This antenna covers 65kHz for 160m band, 75kHz for 80m band and the entire 40m band with SWR under 2:1. The tradeoff is limited bandwidth.
Schematics for Multiband dipole
The coils used are traps for the signal – they act as a notch filters (the windings have capacitance as well).
Construction details are for Registered Users only
We get wire for the coils when we strip the shield from the RG-58 coaxial cable. We thus get only the core – the hot wire, and the first isolation. The coil forms are cut from the PVC pipe.
For the 7MHz trap, we need PVC pipe with 6cm outer diameter. The windings are in two levels: the number of inner windings is 12 turns, and 11 ¼ of outer turns.
The 3.8MHz trap form is made from 3.5” outer diameter pipe. It has 14.3 turns for the inner winding and 13.4 turns for the outer winding.
Use electrical tape to keep the turns in place. Try to make them as tight and closely spaced as possible. Drill the hole in PVC pipe where the winding will enter and exit and secure that hole with hot glue so the coil does not unwind.