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
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