How Deep does a 40' Mast Need to go in the Ground?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KG5NNA, Dec 5, 2016.

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  1. KG5NNA

    KG5NNA Ham Member QRZ Page

    I just bought a 40' telescoping mast (ROHN H40) so that I can put a UHF/VHF antenna on it along with an Alpha-Delta muti-band sloper antenna. I want to put this right nex to my house (bracketed about 20' up at the top). There's about a foot overhang on the roof.

    I figure I'm not going to get the full 40' because I'll need to put it in the ground. The soil here is mostly clay (North Texas). There are occasional storms that will come through with 50mph winds (gusts to 70, rare). I'd like to put it up with no guy wires since I'll bracket it to the top of the roof.

    How deep would I need to put the mast in the ground let's say, given that I expect to get a cement cylinder form about 12-15" in diameter? The mast is 2" diameter at the bottom and 1.75 at the top of the 40'. Mast weight is 36lbs. The antenna will be negligible, say, less than 5 lbs.

    I've looked around on the net and can't seem to find any calculations that fit this application (big towers, yes!).

    73,
    Korky
     
  2. WR2E

    WR2E Ham Member QRZ Page

    To save some people's time, here's your previous thread about this mast:

    https://forums.qrz.com/index.php?threads/do-i-need-to-guy-a-mast-thats-bolted-to-the-house.542829/

    I myself would be following the directions of the manufacturer, and if trying to second guess their engineers, I would at least contact them for their recommendations.

    It's interesting that their product page shows a mast with a small antenna on it, apparently bracketed to the house and no guy wires.

    http://www.rohnnet.com/rohn-telescoping-masts
     
  3. AC0GV

    AC0GV Ham Member QRZ Page

  4. AK5B

    AK5B Ham Member QRZ Page

    Living anywhere of Texas I would guy that mast even if there was NO antenna on top; 5 lbs even at a small wind loading factor is not negligible!

    The good news is that you don't really need to put the mast into the ground so long as you have a sturdy base mount of some sort that is not going anywhere. A pipe flange with a short length of pipe large enough for the mast to slip into would be enough if the flange was securely bolted to a small slab of concrete or thick steel plate.

    Have fun but be safe in the process!


    73, Jeff in South Texas
     
    KC8VWM likes this.
  5. WR2E

    WR2E Ham Member QRZ Page

    I don't think I would embed it in concrete either... unless you absolutely had a way to drain water from inside the pipe.
     
    AK5B likes this.
  6. KC8VWM

    KC8VWM Ham Member QRZ Page

    The only time I wouldn't install any guy wires on a Rohn mast is when all the telescoping sections are entirely collapsed together while laying horizontally on the ground.
     
    AI3V, K7GQ, WB5YUZ and 1 other person like this.
  7. K6BRN

    K6BRN XML Subscriber QRZ Page

    Korky:

    First, I concur with others who are wary of putting up an unguyed Rohn telescoping mast 40 feet with both a UHF/VHF antenna on top and as a support for a wire antenna. That mast is designed to be guyed when extended, will sway excessively if that high with that load and will eventually buckle and fall. If you heavily overlapped the sections (say, 3 feet) you MAY get away with 30 feet total height as a wire antenna only support, without guying. We've discussed this before.

    Then we'd have to consider your first question... how do you mount the mast into the ground. If it is next to your house, Rohn has a simple "spike" plate that can be inserted into the ground at the base, while the body is secured to your home using any one of many side or eve mounts. Still, I wouldn't load the mast too heavily or go up too far without guying.

    If the mast is to be mounted in the open, a simple way to secure it to the ground, if and only if the ground is firm but not rocky, is to drive a 10 foot long galvanized steel pipe whose diameter is just smaller than the Rohn mast lower section, about 5 feet into the ground with a 5-pound sledge hammer. The top of the pipe will be crushed by the hammer, so after driving the pipe into the ground, the top 3-9 inches will need to be taken off with a hacksaw. It is critical that the pipe be kept vertical when being driven in and a plumb bob should be used as a reference. The Rohn mast lower section can then be slid over the pipe and secured with a 1/4 inch bolt driven through both pipes (yes, you will have to drill a hole through both pipes to allow this). The difference in diameter between the Rohn lower section and the ground mount pipe (it should be less than 1/4 inch) can be taken up by winding duct tape "donuts" every 6-8 inches along the ground mounted pipe. The Rohn lower section should then be able to slide over the donuts with just a little friction and play, and THEN you should drill a hole through both pipes and secure them together with a 1/4 inch stainless steel bolt.

    NOTE THAT THIS QUICK AND DIRTY (concrete-free) METHOD WILL NOT WORK UNLESS YOU ARE DRIVING THE GROUND PIPE INTO FIRMLY PACKED AND STABLE SANDY OR CLAY SOIL FREE OF ROCKS! I've used it successfully at a QTH in Southern California (packed sand) and in alluvial clay in Connecticut, with good success.

    With all sympathy, if you are not an accomplished "do-it-yourselfer", it might be better to ask a local antenna installer to work out a good approach for you and put the antennas in. There may even be a simpler way to accomplish your goals. A falling 30-40 foot steel mast can do a lot of damage, injure or kill somebody. This is not where any of us would like you to be.

    Brian - K6BRN
     
    AK5B likes this.
  8. KG5NNA

    KG5NNA Ham Member QRZ Page

    I apologize for double posting. I totally forgot the previous one.
    One of the frustrations I've felt in becoming a HAM, is how much dependence there is on height. And for my location, that's a bad thing. I live in a single story, four-bedroom house. The height to the pinnacle of the roof is less than 20'. I bought the ROHN H40 (they didn't have the H30), on the recommendation of a local engineer that looked at the specs of the antenna itself. For those that haven't seen the antenna, it's galvanized and is a solid mast (not an open, hollow one). Yes is has telescoping sections. And it has guying rings. His suggestion was that I put it in the ground about 4 feet in concrete (he gave me some formula that he figured in the soil force and the wind force. He didn't know exact numbers but with some fudge factors came up with 4 feet that took into effect a 50mph continual wind with occasionally 70 mph gusts (very rare here in Dallas).

    That said, I also count on my fellow hams as they have MUCH more experience than me. I have no trees on the property, except those landscaping kinds like crepe myrtles (they are about 16' tall, however). The wife would be very upset if I just did a freestanding mast in the middle of the backyard! Which was why I wanted to bolt it to the house to where only about 13' would be above the house. At that point you couldn't guy it anyway because there's no path to guy it to ground.

    Believe it or not, I created a 146MHZ dipole out of 14 ga wire, an old pill bottle and some fishing line to suspend it inside my ham room (spare bedroom). I seem to be able to hit the repeater easily, and get reception all the way to Sacramento, CA so far. But it's noisy. Being 5'2 and 64 years old, my climbing days are over and there are NO resources around here that will actually come out and put an antenna up. So I keep searching for more alternatives.
     
  9. WR2E

    WR2E Ham Member QRZ Page

    If it's solid, how can it be telescoping, and how can it weigh so little?

    No, that's not good enough. He MUST know the exact numbers. This isn't something that one should 'fudge'.

    Which is OK I guess, but how do you know which ones KNOW and which ones are full of BS?
     
  10. N2EY

    N2EY Premium Subscriber QRZ Page

    Putting the bottom into concrete is completely unnecessary. In fact, it's counterproductive, because there will be no way for water to drain out, and no way to take the mast down.

    All that's needed at the base is some sort of mounting such as the "spike plate".

    The real strength is being bracketed to the house.

    The problem is not that the mast will fall over - it won't. The problem is that the part above the house bracket will flex in the wind. That's why guy lines are used - because the mast isn't stiff enough not to flex in the wind.

    btw.....you're a ham, not a HAM. It's not an acronym nor a proper name.
     
    KQ9J, N9ATV, KC8VWM and 2 others like this.

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