Motivation

If you want to measure fairly high power or see the spectrum in a spectrum analyzer, you will need to attenuate the output. Normally, spectrum analyzers can only accept a very low power signal as its input. The other factor to remember is that the power is the average power over the entire bandwidth of the spectrum analyzer. You may be watching a part of the spectrum. But there may be harmonics or other signals in other part that is outside your "view port" and if you add all that power together and if that exceeds the specification, the spectrum analyzer would get damaged. Seasoned RF engineers say that one should never terminate a power source with a spectrum analyzer. It is mostly guaranteed to damage the unit.

Using an attenuator is also pretty hard. Let us say you want to attenuate 100W into say 100mW. That is a factor of 1000. The power rating of the resistors in the attenuator is also a factor to consider.

Solution

We will need some kind of a "tap" or a "sampler" of the input power. The principle is somewhat similar to the directional coupler in a transceiver.

I followed this design by Don Jackson, W5QN. There are other designs by W7ZOI and others. TinySA website also links to a design. But I chose Don's design, because it had some analytical explanations that I can follow and also because a friend of mine had built it in the past.

Build pictures.

I used a cheap box (hammond 1590a clone from ebay).

Unfortunately, my cheap signal generator can only show and change level in volts. In the RF world, everyone use dbm. Web calculator to the rescue. I usually use this calculator. 10dbm is 2V peak-peak.

Output port is connected to a dummy load. Sampler output goes into TinySA.