VLF: T106-52 cores on VLF

[DK7FC <selberdenken@posteo.de>]

Hi VLF,

I've done a quick experiment with the T106-52 cores which could give 
some more ideas regarding these cores for a compact VLF coil.
I wound a coil with 0.5mm enameled cu wire, 85 turns at 33mm diameter. 
Inside the coil there are 5 of these cores stacked on another. In 
parallel there is a suitable C of 1 uF.
https://dl.dropboxusercontent.com/u/19882028/VLF/20160427_195530.jpg

The resonance was found at 7.49 kHz. The 3 dB bandwidth is (7.64-7.36) 
kHz = 280 Hz. Q = 27. L = 452 uH.
Without the cores inside, the resonance frequency rises to 14.54 kHz and 
the bandwidth is (15.12-14.07) kHz = 1.05 kHz. Q = 14. L = 120 uH.
Hmm, so in this configuration, the effective µr (ur) seems to be just 
3.75! :-/
That means i still need half of the number of turns for a single layer 
VLF transmit coil?!?
73, Stefan