2013-10-03 Build Dual Band J-Pole Part 2

Meeting Activity Summary

The Romex Dual Band J-Pole antenna project is completed. Last week, in part 1 of the project, several variations of the J-Pole antenna were on display and discussed. Teams went into action and created several components and sub-assemblies. Between meetings, Steve KB8STB created a new prototype with the same wire, coax, and wood blocks available to the group. This week, assembly and test completed work on the Romex Dual Band J-Pole. Critical dimensions from the prototype guided accurate assembly of major components of the antenna. 

Please note that the results may vary if you are building a version of this antenna using the information on this page. Be prepared to make several adjustments. It is necessary to use test instruments to tune this antenna. Do not only cut and solder wire and coax without testing the finished antenna. The antenna may not work due to differences in materials and interpretation of the information below.

Prototype

Dimensions of Prototype

The parallel line impedance transformer section measures inches 16.75 inches. The length from the shorted end of the matching section to the coax stub which includes the second element is 29.25 inches. The coax stub is 5.25 inches. The second element is 18.75 inches. The feedline attachment point is at 1.75 inches from the shorted end of the matching section. The prototype dimensions are only valid for the materials used at the meeting. If other materials are used, a new prototype should  be constructed, tuned, and tested resulting in new dimensions.

At the meeting, members were advised to make the matching section and second element longer for trimming. Some of the dimensions below include the additional wire.

Building the Romex Dual Band J-Pole

Parallel Line Impedance Transformer Matching Section

The parallel line impedance transformer section fabricated from a length of romex or THHN wire spaced by small wood blocks of 1/2 " plywood, nominally 15/32 of an inch thick. The length of the matching section near the feed point is 17.5 inches. The 17.5 inch dimension allows for almost 1 inch of trimming. The feed point is 1.75 inches from the shorted end of the matching section. Two or three wood blocks spaced the matching section just enough to snugly fit into the 3/4" PVC pipe radome after fastening the blocks with electrical tape. The blocks measure approximately 2.875" x .0.490" x 0.468". The 0.490" dimension spaces the parallel lines. The blocks can also be used as a a wire bending form for the shorted end of the matching section. After the matching section is formed, the feed point is measured and insulation is removed 1/4" on either side of the feed point leaving 1/2" of uninsulated copper wire on both legs of the matching section for soldering a coax feedline later. On several examples that tested well, the block centers (of the 2.875" dimension) were placed at 5 inches and 15.5 inches from the shorted end of the matching section.

First Antenna Element

The first element is actually a continuous part of the same wire the matching is fabricated from. The total length of wire from the shorted end of the matching section to the open end of the inline coax stub is 29.25 inches. Note that 29.25 inches is a fabrication measurement and not the length of the first element since it includes one side of the parallel line matching section. The second element would measure 29.25 inches minus 17.5 inches, or 11.75 inches if it was fabricated from a separate piece of wire. 

Coax 1/4 Wave Stub

The coax stub is fabricated from Jetstream JT2008, available from R & L Electronics (Link to JT2008 and JT2008 Data Sheet on Page 2). JT2008 is similar to Belden 8214 Coax - RG-8/U Type (Belden Data Sheet). The stub is open towards the matching section and shorted towards the second element. The coax stub measures 5.25 inches. Last week the stub was cut from coax. The center conductor protrudes from each end approximately 1/2 inch. On the matching section end, the coax outer plastic jacket, braided shield, and dielectric insulator are cut squarely leaving only 1/2 inch of center conductor exposed. On the second element end, only the outer plastic jacket and and dielectric insulator are cut to achieve the 5.25 inch dimension of the stub. The excess braided shield is squeezed towards the exposed center conductor and soldered to the center conductor to create the shorted stub. After soldering the short, approximately 1/2 inch of center conductor should be available for joining to the second antenna element.

Second Antenna Element

The second and last element dimension is 16.75 inches and allows for trimming.

Assemble the Coax 1/4 Wave Stub to the First Antenna Element

The coax stub is soldered to the end of the first antenna element using a 12-10 ga non-insulated tin-plated copper butt splice for support. Yellow insulated butt splices can be used if yellow jacket is removed, but use the best safety practices while removing the jacket. Small copper tube from Ace Hardware, large enough to fit over the wires, may also be used if cut into 1/2 inch sections. Also, small tubing from a hobby shop may be tried. Be sure to include the ends of the wire and coax plus any length the butt splice may add to the overall dimension of 29.25 inches as mentioned above. 

Assemble the Second Antenna Element to the Coax 1/4 Wave Stub

The second antenna element is soldered to the shorted end of the coax stub using a 12-10 ga non-insulated tin-plated copper butt splice for support. Be sure to include the ends of the wire and coax plus any length the butt splice may add to the overall second element dimension of 16.5 inches as mentioned above. 

Assemble the Coax Feedline to the Matching Section Feed Point

Tin the bare feed point area prepared earlier with solder to make soldering the feedline with a minimum of heat.  Strip and prepare coax that will be used for the feedline. Strip only enough jacket from the coax as will be necessary for soldering to the feed point. Tin both the coax braid and center conductor with solder and minimum heat. Use a fine marker or pencil to mark 1.5 inches from the shorted end of matching section. Solder the center conductor to the inside of the long side of the matching section by first heating the 12 gauge wire. Watch the previously tinned solder flow, then attach the center conductor at the proper location.  There should be enough solder for capillary action to draw solder to the joining point, since both conductors were pre-tinned. If not, add a small amount of solder. Try to minimize the melting of the center conductor plastic dielectric. Solder the braided shield conductor to the inside of the short side of the matching section by first heating the 12 gauge wire. Watch the previously tinned solder flow, then attach the braided shield conductor at the proper location.

The feedline attached to the j-pole project varied from RG-58A/U to RG-8X to the larger RG-8 variants. RG-58A/U provides a nice option if only 12 inches or less is needed for feeding to a panel type connector in a PVC pipe cap. The RG-58A/U can be soldered at a 90 degree angle to the matching section as stay away from the feed point.

Testing the Romex Dual Band J-Pole 

Analyzers

Several members own antenna analyzers of various types and made them available at the meeting. The most popular is the MFJ-259B and MFJ-269. Another member has an Anritsu analyzer and was available at the meeting. Last, another member has an HP 8924C (similar in function to the HP 8920), but was unable to bring it. Each analyzer can be used to take measurements in the bands they are capable. Note that the MFJ-259B does not have the ability to measure in the UHF range. 

Connector Adapters

Members chose to terminate their antennas with a variety of connectors. Some preferred to drill a hole for a female panel mount coax connector in the bottom PVC pipe cap, while others elected to exit the PVC pipe cap with a long length of feedline terminated with a male cable connector. In, addition the antenna connector can be the popular varieties, i.e., UHF, Type N, SMA, or BNC, male or female. Adding to the connector mix, antenna analyzers have a female connector that can be UHF, Type N, or sometimes SMA and BNC.

If one analyzer is expected to test more than one antenna, connector adapters are usually a necessity. Another way to adapt/connect between dissimilar connectors is to prepare coax jumpers with every combination of male, female and type (UHF, Type N, SMA, or BNC). Most hams choose the adapter route. A male connector on the antenna feedline may join directly to the analyzer if of the same type, or through an adapter providing a female of the compatible type. When a female panel mount antenna is used in the bottom PVC pipe cap, a short jumper (4 feet to 5 feet) is needed for connection to the analyzer. The following table provides one view of using a combination of adapters and jumpers to join the analyzer to the antenna. The table is not a shopping list. Use it for awareness. Awareness of potential cables and adapters to gather before a test session starts can be helpful.