no ice guards and no security fence and I wonder how many times falling ice has killed tower workers in inclement weather.
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AF6D
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« Reply #8 on: January 04, 2014, 09:52:23 PM »

One of my two meter repeaters uses a Celwave Super Station Master antenna side mounted on a Rohn 65 tower at 350 feet.  I contacted Celwave and gave them as much info as I could.  Their engineering department replied with a tower-to-antenna spacing that would make the tower (almost) invisible to RF.  Having run it like this since 1989 I think Celwave was right.

Your tower has much wider spacing than the Rohn 65 so your situation may be quite different.  I suggest you call DB Products or whoever made your antenna and ask them.

Dick  AD4U

I should have said sooner that we contacted Commscope and received relatively poor assistance from them. Instead of answering the question they simply reminded us that we could buy and use their two foot stand-off kit (that we already have). I was just hoping for something more than what I got. I can experiment but when climbing a commercial tower it might be easier said than done. This vendor charges $25 per visit if more than twice in one month, plus has placed restrictions on using the public road to the commercial site, and, has 4restricted visiting hours to 7a-7p.

Too many rules by the site owner. We moved on. But I will remember 7 feet as an approximation. I checked it in antplot and it showed about the same pattern for 2 feet as it did for 7 feet. Both show a significant null in the direction of the tower.
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AF6D
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« Reply #9 on: January 14, 2014, 01:48:37 PM »

One more thing. I think that since no one that I am aware of makes solder type PL-259's for RG11 cable the using Amphenol crimp-type connectors for LMR400 or possible RG214. I have a crimp kit that I've built from RG174 up to RG214. If one cannot solder it, crimp it!!!
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AF6D
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« Reply #10 on: January 30, 2014, 08:46:44 AM »

One last question if I may, and I sincerely appreciate your assistance:

Does the DB224e have to be mounted 2 feet away from the tower or can its mast be mounted directly to the tower using just cross-over clamps? I can do it either way but with a tapered tower we have to customer make a top sway bar as we did at site 1.

Humble thanks,
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