Finding the resonant frequency of a resonant circuit such as an antenna trap is usually done by coupling a source and power sensor very loosely to the circuit.
A modern solution is an antenna analyser or one port VNA, it provides both the source and the response measurement from one coax connector.
Above is a diagram from the Rigexpert AA35Zoom manual showing at the left a link (to be connected the analyser) and the trap (here made with coaxial cable.
The advantage of this method is that no wire attachments are needed on the device under test, and that coupling of the test instrument is usually easily optimised.
Why / how does it work?
So, what is happening here? Lets create an equivalent circuit of a similar 1t coil and a solenoid with resonating capacitor.
The two coupled coils can be represented by an equivalent circuit that is derived from the two inductances and their mutual inductance. The circuit above represents a 1µH coil and a 10µH coil that are coupled such that 3% of the flux of 5% of the flux of one coil cuts the other (they are quite loosely coupled, as in the pic above. Continue reading Measuring trap resonant frequency with an antenna analyser