This focuses on receiver Noise Figure.

Noise Figure is simply the ratio of So/No to Si/Ni usually expressed in dB. It has the advantage of being independent of receiver bandwidth (Duffy 2013).

The Noise Figure was calculated from measurements using a RFD-2305 calibrated noise source using the Y Factor method. NFM software (Duffy 2007) was used to perform high resolution measurements of noise with the source OFF and ON.

The calculated Noise Figure is 5.9dB.

The Equivalent Noise Bandwidth of the nominal 2400Hz soft filter was measured to be 2040Hz (Duffy 2009). The IC-7410 specifications state a sensitivity of 0.16µV for 10dB S/N with that filter, from which an expected Noise Figure can be calculated (Duffy 2006) to be 8.0dB. The receiver exceeds specification by just over 2dB.

- Duffy, O. 2004. Noise Figure Y factor method calculator. VK1OD.net (offline).
- ———. 2006. Receiver sensitivity metric converter. VK1OD.net (offline).
- ———. 2007. Noise Figure Meter. https://owenduffy.net/software/nfm/index.htm.
- ———. 2009. Measuring receiver bandwidth. VK1OD.net (offline).
- ———. 2013. Noise and Receivers. VK1OD.net (offline).

The noise power of a generator could be expressed as an equivalent noise temperature Ts, but is often given as an Excess Noise Ratio (ENR):

**ENR=10*log((Thi-Tlo)/290) dB.**

An attenuator after a noise source decreases the ENR by the attenuation:

**ENR’=ENR-A dB** where A is the attenuation in dB.

The property can be used to obtain a different ENR from the noise source, a different Thot.

A known noise source can be used with a known attenuator to provide two different input conditions for measurement of a Y factor and calculation of NF. The variation here is that the noise source is on for both halves of the test, and the applicable source noise temperature needs to be entered into NFM.

Lets explain using an example.

Lets take a noise source of ENR=50dB, and 40dB and 50dB attenuators.

Procedure:

- before making measurements, set Tcold in NFM to 290, set ENR to 0dB (50-50) and note calculated 0dB equivalent Thot value, it is 580.0K (this will be used for Tcold later);
- set ENR to 10dB (50-40) and a new value of Thot is calculated (3190.0).
- now enter the calculated 10dB equivalent Thot value (580.0K) into Tcold;
- optional: measure SCZ with the receiver powered OFF;
- turn the receiver ON;
- with the noise source ON and 10dB attenuator in line, measure NoiseLo;
- with the noise source OFF and no attenuator in line, measure NoiseHi;
- NFM will try to calculate NF.

Above, the NFM screen showing such a measurement.

- Duffy, O. 2007. Noise Figure Meter. https://owenduffy.net/software/nfm/index.htm.
- ———. 2004. Noise Figure Y factor method calculator. VK1OD.net (offline).

First step is to recheck the NF of the receiver. The TS2000 is getting a little tired, NF=8.3dB.

The technique calculates ambient noise from the variation in receiver output noise of a receiver of known Noise Figure with the insertion of a known input attenuator. The receiver output noise was measured using NFM (Duffy 2007) which allowed integration over 20s for high resolution measurement.

Above, the results of the calculator on the measurement. Ta=1353K (Fa=6.7dB) which is a little higher than I expected given that it is a large block residential area (low density) with underground HV and LV power, negligible traffic even on the nearby ‘main’ road, and antenna pointing towards Canberra over mostly rural land a few hundred metres away.

A second measurement with the antenna pointing at Knight’s Hill, a TV transmitting site about 25km away gave Ta=1900K (Fa=8.2dB).

Neither of these are huge numbers, Ta in excess of 2000K is not uncommon in residential areas of cities, worse in commercial precincts.

The results obtained are well within range of the predictions of (ITU-R 2009).

Note that Ta will vary with direction, hour of day and from day to day.

- Duffy, O. 2007. Noise Figure Meter. https://owenduffy.net/software/nfm/index.htm.
- ———. 2004. Ambient noise calculator. VK1OD.net (offline).
- ITU-R. Oct 2009. Recommendation ITU-R P.372-10 (10/2009) Radio noise.