I can recommend also the very interesting site of LZ1AQ.
A very recommendable design is from Maarten Hagg.
Practical implementation of a rotating broadband magnetic Alford loop
This transimpedance 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 behaviour.
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 neighbourhood.
The transformer together with the Faraday shield take care of the balancing, the common mode suppression (>40dB). 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.
The gain (antenna factor) is a little bit frequency depended. Because of the transformers coupling factor (k¹1) we see the loop resonance at 28MHz. This gives extra gain at the higher frequencies, but also a better noise behaviour. Not a bad compromise.
The noise contribution of the amplifier is about 6 to 10dB below the ITU´s residential and rural noise levels.
Loop current (ia) to output voltage in dB´s.
The large signal behaviour is measured only in simulation (PSPICE).
See also the picture of the amplifier.
Remember that any noise on the power supply adds to the signal. So we need a noise free power supply.
No PCB or SMD devices used.
Using SMD makes the chance on oscillation (instability) smaller.
The necessary height depends on the location. For optimum result (highest signal to noise ratio) the best advice is to experiment with it.
Last update: July 13, 2012