I can recommend also the very interesting site of LZ1AQ.
The broadband amplifier requirements:
- Very low noise contribution
- Very good large signal behavior (large dynamic range)
- Controlled gain (especially if used in pairs)
- Well balanced and decoupled from feeding coax cable
- Overload protected (your own transmit signal e.g.)
- Optional filtering of overloading out of band (VHF) signals
Practical implementation of an active broadband loop amplifier:
This trans-impedance amplifier is not very complex, but not really a beginners project.
Important is a solid (V)HF construction with short interconnects and VHF decoupling.
The amplifier consists of three stages. The transistor Q1 is optimized for noise. The output stage Q3 must deliver the power to the load. The intermediate stage Q2 and Q22 gives extra amplification to get the desired large signal behavior.
The gain (antenna factor) is fixed with Rgain and Csgain. Csgain is also used to stabilize the amplifier.
The transistors Q5 and Q6 control the DC-levels. It is fast enough to recover from an overload when doing QSK. Remember that your own transmit signal easily overloads the amplifier. The diodes D1, D2 and D3 protect the amplifiers input stage (also from your own transmit signal).
The VHF-notch/LP filter is optional and only necessary when having very strong VHF transmitters in the neighborhood.
The transformer together with the Faraday shield take care of the balancing, the common mode suppression. The coupling factor k of the transformer is about 0.8-0.9.
With a 1.3m*1.3m loop, Rout, k and Rgain the antenna factor (E/Vo) is about 0.33 - 0.5.
Latest measurement of the noise contribution October 8 2018 on a quiet remote location (Rural, may be quiet Rural location), more than 1km distance from the nearest house (location 51.360271, 6.220209):
The ambient noise was well measurable up to 14.5MHz, indicating that the noise contribution of the amplifier was equal or lower than the ambient noise level. At frequencies higher than 14.5MHz the amplifier noise sets the noise floor.
On my home location the noise level is more than 12dB higher on the quietest moment of the day and on the quietest frequencies in the short wave spectrum. Peaking in the spectrum up to 23dB higher. In the evening increasing with at least 6dB.
The gain (antenna factor) is a little bit frequency depended. Because of the transformers coupling factor (k<1) we see the loop resonance at 28MHz. This gives extra gain at the higher frequencies, but also a better noise behavior. Not a bad compromise.
Loop current (ia) to output voltage in dB´s.
The large signal behaviour is measured only in simulation (PSPICE).
The Faraday shielding construction improves balancing. See also the picture of the amplifier:
BiasT power supply:
Remember that any noise on the power supply adds to the signal. So we need a noise free power supply.
A picture of two amplifiers. No PCB layout or SMD devices used, but the BFR93A.
Using SMD reduces the chance on oscillation (instability).
Last update: November 4, 2018