LF: Re: My first ferrite experiments

["James Moritz" <james.moritz@btopenworld.com>]

Dear Daniele, LF Group,

I would agree with others that the presence of the screen is reducing the Q 
in your ferrite rod. One must also be cautious about the possible effects of 
interconnecting wires, dielectric losses in wire insulation, the effect of 
the table and other nearby objects, etc. Also, loading by the scope probe is 
occasionally suprisingly high, where the resonant impedance of the rod 
antenna is high.
For small antennas like this, it is important to maximise the unloaded Q, 
since achieving adequate signal-to-noise ratio is the primary function of a 
receiving antenna and the SNR of the antenna itself is directly related to 
Qu. Screening an H-field antenna against electric field pick-up might be 
important (especially where the antenna is used for direction finding). But 
it will inevitably reduce Q to some extent due to eddy current losses and 
the effect of additional distributed capacitance. Therefore there is a 
trade-off between reducing unwanted E-field pick-up, etc, and degrading the 
sensitivity. Similarly, spacing the winding from the ferrite rod and/or the 
aluminium screen may reduce some sources of loss, but will lead to increased 
winding resistance due to the greater length of wire needed for each turn, 
so it could be an advantage or a disadvantage.
So it is important to know if the screening and winding arrangements cause a 
large reduction in Q - I would suggest doing a simple baseline test by 
winding some wire directly onto one of your ferrite rods, without any 
screening or additional insulation, and see how the Q results compare.
Cheers, Jim Moritz
73 de M0BMU