Had an idea... again, I have not tried this yet.
A magnetic loop antenna, as we all know, are relatively narrowband. For mono-band QRSS, WSPR or WSJT activity this is not a problem at all. But what if one would like to receive (well, let's start looking at RX only at this point) on an additional frequency at the same band or maybe even in another band, such a simple magnetic loop antenna would not be so ideal.
Now that the problem is defined, let's have a look at possible solutions. The basic idea is to build two loops sharing one coupling loop by using heavy gauge speaker twin lead.
RF-principle: Two parameters determine the resonance of a magnetic loop antenna, the diameter of the loop and the capacitor.
For a second frequency in one band one may consider to shorten one cable of the twin lead by just the right amount. This will certainly result in a second resonance close to the first resonance. Since the two leads are somewhat close, there will certainly be some capacitive coupling, what influence this will have, if at all, will be subject to experimentation. The second consequence of the proximity of the leads would be that the lengths cannot be drastically different, hence, using the same capacitor, the frequency offset would be small.
For dual-band use, the two loops should be resonated by individual capacitors. Here, the capacity between the loops will certainly have a dramatic impact on the resonance frequencies.
There is yet another possibility, probably the most messy one... Connecting the second loop to the resonating capacitor of the first loop symmetrically using identical high(er) value capacitors. This will (slightly) increase the resonance frequency of the second loop. I figure, this third possibility is somewhat impractical.
Whatever option is chosen, the (single) coupling loop will couple both loops equally well, since the size of the coupling loop is defined relative to the size of the main loop(s), i.e. 1/6 to 1/4 depending on the loop's environment.
