Low Power FM Transmitter Antenna From Agricultural Tubing

Building an FM transmitter antenna isn't that hard; there are plenty of designs out there. We wanted to make a design from parts you could get almost anywhere in the world for a set of four (soon 16!) community stations we started in Northern Uganda. Luckily, agricultural and garden irrigation systems use a very standardized set of parts that can be hacked into a strong, reliable, and inexpensive dipole side-mount antenna.

Why agricultural irrigation hardware? Glad you asked. It's made to conduct water at high pressure. And it can also conveniently keep high pressure water outside from getting to its interior. The connectors are always plastic but the tubing itself can be plastic or aluminum. Aluminum conducts, so it's great for antennas. Together this means that nearly any rural hardware store, anywhere in the world, has everything you need for a reasonable antenna (except perhaps coaxial cable).

Note the key parts in the last image above: there's a sky pole and a ground pole that are the actual antenna, attached by a T-junction to a side pole, mounted to the tower.

Some key things to remember:

you should use 50 ohm coaxial cable; 75 ohm is more common but won't cut it
don't kink or bend your coax -- those electrons get confused
keep ground and sky in mind
we'll show you how to make the antenna, but lightning mitigation is a whole other ballgame
FM is regulated, you probably need a license
you'll have to calculate the length of the antenna based on the carrier frequency you will be transmitting
you can't tell how well the antenna is working without some sort of SWR meter -- ours with about 100' of cable read at a respectable 1.4:1
FM is line-of-sight: a powerful transmitter and great antenna are nothing without verticality

What you'll need:

coax cable (50ohm)
agricultural tubing (aluminum), 3/4" or larger diameter
an agricultural T-junction for the diameter above
a rivet gun and rivets
two circular crimp-style terminals of greater diameter than the rivets
a drill and bits
UV-safe zip ties or similar
marine sealant or similar
mounting to your antenna tower

How much this all costs depends on what you might have on hand already; in terms of raw material only (minus coax) we have built the antenna for under $20. Three of them have been working with no noticeable SWR increase over a year and a half in tropical conditions.

Let's get started!

Step 1: Step 1: Git Yr Stuff at the Gittin' Place

Here's Jude getting tubing in a market in Kampala, Uganda. How much do you need? Not more than 5' 3.75" (159cm) for the vertical poles (you'll need the side-pole too, see below), if you're broadcasting at or over 88.1fm. Indeed, you'll need to subtract a bit as the plastic T-junction creates a bit of a gap -- the tubes can't meet within it because of internal spacers molded into the plastic. As different diameters and makes of T-junctions vary, we're not going give a number for the length of the gap here, you'll have to measure. Just remember that both the sky (upper) and ground (lower) pole of the antenna should be the same length, and together as close to the frequency length you've calculated as possible.

Which leads us to the diameter of the tubing. The diameter of your antenna tube increases the bandpass of the antenna. So a very thick tube would allow a lot more slop in how you cut your poles, or give you the possibility of upping or lowering your transmission frequency by a bit (though your regulator probably won't be happy). The downside of the thicker tube is weight and wind load -- a thick tube will have more to contend with in a storm. We used 3/4" tube and were careful about lengths.

For the side-pole that sticks the antenna out from the tower (necessary or the tower will ruin your signal), you'll want about 1m of tubing, and a way to mount it. This will again vary by tower, we leave it to you.

Step 2: Step 2: Prepare the Coax

Measure the distance between the molded internal spacers on the top of your T-junction, and add about 1/4". This is the length you'll have to expose from the insulator of the antenna end of your coax. You'll want these parts to stretch like a T as much as possible without bending or kinking.

Be sure to include the length of your crimp terminals too. I did this all by eye, laying the wire and crimps up against T-Junction to test. The interior wire (core) will be the sky wire, while the sleeve wire (ground) will be ground. They have to be matched in length.

Note that I wrapped the ground in some electrical tape just to make sure no bristles could short circuit. Crimp and, if you know how to, solder the wire into the crimps.

Step 3: Step 3: Prepare the Side-mount Rod

Eventually your coax will travel up the tower and to the antenna along the side-mount. Because the side-mount will almost invariably angle down a bit because of the weight of the antenna, its tower end can catch rainwater and potentially channel it toward the antenna end, causing internal moisture and corrosion. We chose to have the coax run through an oblong hole (the cable hole) in the bottom of the side mount. The cable hole is oblong to accommodate a gentle bend in the cable, and it is on the bottom to prevent moisture. You can make it oblong by angling the drill bit back and forth along the axis of the side-mount (fig 1). We added two small holes on the bottom tower side of the cable hole to cinch the cable tight, and two others on antenna side top of the side-mount tube, to prevent chafing against the edge of the hole. Finally, we filed the cable hole edges and covered them with duct tape -- in our case Gorilla Tape (figs 2 & 3). Note the nice, smooth curve when the coax cable goes through the cable hole.

At this point, it makes sense to pass the cable through the side-mount, crimp ends first. You can do this later, transmitter-end first, but that takes longer and your cable may already have the transmitter connector on it. Pass lots more than you need on the tower end... you don't want the side-mount tube getting in the way of the fiddly work you still have to do.

Step 4: Step 4: Prepare the T-Junction

With the cable through the side mount, slide the necessary T-junction hardware on the side-mount. This is basically a screwing end-cap with a plastic o-ring, and an internal sleeve that puts pressure on the o-ring. The tension is mounting and it's time to prepare the T-Junction, sky pole, and ground pole for their inevitable three-way. This is a critical part of the construction process, so allow me describe a bit of what we're doing here.

So far, we have a side-mount tube with coaxial cable coming out its antenna end. Each crimp connectors on the coax need to be connected to the right pole of the antenna: sky for the core of the coax and ground for the sleeve. But the two poles are held together -- and to the side-pole -- with a plastic T-junction.

There may be other ways to do this, but we chose to mount the coax crimp connector, pole, and T-junction together with a rivet. Good rivet bonds are strong and essentially cold-weld metal components, meaning a fair amount of current can pass through the abutting faces. They can corrode, but our irrigation hardware can keep the assembly waterproof. They can stretch with torsion, but the T-junction actually creates its own strong mechanical connection, so the rivet will never be under stress. They are not necessarily waterproof, so we use tape or sealant over the outside of the rivet to keep everything dry.

The steps are numbered in the illustration above:

(fig 5) Use your drill, with the right bit for your rivets, to make holes in the T-junction where the rivets will join the poles, T-junction, and crimp terminals together. This should be about 1/8" away (toward sky on the sky side, toward ground on the ground side) from where the molded ridges stop the tubing from meeting at the center of the T-junction.

(fig 6) Put the sky pole into the T-junction as far as it will go. You should see its aluminum exterior filling the hole you just drilled in the T-Junction. Now use the drill again in that hole to drill through the aluminum sky pole. You'll now have two perfectly matched and aligned holes. You can now remove the sky pole.

Repeat this operation with the ground pole. Insert it, drill, remove.

Step 5: Step 5: Crimp Like It's the 80s

Now begins the fiddly. There's probably no way to do this elegantly.

Somehow the sky pole, T-junction, and coax core with its terminal need to be joined together with a rivet. Blind rivets have a flat outer side that holds the outside of whatever is being connected, and an inside that starts cylindrical but mushrooms to hold the inside of whatever is being connected. In our case, we need the coax terminal to be squashed together against the inside wall of the sky pole. If you do it wrong, they will not be squashed together, and this will significantly reduce the performance of the antenna. So take your time.

Pass the two terminals into the base of the T-junction
Push the sky pole into one arm of the T-junction, and align its rivet hole perfectly with the T-junction's
Align the coax core terminal well enough with those holes, and push the rivet through all three layers. You may need to use a screwdriver, a hemostat, or any number of kludgy techniques to get all three lined up and tight
(fig 7) While keeping all three layers tight against each other, set the rivet
Now write or scratch "sky" on the sky pole

This might have been quite frustrating, and taken you a while to do. Rest assured that the next step will make this look comparatively simple!

Step 6: Step 6: Add the Ground Pole

However you attached the sky pole, you probably used the open arm of the T-junction to see things and hold the layers together. With the ground pole, you have a bigger problem, since there is no longer any access. For this reason, you will need to drill a hole 90 or 180 degrees off from the ground rivet hole. Its placement doesn't need to be exact, and it can be larger than the rivet holes, but it will give you control access to the ground rivet. This needs to be sealed afterwards.

Drill the access hole, and insert and align the ground pole. Now align the ground (sleeve) coax terminal and pass the rivet through the T-junction, ground pole, and terminal. Using a screwdriver, hairpin, or The Force hold all three tightly together, and set the rivet.

Now write or scratch "ground" or the ground symbol on the ground pole.

This is a good time to do a continuity test on the tower end of the coax. There should be infinite resistance between core and sleeve (sky and ground). If not, you didn't do a good job when you trimmed the wire: maybe you should stick to software. I would also test the core of the coax to the sky pole (should beep), and the sleeve to ground pole (should beep).

Step 7: Step 7: Tighten Up and Seal

You should now have good electrical connections for the antenna. You can now slip the T-junction hardware -- end ring and o-ring tightener -- over the sky pole and ground pole to the T-junction. Screw tighten the sky and pole T-junction connectors. Next you can insert the side-pole into the base of the T-junction and tighten it. Note that in (fig. 10) the hole is facing you rather than the ground -- it should actually face the ground. The whole unit should now be very static, and all parts should be mechanically rigid. Finally, you can add zip-ties to hold the antenna cable and prevent chaffing on its hole or stressing the crimp terminal connections.

Now you can seal the hell out of everything. We used "Coax-Seal" [sic] everywhere: the top of the sky tube; the coax hole; the rivets. Marine sealant would work, as would regular silicone sealant available everywhere. The bottom of the ground tube doesn't necessarily need sealant, but in sub-Saharan Africa bees are for realz, and you don't want them angry when you're on top of an antenna tower. We used sealant and a bit of bug screen so that the bottom could breathe and help dry out any moisture that might get into the assembly. But we don't think any moisture is getting into the assembly.

Mount the antenna, but not if it looks like lightening.

Step 8: Step 8: Jam on It

Time to crank the transmitter and cue the Newcleus. SWR should be low, unless your coax is insanely long or crappily thin.

For more information about the stations we are running out of buckets, please visit http://rootio.org web site. Do let us know if you have any questions or especially if you find any mistakes -- we'll revise asap. Lastly: avoid Thor.

-- RootIO team