Preamplifier for beverage antenna

Beverage antenna as a special receiving antenna, it has the disadvantage of lower gain and thus, subjectively, signals weaken when switching from a classic antenna. This phenomenon can be very disturbing and is therefore appropriate supplement the beverage antenna with preamplifier.

The inspiration for the design was the material of Milan OK1VWK http://www.sweb.cz/ok1vwk. Using an MMIC amplifier is a very convenient solution, while we have the option of choosing a circuit according to our own requirements. More than gain, the ability to process strong signals and a low noise figure is essential.

We calculate the value of the resistor in the MMIC power supply according to the formula
R=(Unap – U) / I
R – resistance value in ohms
Unap - supply voltage
U – MMIC supply voltage
I – recommended MMIC current

Since MMICs are very wideband, it is necessary to include a bandpass at the input. Unlike OK1VWK, I reached for a verified connection (I had the entire PCB). These filters have very good properties and can be set reliably. In my design the coils are wound on double hole cores, C' is about 470pF and C12 is 56pF. Tuning using the GDO is simple, we set each LC circuit separately to the upper extreme frequency and set the bandwidth with the coupling capacitor. You can find details and how to calculate the circuit components of the filter at CQ.sk. During the measurement, a bandwidth of 3.6 to 3.98MHz was found for a drop of -3dB.

Such a preamplifier is suitable for an antenna with an impedance of 50 ohms. Since the impedance of the beverage antenna is much higher, it must be connected to the input transformer 9:1. I wound it as recommended W8JI for two-hole cores Amidon BN73-202. I measured out of curiosity PSV such a transformer. With a load of 450 ohms, there was no noticeable reflection, PSV 1:1. When loaded with 470 ohms, PSV was 1:1.1. The power used was 4W and PSV meter MFJ-864.

The preamplifier and input filter were built into the shielding box, the transformer remained unshielded as recommended. There is a supply voltage indicator with an LED diode on the box.

The entire connection is simple, but very laborious for setting and mechanical work. I was able to secure Amidon double-hole cores thanks to the help of Šaňa OM3TY, who recommended the company RLX. Thx!