Custom VHF Preamplifier and Filter description
User specified Custom VHF Preamplifier (LNA) and Filter, model LNAU-0150-648-CF, is a VHF 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-0150-648-CF 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 custom specified Frequency or Band.
The LNAU-0150-648-CF 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 Custom VHF LNA and Filter
This Custom VHF 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.
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-0150-648-CF serves to clean-up the chosen VHF spectrum in addition to amplifying the desired signals. The output filter is particularly sharp, eliminating most of the signals surrounding and far from the chosen frequency and band. The louder intended VHF signal and the quieter background makes the SDR operate effectively and at its peak performance. Under these conditions, the quality of the SDR becomes less distiguishable (you cannot tell much of a difference between a low-cost SDR or a high-end receiver).
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 a batch file or the below console commands (download batch ZIP file here):
rtl_biast -b 1 <--turns bias tee ON (Windows)*
rtl_biast -b 0 <--turns bias tee OFF (Windows)*
* NOTE: The above commands should be run within a Console (Command) window, within the folder/directory containing the extracted contents of the cited ZIP file. Make sure to shut down the SDR-Sharp application first, or turn ON/OFF the bias tee before starting the SDR-Sharp application.
Caution: before enabling any bias tee, make sure that the its output is connected to the target device, and that the output is not a short circuit. The LNAU-0150-648-CF preamplifier is highly efficient, consuming only 8 mA @ 5V typically. However, the RTL-SDR can deliver 180 mA @ 5V, which ultimately comes from your computer's power supply (USB), thus this caution.
For other operating systems, consult the Features Guide, Feature 2: Software Selectable Bias Tee.
LNAU-0150-648-CF Technical Specifications
- Frequency range: Customer specified within 30-300 MHz
- Typical Gain: +20 dB at 150 MHz
- 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)
- Specifications and prices are subject to change without notice