 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. 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 136kHz, 500kHz and 1.8MHz. 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. OTHER LINKS http://www.qrp.pops.net/probe1.asp |