Dear John, LF Group,

If the RX noise level is the same as the antenna noise level, then when you
connect the antenna to the receiver, the noise at the RX output should
increase by 3dB  - this would probably satisfy the criterion of being able
to hear the noise increase when the antenna is connected, but would give 3dB
worse signal-to-noise ratio than an RX with a very low noise figure. I guess
this would be quite significant with a weak signal at the threshold of
audibility, so really you want quite a big increase in noise level when you
connect the antenna - for example, if the RX noise is 10dB below the antenna
noise, the noise level would increase by about 10.4dB with the antenna
connected, and the degradation in overall SNR would only be about 0.4dB.

What is meant by noise is another thing - even quite a deaf receiver can
hear the static crackles on either LF or MF, but what really is the limiting
factor to detecting a signal is the background "hiss" between the cracks and
pops, which can be at a much lower level. Also, at my QTH, even on a
quiet-sounding band a considerable part of the noise can be mains-related -
this can be quite hard to distinguish from the band noise, but is more
obvious if you look at the RX output with an oscilloscope set to "line"
triggering.

So I would certainly agree that a preamp could often be useful in improving
SNR - I think the same is probably true for many rigs on the higher HF
bands, if you are using modest dipole-like antennas. Also, the receiving
antenna arrangements are obviously very important, since the local noise
levels can easily be much higher than the band noise.

Cheers, Jim Moritz
73 de M0BMU