When I was looking at the old time meter, the coupling really falls off as you move down in frequency. At a MHz, I think it was 60dB or more.
I had made some test loads for when I was working on my home made Watt meter. These are only rated for a couple of Watts. I started out a 27MHz where the meter is supposed to work and it wasn't sensitive enough to read the reflected signal, even with a VSWR of 4:1. I think the idea was, what self respecting CBer would only use a few Watts. This meter supports a kW afterall. It's not made for the kiddies.
They had used all 1N4148s for the detectors. I picked up some 1N5711s to try and improve the sensitivity. These one on one tape and they appear all very close. These were not the 100 for a dollar ones on Amazon. It did improve it somewhat. So I tried it with a VSWR of 3, 2 and 1.5 all at 27MHz. 3:1, I would expect would be much closer. There is no adjustment and I suspect would be good enough.
Many years ago, I picked up a CB style SWR meter at an establishment calling itself "Crazy Charlies".
This was in the "returns" tray & was clearly marked faulty.
I thought "might be some useful bits" & picked it up for $A4.00.
I took it home, stuck it on the old 2m "modified taxi radio", & was gratified to see some meter movement.
I took it to work, stuck 8 watts into it from the same radio in line with a Marconi power meter/load & checked the VSWR displayed on the CB meter.
It said, as expected, something around 1.3:1, which was reasonable for something sold as "not working".
Work having an assortment of loads I managed to find a 75 ohm one. The SWR read around 1.6:1- near enough!
It wasn't a "power meter's bootlace" as the "forward power" reading was adjustable to give full scale.
Who cared, as I could borrow the old "Macaroni" one?
From memory, it used OA91 or 1N34 style diodes.
When it came to tuning up 2m antennas, the alternative was to borrow a Grid Dip Meter from work, so it was reasonably useable to "Get in the paddock of where I wanted to be".
Strangely, this particular design of VSWR meter increased in sensitivity at HF, so when I finally got on HF, it was still useful.
Obviously, nothing you would use for work, but ham radio IS just a hobby!
Over time, I have "picked up " several more pretentious CB & even ham SWR meters cheaply, but almost universally the more pretentious, the less likely they were to work.
Finally, the meter failed in the "Crazy Charlies" one & it went in the bits 'n pieces box, but by that time, I had a R-x Noise bridge which was "sorta" useful, & sometimes , access to an MFJ 259.
By dint of not being very active, I went along till I bought a nanoVNA.
Having "short arms & long pockets", I never aspired to possess a Bird.