I'm trying to build the [ARM Radio](http://armradio.weaksignals.com) project detailed [here](http://www.sdradio.eu/weaksignals/code/ARM_Radio.pdf). I routed and fabricated a PCB for the anti-alias filter and preamplifier shown on page 2 of the linked PDF, and reproduced here:

[![enter image description here](https://i.stack.imgur.com/40Z46.png)](https://i.stack.imgur.com/40Z46.png)

Unfortunately there's no mention of what antenna I should use, so I'm trying to use an AM loop antenna taken from a boombox, similar to this one:

[![Loop antenna](https://i.stack.imgur.com/JFmgP.jpg)](https://i.stack.imgur.com/JFmgP.jpg)

Unfortunately, reception is awful (trying to tune to the local AM broadcast stations). Mostly I get noise mixed with interference from a local FM station broadcasting from about 1 km away. At times I get the feeling I'm getting some reception but frankly I'm not sure. Injecting a weak (~50 mVpp) AM signal from a signal generator directly into the ARM board's ADC (bypassing the anti-alias filter) works fine, producing a loud, clear and correct output, proving that everything from the ADC onwards (software, etc.) is working. Reception is great with a car head unit, parked in the same spot where the SDR was tested, so it's not a signal strength issue. My best theory at the moment is that I'm not using the correct antenna to match the circuit (as shown above, the circuit includes a transformer and an inductor/capacitor network for filtering and impedance matching).

I'd rather build a new antenna than design and build a new circuit to match my loop antenna. Thus: **what kind of antenna should I build to work correctly with the circuit above?**