Antenna Mast Support Question

[daschnoz]

I'm setting up the shack. I'm planning to mount the antenna on the gable end of the garage.

 

The specifics:

* Mounting brackets - Skywalker 32812 (4" stand-off)

* Mast - chain link fence top rail (or similar), length to be determined

* Antenna - Tram 1481

 

So the question - How much distance should I have between the upper and lower mounting brackets? Is there some general "rule of thumb", like 25% of the mast height above the top mounting bracket (ex. 8ft mast above = 2ft in the mounting brackets)? Or do I take my best guess, mount it, and see if it pulls the side off of my garage?

[WRAK968]

I'm setting up the shack. I'm planning to mount the antenna on the gable end of the garage.

 

The specifics:

* Mounting brackets - Skywalker 32812 (4" stand-off)

* Mast - chain link fence top rail (or similar), length to be determined

* Antenna - Tram 1481

 

So the question - How much distance should I have between the upper and lower mounting brackets? Is there some general "rule of thumb", like 25% of the mast height above the top mounting bracket (ex. 8ft mast above = 2ft in the mounting brackets)? Or do I take my best guess, mount it, and see if it pulls the side off of my garage?

First, MAKE SURE THE BRACKETS ARE INTO GOOD STRONG STUDS!!! Dont just drive them through the sheeting.

 

Now, I can give you the utility pole rule of thumb, should work for this application. So, you use 1' for every 10 feet of pole. This includes the antenna as well, plus 2 feet to the total. So a 20' pole with a 10' antenna, 30', 3+2= a 5 foot space.

[mbrun]

That is not an easy question to answer. It depends.

 

The greater the separation between the upper and lower clamps the more effective stress the mast/antenna can withstand before bottom bracket fails and when wind is blowing some directions. The top bracket becomes the weak link when wind blows in other directions.

 

In my case, I have some brackets spaced 4’-6” apart on a 10’ mast which is holding a large-format TV antenna at 5’ above the roof. I would feel perfectly conformable putting the Tram antenna on top of that same mast without any additional support.

 

The better you anchor the brackets to the building, the better you will ultimately be. If you have access to the attic area of your garage, consider adding some additional interior wood blocking where the bolts for the brackets will go if all the bolts do not already bite into the existing 2x framing. Consider longer bolts too to ensure you are seated well into the blocking.

 

 

Michael

WRHS965

KE8PLM

[daschnoz]

Yes, I plan to strengthen the anchor points. And no, I do not plan to use dry wall screws (the all-purpose screw of home improvement).

 

 

Using the math above (thank you 968), I worked out the following:

 

* P = the total pole length you plan to use for the support mast

* M = Mast -- the length of P that is above the top support bracket

* A = Antenna height

* C = 2ft (or equivalent in your selected unit of measure)

Use the same unit of measure for all values.

You need any 2 of the 3 above unknown values to solve for the 3rd.

 

P = M + C + ((M + A) * 0.1)

Distance between the mounting brackets = P - M

 

If you find yourself getting negative values, something in your plan is either too long or too short.

 

The solver function on my TI-85 calculator is super helpful for something like this.

 

 

I'm going to use a 10' length of EMT conduit for the support pole (P), with a 16' antenna (A). I worked out 5.7' for the mast portion (M), leaving 4.3' between the brackets (P - M). I'll round it and go with a 60/40 split, with 4' between the mounting brackets.