Some hints/tips on how to assemble nice COAX TRAPS !


Before we start to assemble our traps, here some general info as introduction :

Now let's go to work ! Here are the step by step instructions for

   20m (and 40m) (and 15m) band traps:

  Bill of Materials (for 2 traps):
  • 2 PVC sewage pipes, Outer Diameter (OD) 32 (50)(32) mm, 80 (95)(80) mm long each, thick walled (3mm) and resistant to heat (95) version.

  • 4 bolts with nuts, M5x15 or M5x17 (zinc anodized or Stainless Steel for permanent use)

  • 4 'grower' M5 (teethed washers)

  • 4 washers M5 (zinc anodized or Stainless Steel for permanent use)

  • 4 butterfly nuts M5 (zinc anodized or Stainless Steel for permanent use)

  • 4 big washers M5 , outer diameter 25mm. Use ZINC ANODIZED IRON if you want to solder directly on them, if you use copper or stainless steel , you will need a eye lug to solder the coax and connect to the M5 bolts.(recommended for high power).

  • 186 (306)(134) cm COAX CABLE type RG-58 C/U  (the type is critical, use C/U version, to have same capacitance per meter and power handling)

  Preparation of the pipes:
  • Mark  2 holes in-line  (these are for the bolts), at 13 (15) (13)mm from each end of pipe
  • Mark 2 holes (these for coax), at 13mm from each end of pipe, and for 20m trap 90 on each side of the two holes for the bolts (so the coax holes will be 180 opposite) / For 40m trap, at 32mm distance along the circumference, on each side of the marks for bolt holes. For 15m, same construction as for 20m.

 

  Drill all 4 holes with 5mm and remove sharp edges with a trimmer, especially for the coax holes
  Preparation of the the 4 big washers:
  • These are used for mechanical strength and connecting the coax ends to terminal bolts
  • Remove zinc anodization on 1/2 of washer surface (as soldering is very difficult and unreliable on anodized zinc). Use a grinder or file.

 

   Now this will be the area where we will solder the coax ends
  Using 2 pliers, bend the washer so that it will fit inside the PVC tube. Keep the area for soldering as depicted!
  This will be the result (4 pces), adjust till they nicely fit inside the PVC tube.
  Now insert the 4 big washers in the tube ends, keep the area for soldering on the outer side, and insert the M5 bolts.
  Now put the small washer and M5 nut on the bolts and tighten very well with appropriate tools.

We are now ready to start to wind our coax coils !

  Take 93 (153) (67) cm of RG-58C/U coax for each coil. Insert approx 10 cm in the hole for the 'start'. RG-58C/U will just pass in a 5mm hole.
  Remove insulation on 15mm of the end passed through pipe and prepare tail like shown
 

On the shield, solder a small hookup wire length 120mm about (take at least same gauge as coax center wire) and cover with heat shrink tube. Put a small nylon fastener 3mm after the start of coax outer jacket (here in the picture in red).

  Now gently retract the coax in the tube, while inserting the hookup wire till it comes out of the tube on the other side, and the coax cannot be pulled out any more (nylon fastener)
  Now solder (a 60w iron should be OK) the coax center on the big washer. Make sure you have a good bond, but do not overheat as the coax / PVC will melt...
  Now start to wind the coil !
  • tightly wind the coax around the PVC tube, till you have 7,5 turns (8,7 turns) (5,5 turns widely spaced, spread over tube), where the coax will enter the PVC tube again
  • spread evenly the windings for 20m trap, keep them 'tight' for 40m trap
  • make sure the coax outer windings are at least 3mm from the connection bolts/nuts
  Use the short remaining end of coax to pull is as much as possible inside the tube and have  windings sitting tight. Then cut off the coax so you have 20mm left inside the tube.
  Using a cutter knife delicately around outer jacket, remove insulation on about 15mm and prepare coax ends like shown.
 
  • Solder the coax shield to the big washer

  • Solder the coax centre to the hookup wire , which you will have cut till desired length (make it as short as possible)

  • Cover this solder joint with heat shrink tube, immediately after heating it up, squeeze firmly with pliers the free end to seal off the tube.

  • important: check if both M5 nuts are still well tightened (they will loosen with the heat !)

 Your coax trap is now ready to be tuned !

 
How to tune the traps
?
  • Use a grid-dipper (but must be accurate to 100 kHz at least)

  • Use a signal generator and put the trap in series with a RF voltmeter, a field strength meter input, .... and look for minimum signal (= resonance)

  • Use a transceiver with adjustable  low power (5w ?), connect  the trap in series with a power meter/dummy load, or a small bulb (like of bicycle rear light), and look for minimum lighting up (= resonance)

  • Tune both traps at 15  (7.3) (22) MHz by playing with the coil windings (closer to each other = lower frequency). You will see that the dip is quite broad.

  • Try to distribute the windings evenly till all looks nice - this will as well distribute the dissipated heat evenly and prevent arcing. Make sure  the coax outer windings remain at least 3mm from the connection bolts/nuts.

 

Once the resonance is OK and the trap is looking fine, apply PVC glue (this glue is used for mounting PVC tubing, it is clear, dries in minutes and becomes quite hard after hours) on the windings, especially on the outer turns of the coil, and a couple of strokes in the longitudinal direction. Liberally apply 2 or 3 coats of glue as well on the coil inside (soldering points, coax ends, big washers etc. to weather protect these elements).  The recheck the resonance point : it will not have moved due to the glue.

  Put the 5mm 'grower' washers and the butterfly nuts in place (with some Vaseline grease as protection against rust) : your COAXTRAP is now ready ! Apply a label indicating the band it has been designed for as the 'finishing touch' !


See my '' simple HF holiday antenna's '' page  to make your dipole !
 

Very useful links:

   
     
http://www.w8ji.com/traps.htm   Calculation of trap losses under various conditions by W8JI  Tom
     
http://degood.org/coaxtrap/   The attic dipole with coax traps for 80 to 10m by NU3E John
     
http://www.qsl.net/ve6yp/index.html  

Coaxtrap.zip : Coax Trap calculator / designer by VE6YO Tony (see below)

Found that the number of turns is accurate, using the center of band frequency, but the length of cable is somewhat short for practical construction. Add exactly 30mm coax length to the calculated result to solder tail end in comfort. The result will be easy to tune just above the desired band.




Some additional info...

 

Melted and short-circuited 20m trap... power 800w SSB.
Heat due to induction by coil in Zinc Anodized washer caused
PVC tube and  coax insulation shield melted !

40m high power trap with RG-400 teflon coax