R.F. front ends.
There is much and many schools of thought as to front end design over the years. On the lower H.F. bands the thought is that there is no need for an amplifier, as the noise floor is so high that adding to it would only harm the signal. Other that the signals on the lower bands are so close or local that the need for front end amplification seems not required or perhaps needed. However, in levels of low background noise, the smaller signals of the lower band may be either QRP transmitters, or even perhaps a D.X. signal. In such moments, front end amplification is desirable.
Low levels signal can be classified into the two same categories, D.X. or QRP signals, but also the maximum usable frequency plays apart here. In fact the minimum usable frequency is also a propagation concern, but the maximum usable frequency is determined by the signal propagation performance needed to get to the location needed for contact.
For the upper bands, the back ground noise can be much less a problem, but here the front end signal performance comes into play. If the atmospheric noise is a minimum, then it is not wise to add to the signal noise, so a low noise signal amplifier is desirable.
It is perhaps prudent to have a front end amplifier with a little signal amplification as not to enter into the 1dBm compression point by any margin, so the signal loss through the mixer stage can be averted
A mixer stage is usually a noisy affair, plus also may poses a certain amount of signal loss for a passive mixer stage such as a Double balanced mixer of ring diodes. An active mixer provides some gain, perhaps just enough to overcome any losses, but a front end amplifier can provide enough gain to also overcome any mixer losses, such as the reduction of the radio receiver s/n ratio due to mixer losses of additional signal noise. The low gain front end would in effect equate to a low noise signal boost to the reducing effects of mixer noise.
Once the signal is put through the roofing filter, the main signal amplification of the receiver can be undertaken. By this point, any co-channel or adjacent signal would have been filtered out by the roofing filter, and thus the signal to be amplified would be essentially a clean signal, or just the intended received and tuned input signal.
To provide further signal performance of the front end amplifier, a pre-selection of the intended signal from the unwanted signal is perhaps of some good use. Tuneable front end BPF filters are or could use, but should a high level interference signal drive the variac diode into a modulation cycle by affecting the tuning voltage by comparison, producing an increased 1/f noise response curve, this then would modulate the intended signal at the input of the mixer stage around the carrier frequency of the intended Rx signal.
In any case for the front end amplifier, the modulated interference signal would mix with the intended signal and create undesirable output signal for the mixer stage to content with, and the 1/f noise response would in itself be amplified though out the receiver. It is perhaps best to provide a BPF using lumped components of capacitors and inductors, switching as required the appropriate BPF for the bands intended for use.
Switchable front end attenuators at the input of the front end amplifier are quite usual in radio receivers. A range of switchable attenuators that would reduce a signal in regards to the “s meter” readings would be of perhaps some help. Should the intended signal go over the “S9” meter reading, then the switchable attenuators could come then into effect. The “S meter” calculation would compensate for the attenuators, thus to show the input signal as it is at the antenna terminal socket, with indicators to show the degree of signal attenuation applied by the receiver. A second “S meter” reading could be shown indicating the new attenuated signal as the reading, with a thought of determining if the I.F. and demodulator circuits are been over driven into distortion levels.
Now here is a thought, the lower band signals are deemed to be a ground wave propagated signal. However many antenna designs are cut to a 50ohm match, but a 50ohm stub antenna is about 8% efficient, so 100 Watts transmitter then equates to an 8 Watt radiated signal. The 50ohm stub antenna is cut to 8% of the full wavelength of the signal used, that is a Top Band 160m signal has a 50ohm stub tuned wire cut length of around 13metres of wire.
Would it not be perhaps the in-efficiency of the lower band antenna's that perhaps amount to the ground wave effect, the low value radiated signal relative to the transmitter power output upon the antenna be the cause of the effective low signal propagated range. Well it is a thought?