Due to some inaccurate info on another forum by a poster I wanted to post this for reference. And no I'm not trying to take over from Rich!
uV/M is a measurement of an electrical field intensity in free space and uV is a measurement of a voltage across the antenna terminals typically used to indicate sensitivity with an FM receiver. This can also be expressed as dBm etc.
250 uV/m @3 meters doesn't mean 250uV at the antenna terminals of a radio.
AM on the other hand commonly uses field strength as a sensitivity measurement as the loopstick antenna responds to field strength for a voltage in the coil so here the sensitivity of the receiver directly relates to the electrical field in free space. Not actual voltage at the antenna or input of the receiver.
Here's some info explaining the two. Canada's 100uV/M@30meters doesn't mean you can only be received within 30 meters(98 ft) away. It just means an electrical field intensity in free space taken a certain distance from an antenna. But with AM sensitivity expressed in field strength, the same measurement would mean that if a receiver on AM has a sensitivity of 100uV/M for 20 db quieting(listenable station) that 98ft/30 meters would be the max. distance the radio would hear a signal that would be considered listenable. What the external tuned loop does is greatly concentrates more field strength around the receiver, or amplifies the electrical field around it.
This site gives some info ... https://urgentcomm.com/2009/06/01/good-to-know/
Some time ago, the electronics wizards here on the site calculated the antenna factor for FM. I've looked for the posts, but haven't been able to find them yet. I'm sure I'll run across them while I'm transferring posts out of the temp forum back into their proper places (these are posts that were orphaned in the first migration from Part15.us to Part15.org).
Anyway, I seem to recall it was about 40%, i.e., a field strength of 5uv/m would induce a voltage of 2uv at the antenna terminals for a signal on the FM band (around 100 Mhz). That seems to be consistent with the article Mark posted. In it, the antenna factor was about 25% for a half wave dipole at 160 Mhz. The dipole has a bit of gain, but lowering the frequency to 100 Mhz will mean more voltage to the antenna terminals.
That means if you have a super car radio/antenna system with sensitivity less than 1uv, you'd need a field strength of 2-3uv/m to get a listenable signal (line of sight, no other mitigating factors).
For a Tecsun radio with sensitivity 5uv (and indeed, most good portables), you'd need a field strength of somewhere around 12-13uv/m.
"a field strength of 5uv/m would induce a voltage of 2uv at the antenna terminals for a signal on the FM band (around 100 Mhz)".
That would also depend on the antenna used as that is what gets the signal in uV to the receiver. I remember my brother in the 70s had a Marantz receiver and the tuner on FM was around 1.2 uV in mono. I can't remember how he did it but houses still had roof TV antennas or ones on a tower beside the houses. He somehow got the receiver connected to the outdoor antenna( think he used the neighbours) with coax into the room and to the receiver. He was picking up what you would never get with a normal 300ohm dipole. I think he said he was getting stations from Pennsylvania and as far as London and Chatham Ontario. From Toronto on the second floor of a house. The field strengths of those stations he was picking up would be so small but there. That Marantz tuner would have been considered high end hi fi back then. Talk about FM DXing.
When you consider this think how our BETS range would be if everyone had much better receivers! Here's the picture of that receiver..can't remember the exact model but is the 2200 series. He still has it today! Stuff lasted back then. 50 years later and works fine.
The antenna factor is directly related to the type of antenna being used. A quarter wave antenna would not deliver as much voltage to the antenna terminals as a half wave dipole (which is the equation used in the article linked to).
I have not been able to find the antenna factor equation for a quarter wave - it's in the forum somewhere and eventually I'll run across it.