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I have a simple relative field strength meter that someone built years ago and I inherited. I use it to tune antennas. It is a simple box with a telescoping antenna, a germanium diode, a small rf choke, a capacitor and a 25 microamp meter movement. Within about 6-8 ft of the antenna and with the FS meter antenna collapsed enough to bring the reading on-scale, it works very well. This type of FS meter was popular long ago, but cheap microamp meters are hard to find now.
MFJ Enterprises sells two inexpensive relative FS meters. I don’t know how well they work, but they seem to be the only source that I have been able to find.