The basic design (1 Coil)
In its stock form the Panasonic RF-2800 Radio has a simple coil wounded in a 6 inches ferrite which is not that sensitive because it's a short 6" ferrite and a short winding like the ferrite antenna that's shown in the picture of the Panasonic RF-2600 (Sorry but I don't have anymore the original antenna of the Rf-2800 so I only can give you this picture). As you can see this design consist of having 70 to 80 turns of coils wound in a ferrite which is not fully optimized because THE ANTENNA IS THE WIRE WOUNDED in the ferrite not the ferrite itself and in this design the winding is short despite having a long ferrite, remember the longer the antenna the more signal it gets. In fact this antenna is small ! and so its gain !
The twin coil design
In the Twin Coil design there are 2 coils at the extreme ends of the ferrite and this gives a signal as big as if it were coming from a long winding from end to end and hence the big performance of this design.
BUT there's a problem with this design.....the Twin Coil Ferrite design must compromise sensitivity weather in the low or in the upper AM band because it can't achieve both, that's a problem with the Redsun radio wich many people report something like "My RP2100 is great at low frecuency but the "X" radio is better at the upper part of the band" or viceversa, this issue is directly related to this design.
Despite that, the RP-2100 has great sensitivity limited only by this issue and its 6" ferrite.
The 3 coil design
In the pictures of the 8" 3 coils Frrite Antenna you can observe that the coil in the middle is not exactly at the middle but that's not too important as the performance it's midly affected BUT what's VERY important is that the 2 coils at the ends must be AT the Ends to have the best performance, and this is achieved winding more or less turns of wire, so the more turns, the farther the sweet spot coil position from the center; Actually I haven't counted the windings but I'd say it's about 27-35-27 winds more or less.
The coil in the middle helps eliminate a little the problem of having great performance in only 1 part of the band and this was the reason this radio outperformed the Redsun because it has a bigger ferrite AND a middle coil which helped it to have a great performance in the overall band not just some part.
This is only an empirical experiment without scientific foundation to state these facts, so I might have got the wrong conclusion so I'd really like to discuss it with people with more experience about these designs.
Hugo Huesby Ali