Here's an easy to make trap dipole for 40 and 20 meters. I made this for portable operations, mostly for "to the field" type QRP contests. Being able to switch bands between 20 and 40 quickly and without re-adjusting an antenna tuner is a nice feature. The only problem is it's a little heavy with the feed line and all, and putting it up in the raw New England woods can be daunting. Generally, I deploy it as an inverted "V".
I use #22 Teflon insulated wire. It slips around pine tree branches easialy.
20M section: 17.0 feet. 40M section: 14 feet, 8 inches.
You should add a few inches to these lengths so that the antenna can be trimmed. Easier to cut wire than to add it! You'll need about 6" extra on the 40M section to make a half hitch to attach a support string to. If your using insulated wire anyway, there is no need for an insulator and its extra weight and bulk.
.. Half hitch at end of 40M wire for attaching support string to.
The trap is wound on a T50-2 core, with 24 turns. A 47 pfd mica cap is placed across the coil to tune the trap to about 14 MHz. The trap is then mounted to a piece of PCB board, with the copper removed, which acts as a strain relief when attaching the wires to it. Drill 2 holes close to each other on either end of the board, then pass the wire down into one, and up through the other and solder to the trap. I covered my trap with some black liquid rubber to protect it from the elements and hold it down to the board.
..Trap on pcb board.
There are any number of ways to attach the feed line to the antenna wires. Here's one made from a 1" OD PVC pipe cap. I didn't make this, it was supplied with the N2CX "Gusher" antenna. It makes a nice universal center connector, as one can easially change the wires for different bands or configurations. It is easially duplicated.
The coax (RG-174 in this case) passes through a hole drilled into the bottom of the PVC cap. Its a good idea to put a knot in the coax, so it doesn't pull out. Once the coax has been soldered to spad lugs and connected to the screws on the side of the PVC cap, the cap is filled with 5 minute epoxy. Before the epoxy fully sets up, insert a small screw eye. This gives you an anchor point for a support string, so that the antenna can be pulled up into a tree, without putting stress on the wires.
Trimming the antenna:
Before you go and try to use this antenna in the field, you should set it up and check out it out. An antenna analyzer or "Tenna Dipper" is a good QRM free way of testing for resonance and is a lot easier than running back and forth to the rig and SWR bridge! Test and trim the 20 meter section first, then do the 40 meter ends. Once you get a 1:1 match where you want it on both bands, your ready to go out into the field with it!