Coaxial Traps
This is a guide on how to make a coaxial trap (Inspired by ON7EQ's tutorial).
A big thank you to Richard (G3ZIY) for all of his invaluable help, time and patience.
First of all we need to know what a trap does and why we use them. A trap essentially shortens your antenna to allow it to be more efficient on certain bands.
• A coax trap is cheap and moderately easy to construct
• If the right coax is used, the trap can handle up to 500W +
You will need reasonable soldering skills to make these traps.
- Follow these colour codes with the trap you would like to make. In the pictures I am constructing a 20m trap. -
20m Band 40m Band 15m Band
You will need:
• 2 Plastic pipes with a outer diameter of 32mm 50mm 32mm, with lengths of 80mm 95mm 80mm (can be 3D printed like the ones in the pictures)
• 4 Bolts with nuts (M5x15 or M5x17 - Zinc anodized or Stainless steel for permanent use)
•
4 Washers M5 (Zinc anodized or Stainless steel for permanent use)
• 4 Brass or Zinc anodized washers M5 (to solder directly onto the washer)
• 4 Butterfly M5 nuts (Zinc anodized or Stainless steel for permanent use)
• 12cm of approximately the same gauge of wire as the coaxial core
• Some heat shrink (Optional but recommended)
1. Preparing the pipes:
- The holes for the bolts -
Mark with a pen two holes in-line with each other, 13mm 15mm 13mm from the end of the pipe on each end.
- The holes for the coax -
Mark two holes 13mm 90º from the bolt holes that the coax holes are 180º from each other 32mm along the circumference on each side of the marks for the bolt holes The same construction as the 20m one.
Drill all holes with a 5mm drill piece and trim off sharp edges.
2. Preparing the coax:
Pull about 10cm of your chosen coax type ( 30pF / Ft !) through one of the coax holes and remove shielding on about 15mm - 20mm of the coax end passed through the hole and prepare the coax shielding into a tail as shown.
Then solder a wire of about the same gauge as the coaxial core (I used a UK neutral plug wire) to the coaxial shield tail (The hookup wire should be about 12cm). It is recommended that you use heat shrink to stop shorts as the wires can be close to each other inside the tube.
Continuously check the cable for unwanted short circuits eg. a whisker of the coaxial shield touching the inner core.
3. Bolting down:
Retract the coax and hookup wire back into the tubing making sure not to damage both the coaxial cable and the hookup wire.
On one of the Brass / Zinc anodized washers apply solder to one of the sides which you plan to solder on the inner core.
Carefully cut off about 5mm - 10mm of the plastic covering the inner core and then Tin / Solder the core to make it easier to solder onto the washer.
Next place the washer and the coax core one one over the other and solder together like picture shown.
Apply the M5 bolt, washer, nut and the connector you have just made through the bolt hole like shown in the picture.
The Brass / Zinc anodized washer should bend to place when bolting it down.
You will need to have some sort of spanner to hold down the bolt when turning the nut.
Do not over tighten the bolt as you can break the tubing. A good way to test if you have tightened the bolt enough is that if you try and twist it by hand it should not move.
4. Winding the coax:
Now it's time to wind the coax.
Wind as tightly as possible 7.5 8.7 5.5 (widely spaced over the tube) turns.
Now the tricky part (it is recommended to have someone else to help)...
While holding the coax around the tube measure (or estimate) 15mm - 20mm from the end of the coax and remove the plastic covering around the coax shield (and prepare it into a tail) and 5mm - 10mm of the cover around the coax core and carefully push both through the coax hole.
Much like before, apply solder to one of the Brass / Zinc anodized washers ready to solder to.
Place the bolt and washers through the hole and secure with the nut on the other side and tighten.
Next solder the coaxial shield to the washer (be generous) as this is stopping the coax from slipping.
Before continuing check the trap for any shorts.
Finally wrap the hookup wire around the coaxial core as shown and solder together.
It is recommended to add heat shrink over this joint.
5. Testing the trap:
It is always best to test the trap before attaching it to the antenna.
Here I have my GDO (Dip meter) to check if the coaxial trap is resonating on the correct frequency. - It is advised to have mentoring on how to use a GDO or on some sort of familiar device.