Recently I have been helping Tim, KC8UHE, with his "Big Antenna Project." This project entails removing his current HF Tri-bander and 6-L CushCraft 6-M yagi, removing his Hazer system, adding a 10-foot tower section (tower will now be 57-feet), replacing the 1/4-inch galvanized guy wire with Phillystran, replacing his mast with 21-feet of Schedule 80 and building/installing a Mosley Pro-57B at about 50-feet and a full-size 2-L 30-M beam at about 60-feet. Yes, Tim plans to move up a notch or two in his ability to work the elusive DX!
Below you can see a photo taken the day Dwight, KC8WDT, Tim and I completed the construction on the Mosley Pro-57B (with a timely assist from Rick, W8ZT.) Tim's old antennas are still on the tower in the background. In the center is a photo of Charlie, N8RR, holding the 2-L 30-M beam. Charlie took my old CushCraft 2-L 40-M beam and modified it into a full-size 30-M beam. The boom is 16-feet long and the reflector is just over 50-feet - YIKES!
Since Tim's "Big Antenna Project" adds length to his tower, he needed to replace his coax. He has installed a remote antenna switch just under the eave of his back-porch so only one coax needs to run to the radio. Then, individual runs will be made to each antenna. Also, RF Chokes needed to be made for both the Pro-57B and the 2-L 30-M yagi. That's what this post is about. I began constructing the 30-M choke first and wound 12 turns of Belden 8267 (RG-213) on a 6-inch diameter form. This was held in place with ty-raps and Scotch® 88 electrical tape. You can see the completed 30-M choke below on the right. Click on any photo to enlarge it and use your BACK button to return to this page. The RF Choke for the Pro-57B was only 5 turns on a 6-inch diameter form.
The "Big Enemy" of coax and antennas is WATER! You need to go to great lengths to keep any moisture from reaching the coax as it will corrode the wire and destroy the efficiency of the cable. Toward this end and with a view to the fact that I never want to hear "My RF Choke failed because W8TN did not properly keep out the water," I took great pains to see that no moisture could reach into the cable at the point where the braid and the center conductor are separated for attachment to the beams.
Below you can see the beginnings of the process. First you separate the braid from the center conductor. I chose to slide the braid back about an inch which causes it to bloom out. You can then "make a hole" in the braid by just pulling the fine wires aside and then you can fish the center conductor through the hole by bending it. Then smooth out the braid and slip some heat-shrink over it. Now I crimp a couple of ring terminals on both ends and then solder the lugs onto the center conductor and the braid. When soldering I make sure the solder fills up the center of the crimp connector so no moisture can invade at this point. Next I used some self-fusing water proof tape I purchased at Dayton called Rescue Tape®. You pull this tape to twice it's length as you wrap it and this begins the fusing process. I used this tape to wrap the joint where the braid separated from the center conductor.
Remember, the point of this is to keep ANY moisture from getting to the wire. After the Rescue Tape was applied, I tightly wrapped all the pigtail with Scotch® 88 electrical tape. This wrapping was done with some pressure on the tape so that it's width was reduced to about 3/4 of normal and each successive lap over-lapped the prior lap by 1/2. This stretching of the tape forms a very tight seal and in years past, I've had great success with nothing more than 1 or 2 layers of Scotch® 88 tape wrapped in this manner. Some folks would stop here and call it done. However, I wanted to make absolutely these connections literally last a lifetime so I continued.
On the 30-M RF Choke I then coated the entire pigtail with 3M™ Scotchkote™ Electrical Coating. I've used this many times in the past to make certain that the taped joint is completely water proof. I actually let the first coat dry over-night and applied a second coat the next day. While planning on how to do this water proofing properly I found several references by hams on the Internet to their use of Plasti Dip® or Liquid Electrical Tape. I was not able to find the Plasti Dip at Home Depot so I bought a bottle of Gardner Bender's Liquid Electrical Tape. That is what I used to cover the Pro-57B pigtail (see center photo below.)
Finally, as you can see in the far right photo, I again wrapped the entire pigtail with Scotch® 88 electrical tape. This is necessary as the Scotchkote™ can become brittle and flake off due to Ultra Violet light. The RF Chokes are now ready to be attached to their respective antennas. Using heat shrink, soldered ring lugs, self-fusing Rescue Tape®, liquid electrical tape (or Scotchkote™), and TWO layers of Scotch® 88 electrical tape might be considered a bit over-kill but I'm now willing to bet some serious coin that these pigtails will NEVER see any moisture. But considering that you are now paying more than $1 a foot for good coax, it makes sense to take some time and effort to protect your investment so that it lasts as long as possible.