Tuesday, 5 January 2016

Loop Comparison

LW, MW and HF listening did not become practical at my current location until I got my first Wellbrook active loop antenna (an ALA1530). Even when I just had it sitting out on the wood-frame balcony outside my home office it was a huge improvement over the indoor random wires I had been using up to that point. The noise level was low enough that I could rely on it for reception of much more than WWV and CRI. When I moved it away from the house to be mounted NE-SW on a nearby fence the noise level dropped a further 5 dB to 10 dB on most of the frequencies I tested. I call this Loop A.

A few years later I added a second Wellbrook loop (an ALA100) with four turns of wire wound on a wood frame and mounted on a different fence panel so that it favours NW-SE. I call this Loop B. It's centre is about 5 metres away from Loop A.

The plan was for Loop A to favour Europe and the South Pacific on HF while nulling the MW broadcasters in northwest WA. To compliment that, Loop B was to favour Asia, Alaska, the Lower 48, and the Caribbean on HF while nulling the MW broadcasters in the lower mainland of BC. That has all turned out as expected and the spacing between them has also been adequate for creating a deep electronically steerable null on MW using a DX Tools RF+ Quantum Phaser (not nearly as lethal as it sounds). I also hoped that on HF either loop could serve as a reference for the other when connected to an MFJ-1025 noise canceller. Unfortunately that turned out to be a complete waste of time and money because I did not find a single signal that benefited from use of the device.

After a recent post to the Shortwave Radio Station Listening group on Facebook I was asked about how well Loop B works. Such things are difficult to quantify so the best I can add to what I have said above is to refer to the following two frequency response curves from yesterday afternoon.

Loop A

Loop B

At almost all frequencies the output of Loop B is lower than that of Loop A but this is to be expected because, even with four turns, Loop B is built with much less "capture area" than recommended by the manufacturer. What else do the two images reveal? As expected, my only local MW broadcaster is clearly weakened by being in the null of Loop A and multiple Vancouver stations are brought down by being near the axis of Loop B. Aside from that, the two curves show differences in the sizes and locations of their peaks and valleys but there is no way to know, from this experiment alone, the extent to which that is due to differences in their frequency response or to differences in their location and orientation. For example, the electromagnetically noisiest place in my house is the service panel where AC power, cable TV and ADSL internet all converge and it is almost directly in the null of Loop A while the orientation of Loop B favours reception of noise from my nearest neighbours.

Switching from Loop A to Loop B

This last image shows what happened in the vicinity of a broadband noise source when I switched from Loop A to Loop B. I am sure that the frequency shift means something but I currently have no clue about what that might be or how to use the information.

No comments:

Post a Comment