Building a Dipole

By N6XN

Click any photo to enlarge

The above photo shows the completed center support / insulator.  This one is cut from a piece of 1-1/2 PVC schedule 40 pipe.  The piece upper left is a slip coupling, about .90 cents that might work even better.  The eyebolts are 3/16" and have been cut off.

The photo above shows the completed center support with the two dipole legs attached and the hoisting line connected.  If your dipole is to use end supports only, omit the hoisting line and the eyebolt.  The hole in the bottom of the center support is for the coax. RG8 type is recommended which will take about a 7/16" drill.
About 5" of jacket is stripped off the coax feedline as shown.  The piece in the photo is RG-58 from the junk box.  RG-8 style is recommended for reduced loss but the preparation is the same. An easy method of separating the braid from the center conductor:  Push the braid back toward the jacket until it bunches up.  Using a small nail or other pointed tool, open a hole in the braid and hook the center conductor.  Bend it sharply and pull it out through the hole.
Pull the braid tight and flatten it out.  Feed it and the center conductor through the hole in the support as shown on the right.  Before you make up the ends, seal the coax where the braid and the center conductor leave the jacket.  Silicone caulking and vinyl tape work well.  It's important that the coax stay dry.  If rain gets into the cable it will be ruined.  

The center conductor insulation is removed  and the braid and center conductor are brought out, through the eyebolt and wrapped around the dipole legs.  Solder in place.

 
Now that we've demonstrated a quick 'n easy method (and cheap) let's consider the downside.

Even with simple tools and a small amount of skill, making your center support in the manner shown will take a couple of hours and two or three dollars. You will be feeding a balanced antenna with an unbalanced line (the coax) which is not the best way to do it.  Here's a method that will produce the same dipole for a few dollars more, but take less time overall.  Plus you'll end up with a balun fed antenna.

This is from an article on the "Slinky Antenna" from the June 07 newsletter.  The device on the left is the "Unadilla / W2AU" balun, available from most ham dealers for around $25.  They come in a couple of ratios: 1:1 for flat top dipoles like this one, or 4:1 for folded dipoles.*  Using it in place of the homebrew center support is pretty much the same but with the added advantage of a pre-connected attachment point on the bottom.  Simply install a PL-259 on the end of your feedline and screw it in place.  Be sure to tape it up to keep it dry.

* Folded dipoles have a characteristic impedance of around 300 Ohms vs the flattop which will be closer to 70 Ohms.  A 1:1 balun will provide the same impedance out as the feedline connected to it.  A 4:1 balun will provide an output impedance of 4 times the input... Using 70 Ohm coax will produce an output of 280 Ohms; pretty close for a folded dipole....But Twin Lead would be even better and no balun required.