Hi Daniele,
You wrote:
...
Yesterday evening a purchased a bunch of 7 x MU400HH ferrites (10mm x 200mm each) on
eBay, planning to join them together "in parallel" to form a fatter rod, no
idea if it will be a good choice but let's play with this new toy ;-) Meanwhile I'm
collecting ideas and hints from the web about construction (shielding,
computing/measuring parameters, ect.). Projects are stacking as usual :-)
...
These rods are working fine even at VLF. But according to my own
experiences in the late 90th bundling is not worth the effort. Two tight
(!) stacked 15cm rods achieved better SNR compared to a bundle of 7 rods.
PELGRUM (2006) confirms these findings:
"...
The rod-length has a larger influence on the noise-performance than the rod
diameter. Therefore, in weight-limited applications, rod length is
preferred over rod diameter. However, this may conflict with the
requirements of volume-limited applications, such as mobile/handheld.
..."
and:
"...
In applications where the antenna size is limited, it can be beneficial to
use multiple small ferrite rods instead of a single larger rod...
...it can be concluded that two 10 cm rods experience virtually no mutual
coupling when placed 10 cm apart, and therefore, result in a 3 dB increase
in SNR compared to a single 10 cm rod.
..."
And for the coil length:
"...
the best noise performance is achieved for a fully wound rod: an
improvement in SNR of approximately 2.2 dB is obtained when a fully wound
rod is used instead of a narrow coil at the center of the rod.
..."
Source: Wouter J. Pelgrum, "New Potential of Low-Frequency Radionavigation
in the 21st Century", Delft, November 2006
These are empirically obtained findings!
According to that and what I know from professionals (submarine) stacking
works fine up to length/diameter ratios of approx. 60. Above that ratio
bundling may be an option. Of course the challenge is to fix those stacked
rods in place tightly...
Peter, df3lp
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