{"id":42595,"date":"2020-04-05T11:39:37","date_gmt":"2020-04-05T15:39:37","guid":{"rendered":"https:\/\/swling.com\/blog\/?p=42595"},"modified":"2023-01-18T07:45:28","modified_gmt":"2023-01-18T11:45:28","slug":"diy-how-to-build-a-noise-cancelling-passive-loop-ncpl-antenna","status":"publish","type":"post","link":"https:\/\/swling.com\/blog\/2020\/04\/diy-how-to-build-a-noise-cancelling-passive-loop-ncpl-antenna\/","title":{"rendered":"DIY: How to build a Noise-Cancelling Passive Loop (NCPL) antenna"},"content":{"rendered":"

\"\"<\/a>I’ve gotten an number of inquiries from SWLing Post<\/em> readers asking for a step-by-step guide to building the passive loop antenna I’ve mentioned in a number of previous posts<\/a>. This antenna is the homebrew version of the commercially-available\u00a0 Airspy Youloop<\/a>.<\/p>\n

It works a treat. And, yes, folks…it’s fun to build.<\/p>\n

There are a number of loop designs out there, and to distinguish this one, I’m going to henceforth refer to this loop as in the title above:\u00a0 the Noise-Cancelling Passive Loop (NCPL)<\/strong> antenna.<\/p>\n

Before we start building, a little antenna theory…<\/h2>\n

I’m neither an engineer nor am I an antenna expert, so I actually turned to Airspy president and engineer, Youssef Touil, to learn how, exactly, this passive loop works. Youssef was the guy who experimented with several loop designs and ultimately inspired me to build this loop to pair with his HF+ Discovery SDR<\/a> and the SDRplay RSPdx<\/a>. “The main characteristic of this loop,” Youssef notes,\u00a0 “is its ability to cancel the electric noise much better than simpler loop designs.” Got that!\u00a0 <\/span>[See loop diagram below]<\/p>\n

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“The second characteristic of this loop antenna is that it is a high impedance<\/em> loop, which might appear counterintuitive. This means it can work directly with many receivers that have a low noise figure, in order to mitigate the impedance mismatch loss.<\/p><\/blockquote>\n

\"\"<\/a><\/p>\n

Note the resonance lobe near 4MHz. The resonance frequency is controlled by the diameter of the loop, the parasitic capacitance of the cable, and the loading from the transformer. It happens to be located right where we need it the most.<\/p>\n

The transformer is basically a 1:1 BALUN that covers the entire HF band with minimal loss. Our BALUN has typically 0.28 dB loss.<\/p>\n

[…]By connecting the center of this outer shield to the ground of the transmission line, you effectively cancel all the electric noise. The BALUN is required for balancing the electric noise, not for adapting the impedance.<\/p>\n

[…]If you want to boost the performance in VLF, LW and MW, you can try a different impedance ratio, but this will kill the higher bands.”<\/p><\/blockquote>\n

What makes this loop so appealing (to me) is that it can be built with very few and common parts–indeed, many of us have all of the items in our junk boxes already. As the name implies, it is a passive design, so it requires no power source which is incredibly handy when you’re operating portable.<\/p>\n

When paired with a high-dynamic range SDR like the Airspy HF+ Discovery<\/a> or SDRplay RSPdx<\/a>, you’ll be pleased with the wide bandwidth of this antenna and noise-cancelling properties.<\/p>\n

If you don’t care to build this antenna, Airspy sells their own version of this loop<\/a> for a modest $35 USD.<\/p>\n

But building an antenna is fun and you can tweak the design to customize performance, so let’s get started:<\/p>\n

Parts list<\/h1>\n