How To Measure A Trap


Using a NanoVNA to measure the resonant frequency of a trap from a hf antenna.

This post is using a trap from a Hustler Model 6BTV. The entire manual is at this link:

Note this caution from page 45: “You must adjust each trap with the antenna completely assembled – traps cannot be adjusted before assembly.” Therefore the readings taken in the top picture are useless. The picture should be titled, “How NOT to measure a trap”.

The unmounted components from the top picture have been mounted in a small enclosure:

The resistors are all 120 ohm, 1/4 watt or smaller wired as shown here. A goal of this arrangement is to present 50 ohms to the NanoVNA but at the same time the trap is loaded as little as possible.

In practice this method is good for determining if a trap is defective — are turns shorted, for example. For tuning a trap vertical, adjusting the trap by sliding the sleeve up or down and measuring the SWR of the antenna in place is much more effective .