Saturday, February 11, 2012

Building the G3TXQ Broad Band Hexagonal Beam

Building the G3TXQ Broad Band Hexagonal Beam

Six Steps to build it

The electrical design of this hexagonal beam is the same as the beam featured in the March 2009 QST article by the author. However, the mechanical details here differ somewhat and are an improvement, in my opinion, over the mechanical design in the QST article. The guidelines here are for a six band hexagonal beam. 
However, if only three bands are desired or five bands, the same specifications apply for the bands you want to equip. The cost for materials for this beam are around $300 retail and sources for some key parts are on line while most of the small parts are obtainable from Home Depot or Lowes.

The approach in this site is to explain five steps for building the components shown below and then Step 6 outlines how they are all assembled into a hexbeam

1 Base plate

The baseplate is made of a 12 inch square type 6061 T6 aluminum plate that is 3/16 inch thick. This particular type aluminum is harder than pure aluminum and less likely to bend while being resistant to weather. U-bolts attach the spreaders to the baseplate.

Two square base floor flanges normally used for handrails are used to mount the center post to the base plate. One is on the top for mounting the center post and the other is on the bottom for insertion of the mast. These flanges are made of aluminum-magnesium alloy.

Stainless steel hardware can be used to minimize corrosion especially in saltwater environments although it is more expensive. Lock washers should be used; otherwise movement of the hexbeam by wind and rotation will eventually work the fixtures loose.

Don't want to cut and drill and chase down all these parts? Buy the plate already cut and drilled with two flanges and all hardware, and stainless steel at that. 
Your cost for parts and shipping will be close to this if you go ahead and built it yourself so why bother? See the details.

2 Spreaders

Fiber glass tubes are recommended for the spreaders. These can be purchased from Max Gain Systems.  Max Gain provides tips for attaching them together. 
Three thicknesses are needed; 1" O.D., 3/4" O.D., and 1/2" O.D. The smaller sizes fit perfectly into the next larger size.

While it is not necessary, you can spray paint on the spreaders to achieve the color you want.

Some have tried to build spreaders using PVC but I have heard of no one who has done this successfully. PVC is simply too limber and does not have the "spring tension" needed to maintain the hexagonal beam shape. I recommend against its use for a hexagonal beam.

3 Center post

The center post is for mounting the terminals that connect the various bands' driver wires to the coax feed line. It is made of basic one inch PVC plumbing that is a perfect fit into the baseplate described in Step 1.

The terminals are located on the center post and are European terminals. The terminals are connected with 50 ohm coax pieces from the bottom terminals to the top where they are connected to the feedline from the transceiver.

All bands are connected together via the coax links and are all fed by a single feedline.

The terminal spacing on the center post and the length of the post itself are set forth below along with steps for constructing the center post.

4 Wire Sets

The driven element will consist of two pieces of wire for each band. The reflector will also consist of one piece of wire for each band. Each driven element piece will be shaped by the hex beam structure into an inverted "V" and when arranged next to the other piece, will form a "M." Each Reflector piece will be wrapped around the four spreaders and attached to the tip spacers which in turn are attached to the driver wires. Be precise with cutting the wires as an inch can make a difference in the resonant frequency of that band.

Each outer half of the driver wires has a non-conducting line that connects that wire to one end of the reflector wire. These non-conducting lines will be called "tip spacers." The length of the spacers is important to proper performance of the hex beam and they are designed here so they can be adjusted precisely.

The two ½ driver wires, the reflector wire and the two tip spacers should be measured and assembled as single end to end sets before  installation on the spreaders using the approach below for each band.

The higher frequency bands are nested inside the lower frequency bands but only one band is shown here for the sake of simplicity. The wire lengths are for #14 or #16 gauge bare wire. Insulated wire can be used in salt water environments and the lengths will be shorter. Tables for the lengths are on the Specifications page

5 Support Cords

Support cords from the center post to the ends of the spreaders along with two other cords between the ends and the middle of the two front spreaders establish and hold the shape of the hexagonal beam. When installed properly, very little tension is on the wires as these cords do the physical work.

Measure six radial cords 128 inches including the hooks. You can choose to use knots instead of crimps.

Measure one perimeter cord 128 inches to be used between the two front spreaders because they tend to be pulled apart by the wires and need this counter pull provided by a cord.

Add another short cord about 90 inches between the same two spreaders located in toward the center. Don't use hooks on this short cord; use a cable tie on each end to wrap around the spreaders.

6 Assembly

Now that you have built the components for your hexbeam, all that remains is to assemble them into a completed beam. Just follow these steps.

1. Set the base plate on a table, a 10 gallon paint bucket filled with water or rocks to serve as an assembly foundation.

Assemble the spreaders and insert them into the baseplate using the U bolts.

All the P clips on the spreaders should be on the top side of the spreaders. Make sure each spreader is in line with the opposite spreader and if not, then loosen the U bolts and readjust. Do not over tighten the U bolts as you can easily crush the spreaders. If the spreaders are starting to flatten, you have tightened them too much.

2. Install the center post into the baseplate and tighten the two set screws snugly. Do not over tighten as they will penetrate the center post.

3. Install the support cords between each spreader end and the anchor eye bolt on top of the center post. The easiest way to do this is to hook one cord on the end of one spreader and another on the end of the opposite spreader. Then, standing in the middle, pull both cords up simultaneously and hook them on the center post eyehook. Repeat this process for another spreader pair and then another as shown here. Fasten the cords to the ends of the spreaders as shown in step 5.

4. Install the wire sets starting at the highest frequency band. Attach one end to a terminal for that band and then fish the other end through each of the P clips on the spreaders. Then attach the other end to the second center post terminal. If the wire will not reach the terminal without excessive tension, then loosen the P clips for that band and let them slip inward about 1/2 inch each or until the wire reaches. Take small steps in this for better results. Do not have the wires over taut. This only distorts the shape of the beam and makes the remaining wires harder to adjust. Tautness does not improve beam performance at all. A little slack is better.

5. After installing all wire sets, make adjustments so that all wires have generally the same slack and the overall shape of the beam is balanced and symmetrical.

6. Connect the coax to the top of the center post to the 20 meter terminal. Use a small piece of coax at first in order to make SWR tests before going further. You should see an SWR dip for each band but the lowest SWR will not be achieved until the beam is raised  20-30 feet.
If you do not get a dip in SWR, make sure all connections of the wires are tight in the terminals including the coax links.

When finished, be sure to seal the terminals and coax ends to keep moisture out as contaminated coax will not perform correctly on the beam.

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