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Very often especially in the low bands (3.5 and 7 mhz) we are bothered  by various QRN like  static rain ,local power line noise , distant thunderstorms , local switching power supply ,the light dimmer of your neighbour ... .One way to get rid of these  is by using a loop antenna which is able to nullify a signal in one direction without affecting too much the desired signal on the same frequency coming from another direction  .Various designs appeared in the open ham litterature .The choosen  design is taken from a description done by  W9SR which appeared in the June 2006  issue of QST Magazine  with sone changes about the shielded loop and the amplifier .

The antenna consist of a loop made of a standard coaxial line (50 ohm or 75 ohm impedance ) .Its shape is not very important .As constructed it is almost a square having a 1 meter diagonal.THis loop is tuned by a variable capacitor  (recovered from an old BCL ) having a capaciiy between 90 and 500 picofarads .With these value the loop can be tuned from 3 to 9 mhz .It is followed by an amplifier .The fisrst stage is a dual gate  field effect transistor (MPF 131 or similar ) followed by the well known 2N2222 .With this arrangement  the receiving sensiivity of the loop is almost equal to the one obtain from a  full size regular dipole .The total consumption is 4 mA .The amplifier is close to the loop  , its output going to the ATLAS with a piece of regular coaxial line . If the loop antenna is not far from  the transmitter antenna it is safe to protect the input of the MOSFET  by 2 diodes as shown on the schematic  .As is the amplified loop antenna can be used for 3.5 and 7 mhz  ham band

The null of this amplified loop antenna is  around 30 db below  the maximum signal .The null is very deep for signal coming from nearby  , but not as deep for signal reflected  by ionized layers
schematic of qrn killer
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