RF Trap Resonant Frequency

I am constructing a lightweight 40/30/20/17/15m trap style inverted V dipole antenna for SOTA purposes.

Can you use an antenna analyser to check the resonant frequency of a HF trap? Yes you can, here is a practical solution.

The purpose of this post is to demonstrate how you can use a modern antenna analyser to find the resonant frequency of a HF trap. In this case I have constructed two identical lightweight 20m (14 MHz) pico traps, now I want to know the resonant frequency. For argument sake, let’s say I don’t know the values of the inductor toroid winding (uH) and the capacitor (pF).

Two identical 20m pico traps

Equipment:

  • Antenna analyser
  • BNC to Banana Post adapter
  • Short length of 18 gauge copper wire 95 mm or 3.5 inches
  • RF adapter to suit your antenna analyser
RigExpert Stick 230

Remove the nylon screw, nut and washer from the pico trap. Thread the short length of copper wire through the trap and connect each end to a separate binding post.

Connect the binding post adapter to the antenna analyser and turn the antenna analyser on 🙂

Hold the antenna analyser away from metal objects, make sure the trap is in the clear.

Adjust the antenna analyser frequency, make course adjustments as required until you observe a dip in the VSWR display. The resonant frequency of the trap is displayed at the minimum VSWR value or dip. Remember, this is not a measure of the VSWR, it is a visual indication of the trap resonant frequency.

Making fine adjustments to the trap resonant frequency:

  • to lower the frequency, increase the inductance by compressing the toroid windings
  • to increase the frequency, decrease the inductance by opening the toroid windings
  • or change the value of the capacitor.

RigExpert Stick 230

20m trap resonant frequency

RigExpert AA-600

RigExpert displaying the trap resonant frequency

P.S. The 14 MHz pico trap values:

  • Toroid Core T50-6 (yellow)
  • Winding 17 turns
  • Capacitor 100 pf silver mica