The maximum achievable signal-to-noise ratio (SNR) in any receiving system is fundamentally limited by the receiver’s noise floor. In low-frequency receiving systems–typically frequencies at and below High Frequency (HF)—the realized SNR is often limited by external background noise, which includes galactic noise, atmospheric noise, lightening, and man-made noise. In these low-frequency receiving systems, it is often assumed that the mismatch loss (τ) associated with high voltage standing-wave ratio (VSWR) is not significant because it equally attenuates both the received signal power and the received noise power.”
Do you wish to quickly determine your receiver Noise Figure? Do you wish to know how your antenna is performing with respect to local or far away noise? Would you like to know whether your location is really suitable, or not suitable for HF reception? You would, but until now you did not have the terms of comparison, now you have them, just read on…..
In this thread we shall provide methods and data useful in determining antenna and receiver performance in the real world. The following parameters will be easily found:
- Receiver Noise Figure from receiver’s Noise Floor with 1 Kohm and with 50 Ohm input termination.
- Antenna Noise Figure from receiver’s Noise Floor when connected to your antenna.
- Your Antenna noise predicted performance dependent on the characteristics of the receiving site
Today we are bombarded with horror tales about man made noise lurking around and within our antenna space, we read many articles on so called (….science fiction…) low noise antennas, but we have no terms of comparison to really know and understand if the noise floor we are experiencing and which limits our weak signal receiving capabilities is the best we can have in our location, or if there is, at least theoretically, room for improvement. In other word we need to precisely assess the performance of our receiving system not in qualitative, but in quantitative terms.
Today it is easily possible for the radio operator to achieve this level of knowledge because we have at our disposal:
- The RSP class of communications receivers, which provide accurate & calibrated noise floor data.
- ITU Recommendation ITU-R P.372-13, which provides real world noise data and noise level approximation
Tabular information will be provided first to provide simple and easy assessment without the impediment of mathematics. In later posts the mathematic gears behind the show will also be described.
1. RECEIVER NOISE FIGURE FROM TERMINATED NOISE FLOOR
Table 1 shows HF noise floor readings of my RSPduo receiver, S/N 180602D532 with the HIGH Z input terminated with a 1 KΩ resistor and the coaxial inputs terminated with a 50 Ω resistor. All operating parameters are also tabulated for easy reference.
General measurement conditions are shown, so that they can be actually reproduced for other receivers. The RF gain shown in the last column for the two coaxial inputs is the lesser of the two gains found, when different. It is important to do the noise floor measurements using the same SR and DEC values, otherwise inconsistent data will be obtained.
Data in Table 2 has been calculated to quickly determine the receiver’s Noise Figure in each band for any of the three inputs and for a given detection bandwith.
EXAMPLE 1, using noise floor data (Table 1) found for my own RSPduo.
Our receiver, on the 50 Ω terminated coaxial input reads a noise floor of – 135 dBm on Tuner 2 on the 160 m Band ( 1,950 MHz). With a detection bandwith of 2200 Hz (SSB) in Table 2 we read a Noise Figure of : (6.6+4.6)/2 = 5.6 dB.
EXAMPLE 2, using noise floor data (Table 1) found for my own RSPduo.
Our receiver, on the 1 KΩ terminated balanced input reads a noise floor of – 140 dBm on Tuner 1 on the 70 m Band ( 7,200 MHz). With a detection bandwith of 1800 Hz (SSB) in Table 2 we read a Noise Figure of : 1.4 dB.
All in all my receiver shows excellent performance, with the worst value of Noise Figure found on Tuner 2, coaxial input, at 3.750 MHz, with a noise floor of -125 dBm corresponding to a Noise Figure of 16.4 dB. As we shall see, the value of the receiver's Noise Figure, even at this level, impacts little on the overall system Signal to Noise Ratio, because unfortunately the antenna noise figures are much higher due to noise received.
In the next post we shall analyze antenna noise behavior using antenna noise floor data.