Re: LF: Ferrite RX antennas

[Markus Vester <markusvester@aol.com>]

Stefan, Jim,

 

you could increase the signal bandwidth without compromising SNR by connecting a low impedance preamplifier. This technique has been used widely and successfully in magnetic resonance imaging ("preamp decoupling"). The preamp is designed to have an input impedance that differs largely from the noise-optimum source impedance, so that you can preserve the noise match but create an intentional signal mismatch.

 

In practice, you would still want to use a low-noise FET connected to the high impedance point of a parallel resonant antenna. Normally the gate input impedance (megohms) is higher than the noise optimum (tens of kiloohms), so you would have no preamp damping at all. The trick is then to lower the input impedance by lossless feedback, which has neglegible effect on the noise parameters.

 

One configuration is a compromise between common source and common gate circuit configuration ("Zwischenbasisschaltung"), which can be realised by inserting an additional negative-feedback winding in the source-to-ground path. This is similar to the old regenerative audion, but with the feedback coil polarity reversed. Another configuration is parallel feedback from drain to gate, by intentionally increasing the Miller capacitance.

Best regards,

Markus (DF6NM)

-----Ursprüngliche Mitteilung-----
Von: James Moritz <james.moritz@btopenworld.com>
An: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
Verschickt: Mi, 17 Aug 2011 1:13 am
Betreff: Re: LF: Ferrite RX antennas

Dear Stefan,

Looking good so far...

A Q of 486 is certainly reaching the point where it becomes inconvenient - 
but remember that it is easy to reduce Q (probably connecting a preamp, 
putting it in a container, etc, will reduce Q a bit anyway), and that the 
higher Q is, the better the SNR. So I would test it as an antenna with a 
preamp - if there is more SNR than you need, you could experiment with some 
damping resistance.

Cheers, Jim Moritz
73 de M0BMU