HF antenna preselector is an electronic device connected between an HF radio antenna, and a radio receiver. Its primary function is to act as a tunable band-pass filter, selectively allowing desired frequencies to pass through, while significantly attenuating unwanted out-of-band signals. This means it only passes the desired signal and blocks others. The device presented in this project is basically intended for SW listeners who most often use a so-called Long Wire antenna. This type of antenna generally receives all signals in the HF range equally, which leads to many unwanted effects, such as Interference from strong local stations, Image responses and Intermodulation distortion. 

  The antenna tuner is especially useful in simple DIY radio receivers where some important elements in the input part of the receiver are omitted for the sake of simplicity. In this case, using such a device drastically increases the quality of reception. Otherwise, I took the project from the sv3ora (http://qrp.gr/) website where you can find many amateur radio projects and all the credit goes to the author.

   The device is very simple to make and consists of several components:

    - Transformer with rectifier that provides DC voltage with a value of 10 to 12V (you can also use standard 9V Battery)

     - j310 mosfet

     - bb112 varicap diode

     - 10Kilohm potentiometer

     - and several passive elements, resistors, capacitors and inductors

  As seen, circuit is simple and cheap to build and you do not need any inductors winding skills, as it uses molded chokes. We need to place the molded chokes in relative positions that are perpendicular to each other, or far away from each other, so as to avoid unwanted coupling between them. 

 The tuner has only 2 ranges that cover the entire HF band. The potentiometer changes the voltage applied to the varicap diode and thus its capacitance, and in conjunction with the appropriate group of inductances we obtain the required range. At the input there is another potentiometer with which the RF gain is regulated. The inductor with a value of 100 Nanohenry is hard to find, so I made it with 4 turns of insulated wire on a body with a diameter of 7mm. 

The PCB is built into a suitable box that is completely covered with a metal cover connected to the ground to avoid unwanted external influences. 

 The tuner has only 2 ranges that cover the entire HF band. The potentiometer changes the voltage applied to the varicap diode and thus its capacitance, and in conjunction with the appropriate group of inductances we obtain the required range. At the input there is another potentiometer with which the RF gain is regulated. The inductor with a value of 100 Nanohenry is hard to find, so I made it with 4 turns of insulated wire on a body with a diameter of 7mm. 

The PCB is built into a suitable box that is completely covered with a metal cover connected to the ground to avoid unwanted external influences. 

  I connected the long wire antenna to the input of the preselector, and I connected the output of the preselector to the SDRPlay receiver.

 As I mentioned, I will start by using the SDRUno software because it is the best way to visually understand the function of this preselector. Now I will move the Frequency knob until I get the strongest and clearest reception. It is immediately clear that the difference before the adjustment and now is huge. I try this on other bands too. After a short practice, we can almost instantly set the Frequency potentiometer to the ideal position in which we will get the best reception with the lowest noise level. Unless we have a particularly strong shortwave station nearby, the Gain potentiometer should always be in the position for the strongest reception.

  In conclusion, this simple and inexpensive HF antenna preselector significantly enhances reception quality, especially for SW listeners using Long Wire antennas and basic DIY radio receivers. By effectively filtering out unwanted signals, it allows for a much clearer and stronger listening experience across the entire HF band.