Amateur Radio 2m Preamplifier and Filter description
Amateur Radio 2m Preamplifier and Filter, model LNAU-0146-648, is a 2 meter (144-148 MHz) LNA and Filter combined into a single device. It was designed with Software Defined Receivers (SDRs) in mind, which are notorious for performing poorly where there is a high density of signals in close proximity. This is particularly true of low-cost USB SDR dongles, which cover a wide spectrum of frequencies with a few components. The LNAU-0146-648 provides the signal conditioning needed to provide high-end performance to a low-cost SDR. It does so by filtering the signals twice, by adding 20 dB of gain (100 times magnification), and providing a low Noise Figure of 1.5 dB to the receiving system. The output filter is very sharp, completely removing unwanted signals outside the immediate vicinity of the Weather Satellite spectrum.
The LNAU-0146-648 has built-in protection against unintended large RF power levels at either connector. It can survive up to 2 Watts of continuous in-band power at either port; however, while large RF power is present, the Amplifier will either operate at reduced capacity, or disconnect itself, depending on the magnitude of the injection. This makes the LNA very robust. Most preamplifiers will not survive input signals larger than 10 mW (milliWatts). This feature is unique to our LNAU-xxxx-648 family of Low Noise Amplifiers. Variants for other bands and applications are available.
Proper Installation of 144-148 MHz LNA and Filter
This 144-148 MHz LNA and Filter, like any other preamplifier, should be connected directly to the antenna, or as closely as possible, keeping the length of the connection short. This minimizes the loss of the input, and therefore the receiving system's Noise Figure. This also means that the sensitivity of the receiver is maximized. Once the LNA contributes its gain, 20 dB or 100 times magnification in this case, the output can be connected to the receiver through a long coaxial cable, if needed, without impairing sensitivity.
For the amplifier to operate, DC power must be provided at the output SMA (Gold) connector. To provide such power through the coaxial cable, a Bias Tee is generally needed, for example our model BIT-1500-385. Normally, the Bias-Tee is located near the receiver and has 3 terminals: one RF Output/Input, one DC input and one common RF + DC port,. The common port is the one that should be connected to the LNA's output.
The LNAU-0146-648 was designed to work with the popular RTL-SDR USB Dongle receiver. The RTL-SDR has a built-in Bias-Tee, supplying 4.5 VDC @ up to 180 mA, that will support full operation of the preamplifier and filter. Thus, the LNAU-0146-648 output will connect directly to the RTL-SDR through an SMA Male coaxial cable assembly (also part of the kit).
Integration with Software Defined Receivers (SDR)
As indicated above, SDRs are designed to cover a broad range of frequencies, making them vulnerable to large signals over the entire spectrum to a certain degree, depending on the design's sophistication. Most low-cost SDRs have very simple front-ends (the input circuit), which further exasperates the receiver's ability to handle a polluted RF band.
The LNAU-0146-648 serves to clean-up the spectrum in addition to amplifying the desired signals. The output filter is particularly sharp and narrow, eliminating most of the signals cited above. The louder 2 meter signal and the quieter background makes the SDR operate effectively and at its peak performance.
When the LNA is used with an RTL-SDR USB dongle, please pay attention to the version number V2 and below will require that you open the dongle's housing and solder a jumper to engage the internal Bias Tee. For version 3 (V3) dongles, you will need to run batch file command (download batch ZIP file here):
rtl_biast -b 1 <--turns bias tee ON (Windows)
rtl_biast -b 0 <--turns bias tee OFF (Windows)
For other operating systems, consult the Features Guide, Feature 2: Software Selectable Bias Tee.
LNAU-0146-648 Technical Specifications
- Frequency range: 144-148 MHz
- Gain: +20 dB
- Nominal Impedance: 50 ohm (RF) IN/OUT
- Normal Operating Input RF Power: 1 mW (+0 dBm)
- Survivability Input RF Power: 2 Watts CW in-band
- VSWR: 2.0
- Noise Figure: 1.5 dB
- IIP3: +6 dBm (4 mW)
- P1dB: +10 dBm (10 mW)
- Current: <10 mA
- Supply Voltage: 3.3-15 VDC (internally regulated)
- Connector: N Male (Input) / SMA Female (Output + DC)
- Dimensions: L 38 mm (1.5") x OD 28 mm (1.1"), not including connectors
- Mounting: Direct mount
- Operating Temperatures: -40°C to +85°C
- Weight: 100 g / 0.22 lbs
- Shipping Weight: 150 g / 0.3 lbs
- Color: Black (body), Silver and Gold (connectors)
NOTES:
- Specifications and prices are subject to change without notice