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LNA w/bypass relays and RF VOX for 70cm band

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LNA w/bypass relays and RF VOX for 70cm band
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In this project we are present a new version of the LNA with bypass relays and RF VOX. This construction is different from the previous version. It has different bypass relays (up to 100 Watts) and RF transceiver control circuit. Area of the application is utilization of the FM transceivers and radio stations (LPD, PMR). It’s possible to use LNA for civil LPD frequency band (433 – 435 MHz) and PMR frequency band (446 MHz) with usage of the feeder and stationary remote antenna. 

The LNA is manufactured based on PGA-103+ Mini-Circuits Monolithic Amplifier with high dynamic range.

RF VOX is designed for protection from accidental power supply to the LNA output. Because of RF VOX it is possible automatically switch LNA in TX mode (till 50 Watt) for the FM application. For the owners of the portable FM (LPD, PMR) radio stations it is sufficient to connect LNA to feeder and turn on power supply. RX/TX switching is automatic. 

According to the simulation results: NF < 0.3 dB, VSWR on the input and output is less than 1.5. Gain equals 23 dB. LNA is absolutely stable.

LNA design features

LNA consists of the matching transforming circuit, MMIC PGA-103+, 6V voltage regulator, band-pass filter, bypass relays AXICOM IM2, RF VOX elements and power supply circuit by feeder reduction (Fig. 1). Values of the input circuit elements and band-pass filter depends of operating frequency range. Power supply of the LNA is 12 V DC. There is opportunity to connect power supply using RF feeder from transceiver or using power supply injector.

Block-diagram of the LNA is shown on Fig. 1

LNA 2m RF VOX schematic circuit

Fig. 1 LNA schematic circuit

Without power supply LNA is working in «bypass» mode – power of the transceiver is directly going through the LNA to the antenna. In TX mode the LNA is turned off. RX mode is on in the case if power supply is turned on. For LNA switching it is necessary to disconnect power supply before transceiver power comes to the output. If TX mode is turned on without power supply disconnection the switching is performed using the RF VOX. It is necessary to understand that bypass relays are switching under RF load in the case of RF VOX is operating. In this case transceiver power should not exceed the 50 Watt.

LNA was successfully tested operating with 100 Watt transceiver IC910H. However it is not recommended permanently using RF VOX with this big power. Bad VSWR of the antenna can cause some issues with relay pins.

The PCB of the LNA has 60×32 mm dimensions. It is manufactured with 1 mm thickness FR4 substrate material with plated via holes and soldering mask. Inductances of the band-pass filter are high quality SMD 0805. The input inductance is designed with 0.8 mm wire.

LNA PCB is shown on Fig. 2

Fig. 4 LNA 2m RF VOX PCB

Fig2.  LNA PCB

The housing of the LNA is manufactured from aluminum plate D16 by milling on the CNC machine. It has N-female input and output connectors. Power supply is designed through the feeder reduction or capacitor that mounted on the surface of the housing.

Easy installation while working without PA with transceiver or FM radio station. Protection against accidental flow transmitter output power to the LNA enhances operational reliability. RF VOX helps to work without RX/TX switching in FM mode. It’s possible to use LNA for civil LPD frequency band (433 – 435 MHz) and PMR frequency band (446 MHz) with usage of the feeder and stationary remote antenna.

Technical specifications of the LNA-70cm

  • Model: LNA-70cm-100W-RF-VOX
  • Active element: PGA-103+
  • RF Power TX mode: 100 W (max), 50 W RF VOX
  • Gain: 20 dB
  • Noise Figure: < 0.6 dB
  • Output IP3: 39 dBm
  • P input: 21 dBm (max)
  • DC voltage: 12 V
  • DC current: 150 mA
  • Impedance: 50 ohms
  • RF connectors: N-female or SMA-female
  • Dimensions: 110 mm x 43 mm x 20 mm (with connectors)
  • Weight: 0.15 kg

Package includes

  • 1 x LNA-70cm-100W-RF-VOX

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