21 cm band also known as the Hydrogen line is one of the important bands in radio astronomy.
Please read this Wikipedia article if you not familiar with the Hydrogen line.
Nowadays even radio amateurs can observe this signal from outer space, thanks to modern hardware.
One of the most important parts of any receiver is the antenna and input stage. Typically input stage is some kind of the amplifier with extremely low noise and high gain due to weakness of the space signals.
In this article, I want to describe a simple and cheap amplifier for the first stage of the receiver.
This amplifier is a part of my DIY radio telescope project.
Please watch this intro videos from the last year.
In the following article, you can read more about this antenna.
But now let’s see how to build first stage LNA (low noise amplifier).
This amplifier is a two-stage device with a high pass filter.
As a first stage amplifier, I’m using GRF2071 from Guerilla RF.
Thank you guys from Guerilla (Eugene Proenza, Alan Ake, Frank Nicosia, and others) for the great technical and information support.
Components surrounding GRF chip (caps and inductors) was selected for the best performance at 1400 MHz.
Here is the frequency response of this part
Between amplifier stages, I’m using HFCN-1200 high pass filter which can reduce unwanted low frequencies and determines the frequency response of all device.
Filter frequency response:
The second stage of the LNA is performing additional line amplification of the signal (which is a little bit attenuated by the HP filter) before sending this signal to the feeder. This part is based on the popular SPF5043Z IC.
A full schematic of the LNA:
Total theoretical gain and noise figure of the whole LNA can be calculated using this very nice tool: https://www.pasternack.com/t-calculator-noise-figure.aspx
For my schematic I got these values:
Total Noise Figure: 0.3052 dB
Total Gain: 38.00 dB
which is very good! 🙂
Of course, real-world devices can’t get such characteristics due to many reasons but anyway real params should be very close to the theoretical and that’s great.
LNA board, arrangement on the two-layers PCB:
Please contact me if you wish to get Eagle project files.
The whole board was placed inside the metal box which was hermetically sealed. As the input and output connectors, I’m using the N-type connectors which were bolted to the case. For the power delivery is used 4700 pF feedthrough capacitor. The optimal supply voltage is 6-9 volts.
LNA noise very depends on the system’s total temperature. So it’s always a good idea to cool the device.
For the cooling purpose, I’m using Peltier and cooling system (radiator + fan). To get better performance I put whole construction inside the aluminum can which is surrounded by the thick layer of foam.
Thanks for reading!