VA3MP's trap
This is all you need to build a trap... (plus of
course, the wire) All the parts are available at
your local hardware store, Canadian Tire, Rona,
Home Depot, etc.
The parts list is as follows:
2 pieces of 1/2" copper coupler (3pcs for 7Mhz)
1 piece of 3/4" copper coupler (2pcs for 7Mhz)
1 piece of 11 cm 1/2" white PVC pipe (14cm for
1 piece of 4.5 cm 3/4" white PVC pipe
The process described here is the same for any of the traps. Please notice however, that
for the 7Mhz trap, you need 3 pcs of 1/2" coupler, 2 pcs of 3/4" coupler and longer piece
of the 1/2" plastic tube.
The first step is to solder the two (3 for 7Mhz) 1/2" couplers together. It is best done by
pushing them onto the 11 (14cm for 7Mhz) cm long plastic pipe so, that they join together.
Position them in the middle of the plastic pipe and solder them together with a 100-200W
soldering iron. It is a good idea to shamfer off the joining edges of the couplers so that
more solder can get between them. Try to do the soldering quickly so as to not overheat
and melt the plastic pipe. You might want to use acid paste flux, the same stuff plumbers
The pictures above show the 1/2" couplers soldered together on the 1/2" plastic pipe.
Once done, file off the excess so that the surface is even and smooth. Make sure that
the 3/4" plastic pipe can easily slide on, that will be the next step as shown below.
Slide on this 3/4", 4.5 cm plastic tube (9cm for 7Mhz)
just 3-5 mm off the edge of the 1/2" coupler. If you did it
correctly, you won't see the edge of the inner coupler's
edge. At the other end there should be a healthy piece of
the inner copper coupler visible. See picture at the left.
Line up the edges of the inner and outer couplers -at one side- making sure that the 3/4"
plastic pipe portrudes out by about 3-5mm. The inner and outer couplers form the two
electrodes of the capacitor with the plastic pipe as a dielectric. It is therefore important
that the 3/4" plastic pipe portrudes beyond the edge of both the inner and outer copper
coupler, otherwise the high voltage will arc over. To make small adjustment, place the
edge of a larger flat screw driver on the rim of the piece you want to move and genly tap it
to move it to the desired direction
28Mhz = 5-3/4 turns
24.9Mhz = 7 turns
21Mhz = 9 turns
18Mhz = 10.5 turns
14Mhz = 14 turns
10.1Mhz = 21 turns
7Mhz = 26 turns
Bend the end of the coil so, that when you solder the two ends to the inner and outer
copper couplers, the assembly will be at the center of the inductor. You may have to adjust
it slightly with a pair of pliers after soldering. You now have a nice trap... Tune it to the
desired frequency pushing or pulling the turns. You may have to move the end of the
inductor that is soldered to the outer coupler. Although the photos show the wires that
connect the trap to the antenna wire (see the two ends of the assembly)-
it is very
important that at this stage you do not install these wires as they de-tune your trap!
Use a GDO or -as I did- my MFJ259B to fine
tune the trap, then secure the coil with hot
melt glue. If you put too much glue on the
turns, you may have to shave it off with a
soldering iron otherwise it will not fit in the
housing. Now, install the wires that will make
connection the antenna wires. Any method is
good as long as it will provide good RF
connection; do not use thin wires. Please note
that it is the innermost 1/2" plastic tube that
will take the pulling force...
The housing for the trap is made out of 2" PVC central VAC pipe and two end caps. The best
way is to cut it to length once your assembly is complete. You may have to file a small
notch for the wire on the end cap at the two opposite sides of the hole as shown on the
photo. Contact me if you have any question. 73! Mike VA3MP
Click the above picture for a higher resolution PDF-file
Wind your coil using your best method. The inductor can be wound on a 1.5" plastic
water pipe, or any similar size of tubular material. When you release the coil, it will
expand to about 1.75". Make sure it fits in the 2" central vac tube, with sufficient gap for
the hot melt glue. The hot melt glue is used to hold the turns firm, see picture below.
You should get someting like this.
This is the inner two 1/2" couplers
for the higher frequency traps
This is showing the inner 3 piece
and the outer 2 piece coupler
assemblies for the 7Mhz trap.
Now you need to slide the outer 3/4" coupler(s) on the
3/4" plastic tube. The whole assembly should look like
the one on the pictures below