Here is a very simple continuously tunable RF Pre-amplifier for 40/20/18/15/10M bands. Front-end tuning is done with wide tuning range capacitor ( Varactor ) 1SV149. Monolithic amplifier ERA-3 from Mini-Circuits forms the wide band RF amplifier. The maximum gain of the circuit varies from 12.6db at 7Mhz to 16.8db at 28Mhz. Front-end attenuator controls the pre-amp gain over the entire range.

This amplifier has been developed as a part of 100W HF Booster, to be described separately on this site, for low power (10 to 15W) transceivers abundantly available in the market.

VR1: 1K Pot                          R1: 270E

VR2: 10K Pot                        R2: 100K

C1: 1nF                                R3: 1K

C2: .01uF                             D1,2,5,6: 1N4148

C3: .1uF                               IC1: ERA-3 Mini-Ckt

C4: .1uF                              D3: Varator 1SV149

C5: 2.2uF/35V                     D4: 9.1V Zener

C6: .1uF                             C7: .01uF

C8: 1nF

 L1: 1uH, T30-12, Light Green-White, primary 25T #35, Tapped at 5T from ground end, secondary 5T # 35, 

RFC: .65uH, T-37, Green-White, 20T # 32

 The circuit is self explanatory. VR1 is the front-end attenuator. Diodes D1, D2 have been used for protection from stray TX pulses. L1 together with varactor D3 forms the input tuned circuit. 12V to 13.8V power supply from the transceiver is stabilized to 9.1V through the Zener D4. Tuning pot VR2 supplies 0 - 9.1V to varactor for covering the entire tuning range of frequency from less than 7Mhz to a little over 28 Mhz thus covering 40 through 10M bands.

ERA-3 is a monolithic wide band RF amplifier. Value of RFC is not critical. R1 biases ERA-3 to about 35ma depending upon the supply voltage. Again diode D4 & D5 are used for protection from stray RF coupling from output side.

BW of the pre-amp is fairly wide. It does not require frequent tuning / tweaking while working on any band.

 The above picture shows the set up for testing the pre-amp. MFJ-259B, Antenna Analyzer is used as a signal generator. Its O/P is terminated into a 40db attenuator pad, followed by 12db HP step attenuator Model-355C. Pre-amp O/P is connected to HP 3585A, 40Mhz Spectrum Analyzer. Preamp is powered by a 0-30V home brewed power supply.

 Shown below is the Top-view of component layout over a double sided PCB with bottom side acting as the ground plane.

 For a quick fabrication of PCB, isolated islands were created with the help of diamond tipped metal scriber tool.

 Coil winding details of L1 & RFC:

Tinned and components mounted on the PCB:

 Completed circuit board ready for testing:

        About 1SV149

             (Toshiba make)

It is a variable capacitance diode having a very large capacitance ratio of C1v / C8v >15. It has a high Q and recommended bias voltage operating range is from 1 to 8VDC.

To cover the entire frequency range from 7Mhz to 28Mhz, bias voltage range has been increased to 0 to 9V.