preselector and antenna tuner for wideband SDR

One of the disadvantages of cheap wideband SDR-tuners are their poor input signal filtering: Strong signals can saturate the receiver input stage. Also, weak signals get lost in intermodulation products and reduced sensitivity. To prevent this and give the receiver an appropriate antenna tuning, we can use the simple invention of old radio experts: the "Z-MATCH TUNER".

With two rotary capacitors and a coil you can tinker a "hand tuned antenna preselector".

L1 is wound on a 18mm plastic tube with 10 turns, a 0,5mm copper wire and a lengh of 19mm. On the 5th turn he has a tap for C1. On the 1th turn L1 is groundet. L2 has 2 windings over L1 and has the groundet tab in the middle.

I made my tuner "prototype" from relics in my hobby room. On the WEB you can find better construction proposals.

Important to remember is that C2 and L1 is a parallel resonant cirquit for the respective frequency. C1 "tunes" the impedance right for the antenna input of the receiver.

Left side input - right side output (antenna).

In this graph you can see the "preselector"-characteristics of the tuner and the bad "Q" of the coil on higher frequencies (it means poor coil quality).

My next "Z-MATCH" will be a better one... :-)