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The 23cm station I put together is now working, and I’ve had some contacts, mostly local (yes, there have been more people on a 23cm sked than we have on a Sunday night). Now work goes on to optimize the system. I’ve been talking to Bob again and we’ve been discussing improvements we could make. So what can be optimized? Obviously producing more power wouldn’t be a bad thing. To this end I have got a PA producing around 60W at 23cm. The other thing that can be done is to give it bigger ears. Space is limited so no more elements on the yagi, but a pre-amplifier would be in order.
What is a pre-amp? The job of a pre-amp is to amplify a signal in preparation for being amplified somewhere else, in our case it would be further amplified in our rigs. There are a couple of criteria that are most important in a pre-amp, Gain and Noise Figure.
Gain: Its got to amplify a very weak signal, ready for sending down the coax line.
Noise Figure: A low noise figure means that it puts as little extra noise onto the signal as possible line. Amplifiers have an inherent problem – they generate noise all the time, one component is thermal noise, controlled by reducing the temperature, this is something difficult to control without complex cooling arrangements (something that is done in radio telesecopes using liquid gases). There are other forms of noise that are generated by semiconductor devices called shot noise.
Why not have a low noise amplifier near the back of the rig. This may not be the best place for it. Of importance in any system where signals and noise exist is the ratio of signal to noise or SNR. The more signal the better. What is certain is that at the input of an amplifier, there is signal and noise, both of these are amplified. However there will also be noise added by the amplifier itself. So the signal to noise ratio further down the system will always get worse. The noise factor tells us how much noise is generated by the amplifier itself compared to the amplifier if it had no noise generated at its inputs. A lower noise figure is better than a higher one, it is always greater than one. In a 23cm Pre-amp of Bob’s the noise figure, F = 0.8dB = 1.2. Pretty good!
Lets start off with a system of antenna, coax and rig, NO pre-amp at the moment, look at Scenario 1. The rig connected directly to the antenna, I didn’t choose the noise figure of 4 for any reason, just as an example. Clearly the noise figure of this system is F = 4
= 6dB
Scenario 2, the noise figure when the receiver is connected at the end of so
me coax, with all its losses. The overall noise figure will be given by the equation below. It’s in the ARRL and RSGB handbooks. Remember a lower figure is better:
=
=16=12dB
Scenario 3, with the pre-amp included.
=
= 1.2 + 0.003 + 0.012 = 1.215 = 0.84dB
This is clearly a huge improvement, even better than having the aerial directly connected to the receiver, all on account of having low noise and high gain to counteract degradation later in the system. Now the ultimate test, what if you put the Pre-Amp directly into the receiver as in Scenario 4:
=
=4 + 0.8 + 0.012 = 4.812 = 6.8dB
This is 6dB down on having the pre-amp at the antenna. So the case still stands, and that’s why you always should have low noise pre amplifiers as close to the antenna as possible to improve the systems performance in response to noise.