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LF: Wider-band ferrite rod antenna

To: <[email protected]>
Subject: LF: Wider-band ferrite rod antenna
From: "James Moritz" <[email protected]>
Date: Sun, 11 Sep 2011 14:16:22 +0100
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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

Attachment: Ferrite_wide.png
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