A simple MW box loop antenna revitalizes a poorly sensitive pocket radio

As one of my summer projects, I decided to build a simple MW box loop antenna, just to put into practice the idea of making it using PVC cable ducts like the ones in the image below:

The construction was fairly simple, as documented shortly by the following series of images:

I used some PVC-specific glue to connect the various pieces together. The inductive section is made by 12 turns of  AWG 14 electric wire, covering completely the 40 mm internal width of the PVC duct. The size of the square loop frame is 60 x 60 cm.

For the capacitive (tuning) section, I recycled a polyvaricon capacitor and a small fine tuning capacitor that I had salvaged from an old transistor receiver. They proven to perfectly fit the purpose of tuning the whole MW band. It was only necessary to add a small switch to put the two sections of the polyvaricon in parallel when tuning the lower end of the MW band. With the two sections of the capacitor in parallel, the lower limit of the tuning range reaches about 375 kHz.

When it came to perform some tests, I was not expecting any difficulties. I thought to use my XHDATA D-808 portable radio to verify how the MW reception would have improved by inductively coupling the internal ferrite rod of the radio with the box loop antenna.

Well, after several tests with different approaches, still I wasn't able to assess that the external antenna was behaving as expected. For example, I wasn't able to identify the typical, narrow peak in signal intensity that corresponds to the selective nature of this kind of antenna. On the contrary, there was a distinct, unexpected, narrow notch, along the tuning range, where the signal completely disappeared.

After some reasoning, I concluded that probably the XHDATA D-808 was too sensitive on its own to take advantage from coupling with such a box loop antenna. Probably the narrow notch that I was finding during tests was caused by overloading of the input stages of the radio when the antenna was tuned exactly to the received frequency.

So I decided to try with a little, very cheap, poorly sensitive pocket radio and finally things started to behave as expected, as documented in this short video clip: