Friday, July 30, 2010

Can I run the 450-Ω ladder line through the PVC pipe with no problems? ...

Q Woody, WD4NSB, asks, “I have a 45-foot pole in my front yard that has my 10-meter Yagi antenna mounted on it. All of my coaxial feed lines are routed to the pole through a PVC pipe buried under the ground. I have just put up a McCoy Dipole (nonresonant random length) and will be feeding it with 450-Ω ladder line from a tuner for operation on all bands (160- 10 meters). Can I run the 450-Ω ladder line through the PVC pipe with no problems? If I can’t run the 450-Ω ladder line directly in the pipe, can I make up a 100-Ω balanced line from two lengths of coax (using the center conductors as the feed line and grounding the braids)?”

A Running ladder-line underground through your PVC pipe is not a good idea. Other than very short lengths, or short points of contact, ladder line needs to be kept about 2 feet from any conductors. Running it through your pipe will place it just fractions of an inch away from ground, not to mention the other feed lines in the pipe, for a considerable distance.

Making a balanced line from two pieces of coax is also counterproductive in your situation. The reason for using ladder line in the first place is because of its low loss—this advantage is negated when using the coax balanced line.

If the feed line must go through the pipe to your dipole, my
advice would be to use parallel multiple dipoles fed together at a common feed point with good quality coax. You probably won’t need to use an antenna tuner. See Figure 5 on page 7-3 of the 18th edition of the ARRL Antenna Book.

From QST August 2000