Re: LF: Re: My first ferrite experiments

[Daniele Tincani <danieletincani@yahoo.com>]

Hello Jim,

 

thanks a lot for your helpful advice. However, as I'm short with free time (because of job & family duties :-)), I'm afraid I will have to stay with current build, that is three stacked 10mm x 200mm rods in a 16mm-diameter PVC tube (about 60cm long), which also acts as the mechanical support for the rods and for the coil. The coil distance from the central rod is then about 3mm. I wound 70 turns of 175/46 litz wire (doing my best to obtain a good winding) and my estimation of resulting inductance - based on measured frequency of resonance at min value and max value of the var cap - is around 670uH.

My next steps will be to give a more stable mechanical arrangement to the set, with short wires and soldered connections in place of crocodile clips of course and then perform new Q measurements. The shield will be added later, if required.

The plan is to connect the LC pair to a Mike Tuggle's homodyne mainly for NDB chasing. I discarded other possibilities (e.g. using it as a tuned loop antenna for a portable receiver) due to need to operate frequently on two tuning knobs, which can be rapidly become a bit boring.

 

Best regards

Daniele

 

Da: James Moritz <james.moritz@btopenworld.com>
A: rsgb_lf_group@blacksheep.org
Inviato: Giovedì 29 Settembre 2011 10:35
Oggetto: LF: Re: My first ferrite experiments

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