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HF Pre-Selector Design. 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!}

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hf pre selector design

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@arrl.net.

Dr. Carl O. Jelinek

N6VNG

very simple circuit
Very Simple Circuit

Output

2 Turns

Input

2 Turns

Parts List:

2” O. D. x ½” TOROID

Red for HF, Yellow for VHF

Power < 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

Dr. Carl O. Jelinek

N6VNG

construction notes
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]

Dr. Carl O. Jelinek

N6VNG

tuning instructions
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 can’t, 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.

Dr. Carl O. Jelinek

N6VNG

comments
Comments
  • VSWR should be better than 1.5:1 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.

Dr. Carl O. Jelinek

N6VNG