Message-ID: <4E6CB7B9.4010003@iup.uni-heidelberg.de>
Date: Sun, 11 Sep 2011 15:29:29 +0200
From: =?ISO-8859-1?Q?Stefan_Sch=E4fer?= <Stefan.Schaefer@iup.uni-heidelberg.de>
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Subject: Re: LF: Wider-band ferrite rod antenna
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Excellent!
Maybe i will try that circuit. 37 mA at 12V (so about 50 mA in sum) is 
probably close to the limit of my USB 5V/12V supply but acceptable.

73, Stefan


Am 11.09.2011 15:16, schrieb James Moritz:
> Dear LF Group,
>
> The 137kHz ferrite rod RX antenna I have been experimenting with in 
> the last few weeks has proved to have more than adequate sensitivity, 
> but the narrow <1kHz bandwidth is a problem. It is necessary to 
> re-tune when changing frequencies within the band, also the 
> power-supply type ferrite used seems to have a high temperature 
> coefficient, which causes considerable centre frequency drift with 
> changes in ambient temperature. Since it is usually desireable to have 
> the antenna some distance from the receiver, and remote tuning is a 
> nuisance, a wider bandwidth of about 5kHz would be useful.
>
> The bandwidth could be increased by adding a damping resistor to 
> reduce the rod antenna Q , but unfortunately this also reduces the 
> SNR. Approximately, the signal voltage is proportional to the Q, but 
> the thermal noise voltage produced by the loss resistance is 
> proportional to sqrt(Q), so the signal level decreases more quickly 
> than the noise level as the Q is reduced. For my rod, the desired 
> bandwidth increase would result in about 7dB increase in noise floor, 
> which would be marginal. What is needed is a loading resistor with 
> reduced thermal noise (and without the need for a liquid helium 
> supply...). As DF6NM suggested, one way to do that is to use a  
> preamplifier with a feedback network that defines its input resistance 
> to provide the loading.
>
> The preamp circuit in the attachment achieves this by using a shunt 
> feedback resistor around an amplifier with a well-defined inverting 
> voltage gain to provide the load resistance. The equivalent input 
> resistance is R(shunt) / (1-A); in this case gain A is -10 and 
> R(shunt) is 100kohms, giving Rin about 9k. However, the noise voltage 
> at the input caused by R(shunt) is also reduced by about 1/(1-A), and 
> is about 10dB less than if a 9k resistor was connected directly across 
> the input. The same principle is used in the lower-noise types of 
> "modamp" gain block ICs, and also in my "bandpass loop" 50ohm preamp - 
> it gives quite a big reduction in resistor noise provided voltage gain 
> >>1.
>
> Compared to the original ferrite rod antenna circuit, the result is 
> that the bandwidth is now increased to about 5kHz, and a preset tuning 
> adjustment is adequate. The output level is a few dB higher than the 
> original unity gain buffer circuit. The internal noise level of the 
> rod + preamp combination is increased by a few dB, but is still well 
> below the external band noise, even with the quiet band conditions 
> existing yesterday morning. So the idea certainly works. Some circuit 
> improvements are possible though, I think, i.e. reduce the current 
> consumption for /P operation, the gain is currently too high for a 
> sensitive receiver, etc. The basic idea is quite adaptable to 
> different gain, impedance levels, frequency and so on.
>
> Cheers, Jim Moritz
> 73 de M0BMU