Here's a link to some inductive RF pickups and demodulators.
http://home.comcast.net/~msed01/RFpickup.htmlYou can adjust the level of signal sampled by winding more or less turns on the transformers.
However, beware, the described (Youtube) method of deriving a "trapezoid" by demodulating the RF and comparing the two is useless! By simple identity the demodulated RF and the RF itself must be virtually identical (save for negligible diode voltages) and will always show a "perfectly linear" trapezoid pattern even under the worst signal conditions!
For a trapezoid to have value it must compare differing signals before and after some stage or component. For example; comparing the modulating plate voltage to the RF waveform indicates if the final has sufficient drive or adequate loading, etc. Most stations utilize a trapezoid to compare the input and output of a linear amplifier to determine if in fact it is being operated in a "linear" fashion; proper load, drive, bias, etc.
Comparing the "RF output" to the detected level of the "RF output"... useless.
In any event, being able to monitor your signal reliably is a valuable asset in the AM shack. You'd be well served by building some RF sampling devices for that purpose.
73,
Mark