Homebew Coax Traps
Traps can be one way round multiband coverage without the need of an atu. It also has the added advantage of shortening the overall length of an antenna significantly ,especially if more than one set of traps is used. Traps simply act like a switch electrically cutting off the rest of the antenna at its design frequency, and function as a loading coil below its resonant frequency. For example if you have a 10m trap and a 15m trap on the same dipole at 28mhz the 10m trap exibits a high impedence and electrically cuts off the rest of the antenna. At 21mhz the trap now has become a loading coil thus shortening the next section of antenna to the 15m trap where this now shows a high impedence and cuts off the next section and so on
Above is a homebrew 10m coaxial trap using rg58 on a 33mm former. Using an antenna analyser it was simple to tune the trap and then run a thin bead of superglue to the coax to hold firmly in place on the former. It is a good idea to sand the former rough to allow good bonding. Another very good tip which i found out the hard way by messing my first set up is to seal any joints and bare wire inside the former before final tuning and glueing.
Above is a diagram of the inside of the former and soldering points.The measurements below are what i settled with in the end. The original article was slightly different in terms of coil turns as i used one size former for all traps and the original used different formers for each trap. Measurements are for the center of each band but can be tuned for any portion required. 10M 4 TURNS. 15M 5.14 TURNS. 17M 5.92 TURNS AND 20M 7.32 TURNS.All using rg58
You will notice in the top photo that the coax turns have enough room to be spread or closed as needed for tuning. make sure both the coax entry holes are drilled in a way that allows for this so the coax turns don’t end up butted too tight against each other.
Original article by M0JHA available at M0JHA.com