Antenna tower base

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K4VBB, Mar 18, 2020.

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  1. K4VBB

    K4VBB XML Subscriber QRZ Page

    Well, the aluminum tower that fell into my lap a few weeks ago now checks out structurally. After a few minor repairs, that is. Mechanically, however, it was a different story. The entire pulley system had to be re-worked, but it wasn't a big issue and I got it done.

    The tower itself did not come with a tower base that was conducive to mounting on concrete. At least not a design I was willing to respect, in any case. Since the tower is aluminum construction, I 'could' use steel for the base, but over time steel will create an undesirable electrolytic reaction between the steel base and the tower, so I'm not inclined to use steel. But alas, I had some scrap 3/8" aluminum laying around and am making the mounting flange from that stock instead. Here are some pics of the beginnings. Cutting the base out with the plasma cutter. I need more Argon before I start welding up the supports, but it'll be coming along shortly.
     

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  2. K5WY

    K5WY Premium Subscriber QRZ Page

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  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I believe the OEM (factory) bases that come in contact with concrete are galvanized steel; at least the ones I've seen are, even if the tower is aluminum.
     
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  4. K4VBB

    K4VBB XML Subscriber QRZ Page

    Thank you, Gentlemen. I had no clue that this could be an issue, although it was never my intention for this base to be in direct contact with the concrete in the first place. This base will actually be supported on bolts/nuts with large SS washers so that I can level it before I bolt it to the side of the house. Between the first nut, the lock nut, and the washer it'll likely be a bit more than an inch off the ground after everything is said and done. But that's good to know. Still learning something every day.
     
  5. K4VBB

    K4VBB XML Subscriber QRZ Page

    On another note, the steel tower is sound with the exception of the bottom 24 inches. There are cracks in the lower portion that I could see once I got the paint off, and the magnaflux revealed many more. Looks like someone had some portion of it bedded in concrete without weep holes. It's likely that water crept in from the top of the tower and froze at the bottom and started to split the tubing. But this is an easy repair after I cut the bottom two feet off of it. I can reconstruct it fairly easily.
     
  6. WB2WIK

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    If the "base" is supported above the concrete pad (using j-bolts into the foundation?) it should be fine.
     
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  7. K6LPM

    K6LPM Ham Member QRZ Page

    Nice work and definately sounds like you have access to some nice fabrication tools in your shop. I assume that the person who uses these types of tools probably has experience and and a good idea of the materials in which they are working with...
    I am not a professional machinist or any type of metal fabrication expert, but it does seem as if 3/8" aluminum plate might be somewhat light duty for a tower base?
    I fabricated my own steel base for a old Hygain styled crank up. I basically copied the design as best as I could compared to what I could gather from the original base. Tajian Towers still builds and services the same model but I for several reasons chose not to purchase their base. I relied on many pictures and design information from past installations and design to build my base. It is difficult to compare my style of base to the style of yours. I think that your tower utilized a style more similar to the Texas towers Tx series bases. with the flanges welded to a flat triangular baseplate. My tower utilized special alloy weldable 3/4" rebar, with flanges welded upon it.
    I constructed those flanges with 5/8" steel.
    I would not consider less than 1/2" plate appropriate especially depending upon the type of aluminum alloy that you may happen to use.
    Take what I say with grain of salt but in my limited opinion I would bump up the thickness of you base plate material, especially with aluminum plate. I am sure that there are some alloys more appropriate than others!
    Given your shop access and tools such as magnafluxing and argon tig welding, I am sure you have experience with such materials....
     
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  8. K4VBB

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    It 'might' be somewhat light duty if I were erecting the tower in a free-standing manner. But this tower will be bracketed to the side of the house, so all this base will need to do is
    1: level the tower
    2: rovide a better base support system for the tower itself
    3: provide a hinge mechanism that can be used to easily erect/lower the tower for maintenance/inspection.

    Incidentally, the tensile yield strength of 7075-T651 is 75,000 psi, and ultimate tensile strength of 83,000 psi. An unsupported tower could reach this pressure on points of the tower base in a good wind. In my installation the wind pressure would need to overcome the shear strength of whatever through-bolts I'll be using to secure it to the house, the modulus of rupture of whatever bracketry I design (haven't gotten there yet), and the shear strength of the guys that will be in place (and their mounting system) before it could impart the totality of this force to the base. It is likely that the tower will yield before that can happen. These are the advantages of a bracketed tower, as opposed to a free-standing tower. There are others.

    It's likely that your tower support structure will be in place after whatever catastrophic event obliterates your tower. Good on you, let's hope it never happens. Once embedded in concrete, these structures cannot be inspected, so it's a good rule of thumb to overbuild in order to have a reasonable level of confidence in the system over a period of time.

    Oddly enough, I hardly ever work with aluminum. I normally work with steel and make parts for offroad trucks. This piece was left over from a job where they wanted some pieces cut out for use as a flange on some kind of aircraft (never did find out what the parts were for). I don't do this sort of thing for a living so I'm sure that there are people much more educated on the subject than I am.
     
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  9. K6LPM

    K6LPM Ham Member QRZ Page

    [] 20170816_004222.jpg 20170816_004207.jpg
    Looks like you have a very good grasp of understanding what you are doing. I am looking forward to watching you progress with this project. I am sure it will be very nicely done once completed.
    Please keep us updated.
     
  10. KC3HUM

    KC3HUM Ham Member QRZ Page

    Nice Job so far. I like the plasma table. But like others here I wouldn't choose aluminum for a tower base. Galvanized steel is the choice even for a aluminum tower. Galvanic corrosion is very very small with aluminum contact with galvanized steel. Aluminum fatigues and its really in the means its secured to your base. Idk, but my aluminum tower is designed to slide into three galvanized steel tubes/pipe that is then welded to the steel base.

    I run and own my own fabrication shop fyi, and I'm very experienced with Aluminum fabrications and weldments. But if your anchoring to the house way up the tower, then Id say your fine if you do a good job attaching/welding the tower to the base. Hope you have a 250-300 amp tig welder if your welding to that base. Rock on...

    Torchforcewelds.com is my business in case anyone is curious.
     
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