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Here is an RF sampler used to monitor HF transmitters. It's typically used with an oscilloscope.
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The RF Sampling Unit connects between the transmitter and the antenna.
RF Sampling Unit Construction
The sampling unit was build using a small Hammond aluminum box. It is very simple in construction as shown below. A piece of RG-58 coaxial cable less the shield was connected between the two SO-239 connectors. A piece of hook-up wire was then Ty-Rapped to it with one end open and the other connected to the BNC connector.
A high impedance load should be connected to the sampling port. The isolation measured across HF was about 39 dB from 80 through 10 meters into a RF voltmeter that had a 15 PF input capacitance and a 2 Meg. Ohm input impedance. A 4 foot RG58 cable was used for these measurements. The isolation will vary when different lengths of cable are used. The capacitance between the main conductor and the sample port is about 4.5 PF. The worst case return loss measured was 21 dB at 28 MHz (1.22 to 1 VSWR) and got better as you went lower in frequency. On 80 meters it was about 35 dB (1.04 to 1 VSWR).