How an LNA can Improve VHF Reception with an RTL-SDR
Over on his YouTube channel Adam 9A4QV has uploaded a video showing how an LNA work to improve signal SNR on VHF, as long as the LNA is placed close to the antenna. Adam is the manufacturer and seller of the popular LNA4ALL low noise amplifiers.
On UHF and high frequencies an LNA can help by reducing the system noise figure, but on VHF this effect is small. But if the LNA is placed near the antenna then the LNA can still help significantly by overcoming any losses in the coax cable, filters, switches or any other lossy components in the signal path. It might also help create a better SWR match for the dongle and antenna. The video has some sound issues in during the demonstration part, but on his Reddit thread Adam writes:
LNA4ALL on the VHF close to antenna effect
Nothing new under the sun. The RF gold rule recommends that before amplifying, you have to filter. And if it is done on foot antenna is ideal. If working with filters and amplifiers with an impedance of 50 ohms, use the coaxial cable of the same impedance to avoid maladjustments and SWR.