To assess the impact of amplifier circuit noise in "active" receive arrays, we only need to be concerned with the contribution of amplifier circuit noise relative to atmospheric noise. The details of how signals are phased in any particular array do not matter. The objective is to keep the total contribution of amplifier noise far below the atmospheric noise so as not to degrade the overall system noise floor in any significant way. However, we need to understand that the combiner circuit that phases up the signals in a receive phased array responds very differently to amplifier noise and atmospheric noise. This makes it less obvious how to determine whether the circuit noise of a particular amplifier is "low enough". Fortunately, there is a simple way to determine that using basic principles.
Let's start with a single amplified vertical antenna. To simplify the analysis, we just set the gain of the vertical to 0 dB. In practice we can do a NEC analysis to calculate absolute gain in dBi, factoring in real losses but that is not necessary and does not change the conclusions. The antenna feedpoint amplifier adds its own noise to whatever signal plus atmospheric noise is received by the vertical. Let's set the circuit noise power equal to one "circuit noise unit" and the atmospheric noise power to one "atmospheric noise unit". Of course we can put voltage (or power) numbers on those units, based on properties of the amplifier, the atmospheric noise, the actual antenna gain, and the measurement bandwidth. However, that makes things unnecessarily complicated, so we won't do that. Next we create an array of N amplified vertical antennas, each one identical to the single vertical we started out with. We feed the signals from all the antenna amplifiers into an ideal combiner circuit that does not add its own noise. The combiner circuit phases up signals to create a directive beam pattern. Now we ask how much atmospheric noise appears in the phased up sum compared to the amount of total amplifier circuit noise. The atmospheric noises received at the various verticals are all correlated. The correlation comes about because the atmospheric noise is the same at each vertical except for time delay differences caused by geometric path length differences to each antenna element. However, as I described in an earlier e-mail, the amplifier circuit noises coming from each of the antenna amplifiers are all uncorrelated. For uncorrelated noises, the combiner simply adds the circuit noise powers of the individual amplifiers as I described previously. For N elements with N amplifiers, the total circuit noise power out of the combiner is then N times one "circuit noise unit" (ignoring any additional gain or throughput loss imparted by the combiner circuit). To determine the total atmospheric noise coming out of the combiner circuit, let's assume the atmospheric noise has a completely uniform distribution in 3-dimensional space. That is, the strength of the atmospheric noise is the same in every direction. This is an idealized assumption, but is often a reasonable approximation to reality. Under these assumptions, the total atmospheric noise out of the combiner turns out to be just one "atmospheric noise unit"! In other words, it is exactly the same as the atmospheric noise coming out of a single vertical. This is because the total atmospheric noise power picked up by the array is just the gain of the array (relative to a single vertical) averaged over all of 3-dimensional space times one "atmospheric noise unit" (the noise picked up by a single vertical). That average gain is exactly 0 dB, so the total atmospheric noise doesn't change in our idealized system. It doesn't matter what the antenna pattern is; the average gain is always 0 dB, which is why we did not need to be concerned with details of how signals are phased up to form a beam pattern. Of course, a different gain applies to actual signals that are coming from a specific direction and are not uniformly distributed like atmospheric noise, which is why we see a S/N improvement when the array is aimed at a signal of interest. So, we have demonstrated that in relative terms, the amplifier circuit noise power in an array of N amplified antennas goes up by a factor N whereas the atmospheric noise does not change. That increase in the amplifier noise contribution relative to atmospheric noise degrades the overall noise figure of the system. However, as long as we keep the amplifier noise contribution small enough, the noise figure degradation can also be kept to a minimum. That is why having more amplified elements makes it more important to design the antenna amplifiers for low circuit noise. 73, John W1FV -----Original Message----- From: Topband [mailto:[email protected]] On Behalf Of Michael Tope Sent: Thursday, March 12, 2020 4:37 PM To: [email protected] Subject: Re: Topband: Hi Z amplifiers for 160m Hi Lee, Yes, if you are combining coherent signals that are not in phase, then the each of the voltage vectors is weighted by cos(phi-i) where phi-i is the angle between the i-th voltage vector and the 1st vector. If phi=0, then you have the case I described previously. I can see how this can get tricky, however, with an electrically short baseline where you are striving for cancellation in the rearward looking direction. It's like you cancel in the rearward direction and almost cancel in the preferred direction :-). This degrades the SNR not because the noise is adding up, but because the signals are subtracting down. 73, Mike W4EF............. _________________ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
