Every site on planet earth has a noise floor. In the middle of Nowhere’s Ville the noise floor could be as low as -174 dB. But if there is civilization, it is likely that the noise floor will be less than -174 dB such as -160 dB.
Introducing a Noise Floor
A known noise floor is very significant in radio communications as this defines the minimum weak signal that can be detected. In the middle of Nowhere’s Vill you have the best chance of capturing and copying a weak signal. But if you move to an industrial environment, the noise floor will be significantly higher which will swamp out the weakest signals.
One method of countering a receiver’s inability to capture weak signals is to employ the use of a preamp at the receiver’s antenna input. This will boost any signal coming in which can therefore bring in weak signals that would otherwise be lost. However, a preamp will also amplify whatever noise floor is present. If the noise floor is too high, there is nothing that can be done (other than moving the receive site to Antarctica).
What About a Preamp?
But the inquisitive amateur may ask, “Why not just use a preamp anyway. It can’t do any harm.” Wrong. Because the preamp boosts all signals coming in (including the environmental noise), it will also boost an extremely strong signal that is coming in. If too strong, the result will be distortion.
Let us also note that preamps are generally not useful at HF frequencies. Preamps generally begin to become worthwhile at frequencies greater than 60 MHz.
Measuring a Site’s Noise Floor
To determine if a preamp may prove useful, we need to measure the noise floor that exists at the receiver’s site. The measurement term will be microVolts (uV). We will then compare that value with the receiver’s specified sensitivity with is also specified in uV. The WD8IEL repeater is a Yaesu DR-2X which is specified to have a sensitivity of 0.2uv. If the noise floor comes in equal to or greater than this value, use of a preamp is very likely a lost cause and should not be considered. However, if the noise floor comes in noticeably less than 0.2uV, a preamp is at least worth a try.
Noise Floor Measurement Method
What we are going to do is combine the antenna feed with a 50 Ohm RF signal generator’s output. We will observe the receiver’s S-meter with the RF signal generator turned to zero. We will then start turning up the signal generator’s signal amplitude until we see the S-meter just barely move. The amplitude of the signal generator output represents the environmental noise floor at the site.
Tell Me About the Wilkinson Divider
The Wilkinson Divider/Combiner is an incredibly ingenious device created by Dr. Wilkson in 1960 and published by the IEEE. It uses two coax cables cut to a quarter-wavelength of the signal frequency and connected as shown in the illustration. The characteristic impedance at all three ports is equal. It is especially useful if a 75 Ohm coax (such as RG-11) can be used if the impedance of interest is to be 50 Ohms. It is easily constructed by the hobbyist and is cheap. It is especially useful for applications such as a repeater with one specific frequency of interest.
For our frequency of interest (145.450 MHz), two sections of coax will be cut to 34 cm in length. A 100 Ohm resistor will connect the ends at one end and at the other end they will be shorted together.
What’s That Attenuator Doing in There?
Notice the attenuator interfacing with the signal generator and the Wilkinson Combiner. Most RF signal generators cannot dial in the required low of amplitude required. The signal generator must be able to output a signal at least less than the receiver’s specified output.
But there is a reason why you will not find such low levels available on most RF signal generators. Recall that we are looking to measure a noise floor. Any noise floor is going to interfere with the instrument’s accuracy if we try to make the instrument itself output those low levels.
Let us suppose that we were to decide on making 1 Volt RMS, coming out of the RF signal generator, downsized to 0.1uV RMS. 1 Volt into the attenuator is to be downsized to 0.1uV coming out of the attenuator. Attenuators are specified in dB. What level of attenuation would be required to know what size to order?
Required dB = 20 log10(0.1uV/1V) = -140 dB
They don’t make them with that much attenuation so we will need to have a string of attenuators on hand to string together in series. The sum of the attenuators in the string will equal at least 140 dB.
Processing the Measurement
Let us suppose that when the RF signal generator output was set such that the VHF receiver’s S-meter began to move indicating the noise floor was broken, the RMS output of the RF signal generator was 0.65 Volts RMS. The next step is to find out what the RMS voltage (eo) is on the other side of the attenuator.
The attenuator is -140 dB.
-140 dB = 20 log10(eo/0.65V)
-140/20=-14/2=-7 = log10(eo/0.65V)
10-7 = eo/0.65V
eo = 10-7 * 0.65 = 0.065uV
The noise floor voltage is 0.065uV RMS which is very much less than the receiver’s sensitivity. That’s a good thing! A preamp, in this case, is advised though, of course, there can be no guarantee of success. But the risk is minimal.
What is the Noise Floor
We would like to be able to express this Voltage noise floor in relation to the theoretical minimum possible on planet earth, -147dBm. We are working with a system impedance of 50 Ohms. We can therefore express this noise floor voltage as a power, dBw.
Noise Floor dBw = 10 log10[ 0.065uV2/50 ] = -161 dBw
How Good is That?
So, what are we supposed to tell people regarding the noise immunity/liability of our receiver’s site? First of all, our “0.65 Volts” value was for illustration purposes. It was not an actual value that was measured. But if this were an actual value we measured, we would have to say that the noise immunity is…DAMN GOOD!