A tiny handful of known-good-quality SMA terminators arrived from eBay:

KDI T187GS – 50 ohm 1 W SMA attenuators

They’re described as KDI Triangle T187GS SMA Female Terminator, 50Ω, 1W, 0-4GHz. A bit of searching suggests MCE (whoever they are) borged KDI quite a while ago (their website, last updated in 2003, has been lightly vandalized) and a datasheet won’t be forthcoming.

In any event, a NooElec NESDR Mini 2+ radio connected to a dual-band VHF-UHF antenna perched near a window shows this for a local FM station:

FM 101.5 NESDR – direct

Zooming to 5 dB/div:

FM 101.5 NESDR – 5 dB steps

Installing the terminator at the end of an MCX-to-SMA adapter cable:

FM 101.5 NESDR – 50 ohm terminator

Haven’t a clue about those tiny little spikes with the terminator in place, but they don’t line up with any of the high-energy inputs and are, most likely, junk brewed up within the radio. That’s with the RF gain set to 49.6 dB and AGC turned off.

The hardware looks like this:

NESDR with SMA attenuators

The MCX connector on the radio isn’t the most durable-looking thing I’ve ever seen, so strapping the adapter cable to the case seems like a Good Idea. You can get an NESDR radio with an SMA connector for about the same price, which I’d have done if were available a while ago.

The terminated input looks to be about -75 dBFS, about 15 dB below the between-station noise, and the carrier tops out around -25 dBFS, for a “dynamic range” of 50 dB. Oddly, that’s just about dead on the maximum dynamic range you can get from the 8 bit RTL2832U demodulator / ADC stuffed inside the NESDR: 8 bits × 6 dB/bit.

It is not obvious to me the signal from a randomly chosen (albeit powerful) FM station should exactly fill the receiver’s dynamic range, particularly without AGC riding herd on the RF gain. Some hardware tinkering seems in order.

The GNU Radio flow graph:

FM Broadcast – GNU Radio flow