Project of the Month

Build a simple dipole for 477 MHz.

I built this dipole to use as a reference antenna for testing the gain of other antennae, such as yagis. The humble dipole is a great general purpose omnidirectional antenna, and easy to build. If you want to get into building antennae, this is a good place to start.

The dipole has a natural impedance of 75 ohms, but we need it to be 50 ohms to match with our radio. For the impedance match we will use what is called a 'sleeve balun'. A sleeve balun is an open-ended tube, of a certain length, around the coax, connected to the braid of the coax at the end that is not closest to the end of the coax. Sound confusing? It's easy. Read on.

You will need:

About 300mm of 10mm aluminium tubing

110mm of 12mm aluminium tubing

100mm of wooden dowel that fits snugly inside the 10mm aluminium tubing

700mm of 20mm PVC electrical conduit

2 end caps for the electrical conduit

Some RG-58 coax, about 3 metres should do

solder tags, small screws, star washers

Glue (I recommend Diggers Fibreglass Resin from Bunnings)

Step 1

Cut the 10mm tubing into two pieces 145mm long. The length is not critical, but try to get within a couple of millimetres. De-burr the tubing, especially on the inside edge, otherwise it may be hard to insert the dowel. Mark the dowel at the half way point. The 10mm aluminium tube that I used has a wall thickness of 1mm, so 8mm dowel fitted snugly inside it. Push the two elements onto the dowel until the elements are 5mm apart in the centre. Use the halfway mark to get the dowel evenly in both elements, but this is not critical. Drill a small hole, say 2mm or 2.5mm, 3mm from the centre end of each element. These holes are for the solder tags for the connections to the dipole. The size of the holes depend on the size of the small screws you have.

Drill a 10mm hole in one of the end caps, close to the closed end. About 8mm from the closed end should put you in the right spot. It will be easier if you drill a 3mm pilot hole first.

Step 2

Slide the dipole into the end cap and screw the solder lugs onto the elements. This will probably be a bit fiddly. Use star washers under the solder lugs to make sure there is good contact with the element. We are going to glue this all up later, so there will be no chance to repair it afterwards.

Drill a hole about 3mm from one end of the 12mm tubing. Attatch a solder lug here so that it can be bent down to almost half way across the tube opening. I used a rivet to attatch the solder lug, but you can use a short screw. The screw must be short, so as to not block the tube, because the coax is going to  pass through this tube.

Step 3

Carefully cut some of the outer insulation off the coax 125mm from the end, so that the braid is exposed. Strip the end of the coax back about 12mm. Trim off some of the braid, and twist the rest up. Soak the twisted-up braid in solder, but be careful not to melt the inner insulation of the coax.

Slide the coax into the 12mm tube, from the end with the solder lug. Solder the lug onto the exposed braid of the coax. If you have done it right, there should be about 2mm or 3mm of insulation extending past the end of the 12mm tube. It is important that the braid does not touch the tube at this end. Try to get the coax entering the tube straight down the centre.

Step 3

Solder the inner and outer of the end of the coax onto the solder lugs of the dipole. This will be fiddly. Try not to melt the end cap. Make sure you get good solder joints, because you will not be able to repair this antenna once it's finished. Check the SWR if you have a meter, it should be perfect. None of the dimensions of the dipole are critical, so there's no need to worry if it isn't exactly right. The first time I made one of these, I cut the 12mm tube (sleeve balun) 5mm too short, and the difference in SWR was 1.05 instead of 1.00. No big deal. If you don't have a swr meter, just check that you can hit a repeater with it, or contact someone on it.

Step 5

Once you have confirmed that the dipole works, you have to insulate it from the weather. If water gets into the wooden dowel in the centre of the dipole, it will affect the performance of the antenna badly, and will probably take ages to dry out. Fill the end cap up with free-flowing glue. Slide the PVC tube over the coax and sleeve balun, and into the end cap. Stand it up on end to dry. Drill a 5mm hole in the other end cap and feed the coax through the hole and slide it up to the PVC tube. Fill this end cap with glue, and slide it onto the PVC tube. Stand it on end to dry. Lastly, pour some varnish into the ends of the elements, to seal the wooden dowel. Do these one at a time, and stand the antenna with the element upwards until the varnish dries. Then do the other element.

Construction Diagram : Click for printable picture.

Final notes

Polish the elements with fine steel wool before assembly. It lowers the surface resistance, and thus increases the velocity factor, which improves performance. It's true. I'm not shitting you. I polish all my antennae.

Do not drill holes into the PVC tube for a mounting bracket. The antenna must be weather proof. Use a bracket that clamps over the tube for mounting. If you're worried about crushing the PVC tube, slide a snug-fitting piece of aluminium tube over the PVC tube before you put the rear end cap on. Glue the aluminium tube to the PVC tube so that the centre of the aluminium tube is 630mm back from the dipole. When mounting, mount the antenna so that the dipole is 630mm away from the mast. This is so that the mast does not affect the antenna radiation pattern.