Building Coaxial Cable Traps by W8WWV

 

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“Introduction

This page describes the design and construction materials  I used to build a coaxial cable trap. A coaxial cable trap is a parallel resonant circuit that  is usually inserted in an antenna element to enable multiband operation. In this case, I wanted to build a 40 meter (7.150 MHz) trap for use in an 80/40 meter vertical antenna.

Coaxial cable traps have become popular in recent years, mainly because they are easy to build, easy to adjust, inexpensive, and coaxial cable is usually far easier to obtain than discrete capacitors and inductors. Since coaxial cable is always designed for outside use, the trap is inherently weather-resistant, except for the connections at the ends. When the trap is constructed from larger diameter coax, such as RG-213, the trap should be able to handle  full legal power (1500 watts). Traps constructed from smaller cable, such as RG-58, should be able to accept 600 watts (on the lower HF bands).

In addition to all of these desirable attributes, some sources state that coaxial cable traps offer good performance, even though they are low Q circuits. At the 2001 Dayton Hamvention, Tom, W8JI, presented information that concluded that coaxial cable traps can be substantially inferior in performance to traps constructed from discrete components. I do not believe that this information is generally available, at this time (November, 2001). Tom’s conclusion was that the trap was particularly poor at the trap resonant frequency, 7.150 MHz in this case. At the lower frequency, where the trap acts as an inductor, the coaxial cable trap performs relatively better. [W8JI has created a web page which explores trap loss]

When this information becomes generally available, it might be time to reconsider the coaxial cable trap. There are certain constructions details that will relatively optimize the quality of the trap. Mainly through luck, my design ends up being optimized.”

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