There has been a lively discussion on this subject so far. Several people have made comments on my posts and I want to clarify some issues.Geographic location
Reception in Europe on frequencies below 30 MHz will be much different than what we experience in most places in North America (where I live). The population density is much higher in Europe and this results in higher signal levels and man-made noise on most of the bands. When I lived in Holland shortwave listeners used to call it "RF alley" and my poor Kenwood R-5000 would sometimes overload without the attenuator on. When I listen to Web receivers in Europe I see more activity on the amateur bands than I do here in the Pacific Northwest. MW stations (540 -1700 kHz.) and FM stations are also not much of a problem unless one lives near or in a big city
. Rural areas and small towns are often many kilometers away from a city and don't have high power AM or FM stations nearby. In North America we don't have longwave radio stations like Radio Algeria on 252 kHz with transmission powers of 1500 kW during the day and 750 kW at night or other powerful LF stations like these >>https://www.hfunderground.com/wiki/Longwave_Broadcast_Stations
The end result is that the band below 500 kHz does not have many strong signals for North American DXers to contend with. I live close to a major airport and the Non-Directional beacons are only transmitting with 25W. Most NDB's are under 150W and the strongest are 2 KW. This means that listeners can use a sensitive receiver and a reasonable antenna not be concerned about overload but only if
MW stations do not cause spurious images below 500 kHz. Spurious Signals
Serious VLF and LF NDB DXers use narrowband radios like the Icom R75. They are sensitive receivers with adjustable bandwith and minimal spurious from MW and other out-of-band signals. The RSP is a wideband
receiver and this can result in overload and/or spurious signals if precautions are not taken. Use of Low IF and a suitable LO frequency is required because this will give fewer MW "ghosts" below 500 KHz. In areas with MW stations nearby an RF low pass filter and/or a MW rejection filter is required. A preselector will also bandpass filter the signal input into the RSP and allow more RF and IF gain and fewer spurious signals. Users with MW spurious or overload problems should read this SDRplay app note https://www.sdrplay.com/docs/SDRplay_Optimising_Performance_RSP1A_LF_MW_HF.pdfReceiver Sensitivity
Several posters questioned whether I was able to use my RSPduo at maximum RF Gain. The answer is yes. However the antenna I used was not designed for LF so signals and noise were lower than one would get with a proper LF antenna. The screenshots posted earlier were taken at my location with RF Gain at maximum to get the best Noise Figure and highest sensitivity possible. The IF gain was at mid-position but this did not affect the NF to a large degree but it did lower the level into the AD in order to prevent overload.
For those living in areas with higher signal levels or better antennas they will have to reduce the RF gain considerably in order to avoid AD overload or reduce intermodulation. Each dB of attenuation increases the Noise Figure by 1 dB and reduces receiver sensitivity. In this situation the benefits of the HiZ port will not be realized and the RSP1A may be a more cost-effective solution. Antenna Considerations
The selection of an antenna will have a considerable effect on the signal levels being received. Higher gain antennas will deliver more received signal and RF Gain will have to be reduced. An external attenuator can be used but I suggest that the RF Gain control be used, if possible, because that way the SDRuno S-meter will still be accurately indicating the receive level at the antenna terminals. Active loop antennas use an amplifier and not too much gain should be used with this type of antenna. The designer of Wellbrook antennas recommended that I not buy the Pro model because there is too much gain for the RSP. Noise - Man made or atmospheric
Other posters have stated that noise levels are sometimes high enough that a high sensitivity receiver is not required. They are correct in this assertion.
Summertime atmospheric conditions raise the noise level considerably on HF and lower frequencies and high S meter readings are common. In the winter months atmospheric noise levels are much lower but man-made noise is still a problem. We live in a digital world and the level of unwanted RFI gets worse with each passing year. Noise cancelling devices like the Timewave ANC-4 or MFJ-1026 can provide some relief but they need to be manually adjusted and can become tedious to operate. Summary
The RSP family of products provide a low-cost way for hobbyists to have fun with a wideband
Software Defined Radio (SDR). The selection of which SDR product to buy depends on the budget and the geographic location, antenna, local noise levels and band(s) of interest of the purchaser. These devices provides a capability that would have cost ten times as much only a few years ago. But to exploit all the benefits will require some experimentation and a thorough review of the documentation available.