On Sun, 18 Dec 2011 08:30:46 +0000 (UTC), Frank Wilson
>I have an old 3.5 inch ID steel tube sunk in 30 inches of concrete. I'm
>guessing this was an old satellite dish cut off flush with the surface
>(unfortunately). No threads but the steel is 1/4 thick (or so) and in
>good shape and it goes down to 30 inches in the concrete.
That would be about the size used for a big C-band 8ft dish. DBS
dishes use smaller pipes, usually 2".
>I simply need an antenna mast of about 12 feet high for a new WISP 802.11
>antenna & radio setup. Winds can be high at times. Soil is rocky clay. I
>prefer NOT to have guy wires as the kids play in that area.
How's your stick arc welding? Jack hammer enough of the concrete from
around the pipe to expose enough of the original 3.5" ID steel pipe to
make an effective weld. Find an internal sleeve that's approximately
the correct size. My guess is 3" steel pipe will have roughly a 3.5"
OD can will make a good sleeve. You might have to do some grinding to
the OD in order to get it to fit. Pound it into the original 3" pipe
in the ground. Make some effort to get the pipe to be exactly
vertical. It makes antenna aiming MUCH easier. Tack weld in place.
When you're done welding the stub/nipple, you'll probably need to mix
a little concrete and fill in the area around the welt to prevent
water accumulation. I suggest removing as much slag as possible and
painting with a rust resistant paint, before applying the concrete.
At this point, you have several choices. If you use a threaded 3"
steel pipe, you can attach any of an assortment of pipe fittings to
extend the pipe to the desired altitude. However, I wouldn't do that.
12ft is a LONG way to go with a single steel pipe. Even with pipe
fitting or pipe union, it's going to wobble. You're also looking at
7.3 lbs per foot for 3" pipe or 88 lbs of pipe.
If you don't like guy wires, you'll need to do a good stick arc
You can go to a smaller diameter pipe, but if your 2.4GHz antenna has
a narrow beamwidth, you're going to have alignment stability problems,
especially when the wind is blowing. For example, the common 24dBi
parabolic dish antenna has about a 6 degree -3dB beamwidth, which has
to be aimed within about a 3 degree range to be usable. That's not
easy on top of a 12 ft pole unless it's VERY stable.
>Q1: If I stick with the original idea, where would I get a 3.5 inch OD
>pipe that has threads that can hold a reducer to more available widths?
Any plumbing vendor. Ask for 3" ID 40 steel pipe. I would NOT try to
pound a 12ft pipe length into the 3.5" pipe that's in the ground
(unless you have a 15ft ladder and find it entertaining to use a
sledge hammer on top of the ladder). Use a welded stub/nipple pipe as
>Q2: If I go with what I have right now, how would YOU shore up the pipe
>but still allow easy removal for maintenance?
Nipple sleeve welded in order to extend the original pipe above ground
level. Use a pipe coupler if you want to make it strong an permanent.
Use a pipe union to make it removable but not so strong. Unions are
not made for this type of service. I couldn't find if it will handle
the bending load. That should be researched before building.
Why do you need to lower the pipe? A 12ft step ladder should be
>The 14 dBi planar 2.4 Ghz antenna at top is roughly pointed at the WISP
>access point; the Bullet M2 radio tells me it has an ACK/Distance of
>55/2.6 miles respectively (I'm not sure if that's important); this radio
>sees a signal strength of -56 dBm (pretty good); with a transmit CCQ of
>83% (just ok).
Ubiquiti products are good. A 14dBi panel antenna means it's probably
a sector antenna (kinda rectangular looking), which will have about a
15 degree vertical beamwidth and about a 60 degree horizontal
beamwidth. That's not very critical or difficult to aim. If your
pipe is really vertical, and you have line of sight, you could
probably preset the antenna and not worry about tweaking it once
>I am stuck at HOW to fix the antenna in the ground so that it won't
>wobble & spin (short of digging a new hole & concreting it in) yet being
>able to service the antenna (I have ladders but not a 12 foot high step
>ladder - plus the ground is sloping so it's unsafe to ladder it).
As previously mentioned, chip away some of the concrete. Pound in a
3" pipe stub/nipple. Arc weld. Add mast.
>Do you have ideas how to affix the antenna mast into the old 3.5 inch ID
>hole while still being able to remove the entire mast for antenna service?
Yep. Get a 12ft ladder. Without guys, the pipe will not support your
weight so forget about slithering up the pipe using foot pegs or an
ascender. If this bothers you, I suggest you forget about the pipe,
and switch to a 10 ft section of Rohn 25 tower. Weld the base plate
to what's left of the 3.5" pipe in the ground. You will also need to
drill into the concrete to secure the base plate mounting bolts.
Hmmm... with small brats climbing everything in sight, maybe not...
>NOTE: In the future I'd like to add a TV antenna but that's not the
You'll have a problem trying to get the 1.625" U bolts that come with
the typical TV antenna to fit the pipe.
Good luck. Try not to kill yourself doing this.
Jeff Liebermann email@example.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558