Active Antenna for 8.97kHz, 136kHz and 500kHz RX
Recently I have been trying out a small active voltage probe antenna to aid reception on the LF bands. I used a PA0RDT designed active antenna (but with MPF102 and 2N3904 devices and a 9V supply from a PP3 battery) to listen successfully to amateur signals on both 500kHz and 136kHz. The probe is also suitable for listening on the 33kms "Dreamers Band" (8.97kHz).
It is recommended that the active antenna should be mounted well in the clear and away from the house feeding the power up the coax). This is to avoid noise pick-up from mains wiring, switched mode PSUs and TV line timebases. However, I just stuck mine onto the inside of my double glazed window - it still worked and heard stations on both LF bands. Remember, this is the entire antenna - no antenna wire is needed down the garden. Bear in mind this is a receive only antenna and it is not magic and does NOT work on transmit (more is the pity!).
It works because the FET has a very high input impedance. The output of the FET is converted to a low impedance using an emitter follower. Although the signal level may be lower than a large outside antenna so is the noise and overall the S/N may actually be better if the antenna is located carefully. Overall, if you are unable to erect a decent LF antenna then this may be a solution which will allow you to hear signals on 8.97kHz, 136kHz, 500kHz and 1.8MHz. For VLF reception there may be some advantage in adding a low pass filter between the FET and the transistor to reduce the levels of LF and MF broadcast signals that might otherwise overload the unit. I've not tried this.
As an experiment I connected the active antenna directly into the FT817 and took a listen on 160m when the antenna was completely within the house and surrounded by noise sources all around. To my delight the WSPR software was decoding plenty of stations and not many dB down on the outside wire antenna.
Roelof PA0RDT has tried the active probe antenna at 8.87kHz and says it works well with his Perseus receiver. Gain at this frequency may be increased (along with the noise though) by increasing the capacitors to 1uF and increasing the power supply feed choke to 8.2mH. This is the schematic from PA0RFT's site. Power for the unit is supplied along the feed cable. It is advisable to separate the ground from the house ground and use a suitable isolation transformer feed. More details in the link above.