Dr. Carl O. Jelinek

1
HF Pre-SelectorDesign

• For Field Day, each band should have one to
  prevent harmonic interference to the other
  stations, especially when running high power.
  e.g. 14.2 MHz x 2 28.4 MHz right in the best
  part of the 10 m band!
• The design is simple, tuning is very easy and it
  can be constructed in about an hour.
• See Note Pages for additional comments.
  n6vng_at_arrl.net.

2
Very Simple Circuit
Output 2 Turns
Input 2 Turns
Parts List
2 O. D. x ½ TOROID Red for HF, Yellow for VHF
Power lt 300 Watts
Use insulated 12 gage wire for the
coils. Variable Capacitor 255 ?F. 2 ea. Female
chassis mount Connectors for Input and output RF
tight AL Box Optional 2P2T switch to bypass
14 Turns
255 m F
3
Construction Notes

• Mount variable capacitor in an RF tight metal
  enclosure so that the plates have good spacing to
  the walls.
• Mount female bulkhead connectors for the kind of
  coax your antenna and rig use on the back wall of
  the box.
• Wire each of the 2-turn loops to one of the
  bulkhead connectors.
• A single pole double throw switch may be used to
  bypass the pre-selector. optional

4
Tuning Instructions

• Terminate one port with a 50-Ohm load that will
  dissipate full power It does not matter which
  port since the circuit is symmetrical.
• Connect an antenna analyzer to the other port and
  set it for the highest frequency band. e.g 10
  meters
• Tune the capacitor to make sure that the
  per-selector has a low VSWR and tunes over the
  full band. If it cant, choose a smaller value or
  reduce the number of turns on the larger coil.
  Test the lowest band that it will tune to. Mark
  the bands on the box to speed tuning.
• Connect your rig, tune to the highest band and
  test at full power to determine that the toroid
  does not get too hot. If it does, you need a
  larger size and perhaps larger wires.

5
Comments

• VSWR should be better than 1.51 over each band.
• Each band could use a fixed tuned pre-selector
  with high quality NPO capacitors. However, I
  prefer the variable tuned approach so that I can
  retune to work a lower band at night on Field
  Day.
• You can use a smaller size toroid and wire for
  QRP rigs.
• Use a Yellow toroid for VHF e.g. 6 and 2 Meters
• Several sections can be ganged together to get
  sharper tuning, if there is a really bad out of
  band interference problem, however, tuning may be
  a little tricky and insertion loss will increase.
  
• Cavities are better at 2 meters and above, but
  they are large, much more difficult to build and
  tune.