What's inside cheap SMA terminators

29.05.2020 14:16

I've recently ordered a bag of "YWBL-WH" 50 Ω SMA terminators off Amazon along with some other stuff. Considering they were about 3 EUR per piece and I was paying for shipment anyway, they seemed like a good deal. Unsurprisingly, they turned out less than stellar in practice.

50 Ω SMA terminators I bought off Amazon.

At the time when I bought them, the seller's page listed these specifications, claiming to be usable up to 6 GHz and 2 W of power dissipation. There's no real brand listed and identical-looking ones can be found from other sellers:

Specifications for 50 ohm SMA terminators.

Their DC resistances all measured very close to 51 Ω, which is good enough. However when I tried using them for some RF measurements around 1 GHz I got some unusual results. I thought the terminators could be to blame even though I don't currently have equipment to measure their return loss. If I had bothered to scroll down on that Amazon page, I might have seen a review from Dominique saying that they have only 14 dB return loss at 750 MHz and are hence useless at higher frequencies.

I suspected what's going on because I've seen this before in cheap BNC terminators sold for old Ethernet networks, but I still took one apart.

Cheap SMA terminator taken apart.

Indeed they simply have a standard through-hole axial resistor inside. The center SMA pin is soldered to the lead of the resistor, but ground lead was just pressed against the inside of the case. According to the resistor's color bands it's rated at 51 Ω, 5% tolerance and 100 ppm/K. I suspect it's a metal film resistor based on the blue casing and low thermal coefficient (if that's what the fifth color band stands for). It might be rated for 2 W, although judging by the size it looks more like 1/2 W to me. In any case, this kind of resistor is useless at RF frequencies because of its helical structure that acts like an inductor.

Again it turned out that cheaping out on lab tooling was just a waste of money.

Posted by Tomaž | Categories: Analog


Make it off two SMD 100 Ohms resistors soldered on simetrical ground points.

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