This article continues from Ambient noise measurement using whip on vehicle – #1 – estimate Antenna Factor with a case study for the active antenna electronics.
For this discussion, I will use the amplifier developed at A high performance active antenna for the high frequency band, but applied to the antenna described at Ambient noise measurement using whip on vehicle – #1 – estimate Antenna Factor.
Let’s assume that the antenna + amplifier will be used with a HF receiver with Noise Figure 6dB, Teq=864.5K.
From (Martinsen 2018) Fig 3.8, the amplifier internal noise at the output terminals is -118dBm in 100kHz @ 3.5MHz. That implies that the amplifier Noise Temperature is 857.93K. The amplifier has 6.4dB voltage gain which needs to be subtracted from the AF calculated for unity gain (at the amplifier input terminals).
Amplifier characteristics:
- Voltage gain 6.4dB;
- AF=6.05-6.4=-0.35dB/m;
- input Z=1MΩ||15pF;
From (Martinsen 2018) Fig 3.8, the amplifier internal noise at the output terminals is -118dBm in 100kHz @ 3.5MHz. The equivalent noise temperature is 857.9K.
The amplifier output terminals will be used as the reference plane for the following calculations.
Total internal noise (amplifier and receiver) \(T’=857.9+864.5=1722 \text{ K}\).
Using Field strength / receive power converter for measurements made in a 1kHz wide ENB,
we obtain:
The Ambient Noise Figure Fa is 57.5dB, similar to that predicted for a Residential precinct in ITU P.372-14.
It may be tempting to assume a simple linear relationship between the measure power and ambient noise figure.
In most cases, the measured power includes internal noise of the receive system (both active antenna and the receiver), and when external noise dominates, the relationship is close to linear.
Above is a plot of Fa vs measured (total) power for this (exact) scenario. In this case, Fa=Pt+174 is a good estimator for Pt>-145dBm or Fa>29dB. You would not expect to measure such low Fa in this scenario (frequency and equipment), but that may not be true of other scenarios… you need to check.
Field strength / receive power converter properly accounts for internal noise.
Links / references
- Martinsen, W. Aug 2018. A high performance active antenna for the high frequency band. Cyber and Electronic Warfare Division
Defence Science and Technology Group DST-Group-TR-3522.