Increase receiver sensitivity and calibration
Sometimes, there are situations that the dongle sensitivity is not enough by the weak signals. Such situations can arise not only due to the weak signals but also due to the losses in the long antenna feeder, losses in the used filters, as well due to the industrial interferences that lowers the signal/noise ratio. In general case, the task is not only to gain the weak signals but also to improve the signal/noise ratio.
This can be achieved with different methods:
- using the appropriate antenna
- addition to the appropriate antenna an antenna amplifier
- installation of the LNA (not the best but still suited solution) that provides the gain of the input signals in the high dynamic range but also has a good coordination with the antenna feed.
As known, the electromagnetic waves consist of two components: electrical E and magnetic H. The stub antenna with electrical length λ/4 and the shortened stub are mostly used antennas in the systems due to the simplicity and low costs. Such antenna is omnidirectional and receives the E component of the electromagnetic wave. However, in the cities where a lot of industrial interferences available that mostly have a E component, the usage of the stub antenna does not allowed to receive the weak signals. It makes more sense to use a magnetic or loop antennas in order to receive the magnetic component H of the electromagnetic wave, that is lesser exposed by the industrial interferences. Additional advantage of the loop antenna is a directional pattern, that looks like a number 8 and enables to make a spatial selection of the useful signals from the noise.
The MMICs (Monolithic Microwave IC) are worked well as the antenna amplifiers that have a low own noise, broad band of operating frequencies, good dynamic characteristics and can be used in the circuits 50 Ohm. Although there is a wide range of the MMIC amplifiers on the market, e.g. series ERA, MAR, MAV from the company mini-circuits, only few can be used with the dongle RTL2832U. The main restriction is a current consumption and the noise level. The most suitable component is MAR-6 (N=3dB, ICC = 16 mA) or PSA4-5043+ (N=0,75dB, ICC = 33 mA) by considering the above mentioned restrictions. Another important point is the simple connection schema and the minimal number of the external elements.
The LNA is a non-lineal element on those output the even and odd harmonics can form up. These harmonics can be useful but also a hindering, e.g. from nearby useful radio stations. The commonly used solution is input tuneable selection circuits. Moreover, sometime it makes sense to use the push-pool amplifiers, that enables the even harmonics suppress more than 40 dB.
The transformation coefficient N depends from the resistance of the used loop antenna and can be calculated as N = sqrt (50/Rloop).