SMA Attenuators vs. Broadcast FM vs. NooElec SDR

Four SMA attenuators arrived from halfway around the planet:

The top line has ATTENUATOR wrapped around the body. They’re rated for 2 W = +33 dBm, suitable for antennas and SDR and suchlike, not real radios or even HTs.

That assortment provides 39 dB of attenuation in 3 dB steps:

• 3 6 9
• 10 13 16 19
• 20 23 26 29
• 30 33 36 39

Sweeping them on the spectrum analyzer shows they’re doing what they claim, to within the resolution of the analyzer, and remain flat through 1.5 GHz, where my cheap N-to-SMA adapter cables roll off by 3 dB. Stacking them produces 38 dB of attenuation, which is certainly the small difference of large values and fine for my simple needs.

Conversely, a quick test with a NooElec SDR shows plenty of hocus-pocus betwixt antenna and display: the RF doesn’t attenuate nearly the way you’d (well, I’d) expect.

Direct from the antenna, with AGC off and 50 dB of RF gain:

3 dB attenuator:

6 dB attenuator:

10 dB attenuator:

20 dB attenuator:

Ain’t nothin’ simple…