How much mast should you "grab"?

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LIScanner101

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The title may be unclear, but it’s not about towers but just regular ol’ antenna masts. I didn’t know where else to post this so I’m starting here – if the mods feel it belongs someplace else my apologies and please feel free to move as you wish:

I’m looking into moving either my Channelmaster Monitenna 5094A or Antenna Specialists MONR31 from the attic to the outside. The location I’m looking at is on a chimney mount. I plan on using a 15’ metal mast – I have seen 5’ and 10’ sections that you can just fit together. Anyway, my concern is how much of the mast needs to be captured by the chimney mounts. Since my home was extended many years ago the builders didn’t extend the chimney a hell of a lot – it might only be about 4’ above the roofline.

Since I want the antenna as high as possible I want as little as possible of the lower part of the antenna mast “grabbed” by the mount. Is there a “formula” that needs to be adhered to when mounting masts with chimney mounts? is it 2', 3', 4' what? I just wanna make sure my antenna is secure and safe. Thanks!
 

Blackink

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Chimney Mount...

This is what I did. It may be overkill but I didn't want the thing coming loose or coming down!!

http://forums.radioreference.com/attachments/scanner-receiver-antennas/42471d1375816423-st-2-antenna-finally-up-st-2-antenna-005.jpg

The gray pole just to the right of the chimney is a dish satellite mount and the base is bolted to my metal roof.
The chimney blocks are 8" high and the set-up/straps are used on 3 of them.
The chimney mounts are: Amazon.com: Channel Master CM-3080 12-Feet Antenna Chimney Mount for TV Antenna Masts: Electronics, I used 2 sets.

There is metal flashing on each corner and for extra reinforcement I used 1"x1" aluminum angle iron on each corner also.

The pole I used is 10' long/high (electric conduit pole) slid down into the dish satellite mount and my ST-2 is at the top of that.
I'm sure I didn't need the dish mounting pole but I wanted it solid and tight.

The cable you see is the guy wires not completely attached at the time of this picture.

I inspected everything 2 days ago when removing snow from the roof and all is secure and still solidly attached.
 

LIScanner101

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Wow Steve, that ain't goin' anywhere :lol: !!! Thanks for the info!

I've seen your setup pictured in your link on another thread and I must say I am very impressed! It looks strong and solid. I was also thinking about at least doubling-up on the straps like you did, can't hurt.

Do you know if I would need to guy a 15' mast (10' and 5' connected together) with either a Monitenna 5094A or a MONR31 up top?
 

LakeMan2

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The answer will depend on the wind load of your antennas. Based on the wind load of your antennas and the height of your mast you will exert a certain load on the chimney and the anchor. If you look at the specs for your antenna or do some searching on them you should be able to find a wind load. With that, your mast height, and the max wind velocity in your area, there are some calculators out there like this one:

Antenna Wind Load - Antennas: Theory: Wind Load

You should be able to estimate the load. I have not looked, but the chimney mounts might have a rating that you could compare to your load.

Bottom line is that the bigger your antenna and the more elements (higher wind load), the taller you get, and the sturdiness of your chimney, you will want more of the mast anchored.
 

Blackink

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Wow Steve, that ain't goin' anywhere :lol: !!! Thanks for the info!

I've seen your setup pictured in your link on another thread and I must say I am very impressed! It looks strong and solid. I was also thinking about at least doubling-up on the straps like you did, can't hurt.

Do you know if I would need to guy a 15' mast (10' and 5' connected together) with either a Monitenna 5094A or a MONR31 up top?
Yes, the one thing I did want was the extra chimney straps. I was originally going to use just 3 but once I got going, I decided to use 4.

I certainly would have used 4 chimney straps without question if I hadn't used the Dish pole.

It serves my purpose and in the future if I decide to go higher, the set-up will handle the extra length.
 

McCroskey

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Four things:

1. How are you coupling your two piece mast? That needs to be engineered correctly to have the right amount of strength. Always use one mast if possible, or really make sure whoever you are getting your masts from knows what they are doing.

2. I haven't looked at those antennas but what are their combined wind load ratings?

3. Channel Master says they would guy any chimney mounted masts over 10 feet high.

4. I've heard good things about this company and these mounts, I'd check them out: Ronard Industries Commercial Chimney Mounts 4424 - Free Shipping on Orders Over $99 at DX Engineering Good engineering comes at a price, and in my mind I'd much rather spend more on a mount and not have to worry about some $10 or $20 bracket damaging my chimney.
 

Blackink

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Looking at those mounts from the link that McCroskey posted, I'm not worrying about overkill on my set-up!!!

Yowser!!! I think that chimney mount will hold a flag pole with the flag on it!
 

n5ims

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The general rule is the further apart your supports are (within reason, of course) the stronger your support will be. One thing you can do to test this is take a broom and hold it at the bottom with your hands close together and see how easily it'll move around. Now move your hands further apart and see how it becomes more rigid as the space between your hands gets greater.

I agree with others that have pointed out that a single longer mast will be much more sturdy than the same length mast made by joining two together. It's easy to find them in 10' sections (the conduit mentioned above for example) but it isn't that hard to find some a bit longer. There's also some chain-link fencing top rail that should be OK as well (http://www.grainger.com/product/Top-Rail-4LVL2?functionCode=P2IDP2PCP for example).

Just make sure that you don't overload your mast or it may bend down in a strong wind and not be too good for your antennas.
 

McCroskey

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Looking at those mounts from the link that McCroskey posted, I'm not worrying about overkill on my set-up!!!

Yowser!!! I think that chimney mount will hold a flag pole with the flag on it!

Wind and water are two forces in the world with far more destructive power than what people typically believe.

Your county publishes a wind speed rating for local towers, so at minimum I would recommend reviewing that and ensuring your chimney mount and attached antennas are capable of withstanding 3-second gusts at those speeds (normally anywhere from 55-90mph). FYI, Champion Radio has a link on their Tower FAQ page to find the speed rating for your county.
 
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