Center post

Print Friendly

General

The center post of the beam is seated in the baseplate flange and serves two functions. First, it serves as a terminal for all the driver wires to be fed by the coaxial cable and secondly, it provides an anchor point at the top for all the radial support cords that bow the spreader arms into the inverted umbrella shape. You can save yourself the trouble by buying an air coaxial, all aluminum/stainless steel center post here.

The center post should reflect a characteristic impedance as close to 50 ohms as possible as that is the impedance of the coaxial feed cable and it is close to the radiation resistance of the tuned wire sets. There are a number of devices used for the center post by commercial hexagonal beam makers but most, though simple, are nevertheless a bit complicated to build. As a result, many home brewers use a pvc pipe with exterior coaxial sections connecting the wire terminals. This actually works quite as well as the commercial center posts but has the vulnerability of water contamination if not sealed. Coaxial cable, when contaminated with moisture, changes its characteristic impedance and no longer provides the good match that optimizes power transfer to and from the antenna wire sets.

Centerpost specs

1. Measure, mark and drill

Measure two longitudinal marks along the length of a 38 inch piece of 1 inch schedule 80 pvc pipe and then measure off marks for the wire terminals following the sketch above. Drill holes through the wall of the pipe for each of the two terminals for each of the six bands. Use a 7/32 inch drill bit for the holes.

2. Install terminal bolts

Using a piece of stiff wire, insert the bolts with external toothed washers from the inside of the pipe through the holes and install another toothed washer and a nut. This technique looks difficult but actually is quite easy. Tighten the nut after pushing the stiff wire forward to remove it from the bolt. Repeat for all the terminals.

3. Make terminal interconnections

Using RG8X coax cable, cut a piece to connect two adjacent terminals as shown. Solder ring terminals on each end of the piece. Install it on the center post. When you are sure it fits, remove it and apply a generous coat of liquid tape to seal up the braid and center connection to prevent ingress of rain water. This is very important as waterScreen Shot 2014-07-31 at 11.13.39 AM contamination of the coax braid will alter its electrical characteristics and degrade the beam SWR performance. Repeat this process for all the links between the terminals. When all links are dry, install them on the terminals using a toothed washer and nut on each. Make sure the braid is on the same side of all terminals.

4. Make center post pigtail

Cut a piece of RG8X coax cable about a foot long, solder ring terminals on one end and a PL259 connector on the other end. Apply the liquid tape on the exposed end of the pigtail to seal it against ingress of rain water. When dry, Install the pigtail.Screen Shot 2014-07-31 at 11.16.15 AM

5. Make the cap

Drill a 7/32 inch hole in the top of the pvc cap and install a one inch eye bolt using toothed washers. Install the cap and drill a small hole through the side of the cap into the side of the post for installing a 3/4 inch metal screw to secure the cap against violent wind conditions.

Screen Shot 2014-07-31 at 11.12.17 AMComments

Some have suggested use of parallel wire links between the terminals as an easier way to interconnect them and avoid the water contamination issue. This is not a good idea because it is nearly impossible to achieve a low impedance in these links without using 50 ohm coax.  Others have tried to install the coax inside the center post for water protection. This is a nice idea but also nearly impossible to achieve. The commercial hexagonal beams all use different techniques than the one described here. But they require considerable manufacturing finesse and skill that can be avoided by the simple method here.

Screen Shot 2014-07-31 at 11.05.03 AMThe RG8X coax recommendedScreen Shot 2014-07-31 at 11.17.40 AM here is much easier to work with than the larger 1/2 inch coax such as RG8 or RG213. This coax will handle 500 watts easily and unless you are going to use higher power there is no need for the heavier cable.

The liquid tape here is not used on this mock up center post but should be liberally applied on your exposed coax cable links.