1. 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 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.
2003/07/04
Dr. Carl O. Jelinek
N6VNG

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 < 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
I have used AMIDON FT Ferrite Toroidal Cores for the HF Bands [80 meters to 10 meters ].
Check the temperature of the torrid under load conditions.
~ 255 m F
2003/07/04
Dr. Carl O. Jelinek
N6VNG

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]
Wide Plate spacing on the variable capacitor may be required for high power.
2003/07/04
Dr. Carl O. Jelinek
N6VNG

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 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.
Connect a dummy load to one port and the VSWR analyzer to the other. Alternately, you can use a rig at low power, if it has a calibrated dial and VSWR meter.
2003/07/04
Dr. Carl O. Jelinek
N6VNG

5. 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.
Send comments to Carl at
2003/07/04
Dr. Carl O. Jelinek
N6VNG