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 Milana OK1VWK material, which hopes for 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=(One p – U) / I
R – resistance value in ohms
Unap - supply voltage
U – MMIC supply voltage
I – recommended MMIC current

Predzosilňovač pre beverage anténu
Since MMICs are considerably 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 two-hole cores, C' has about 470pF and C12 56pF. Tuning with GDO is easy, 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 Bandwidth was determined during the measurement 3,6 up to 3.98MHz for -3dB drop.

Zaťažovací rezistor pre beverage anténu Such a preamplifier is suitable for an antenna with impedance 50 ohms. Since the impedance of the beverage antenna is much higher, it is necessary to connect a transformer to the input 9:1. I wound it according to the recommendation W8JI for two-hole cores Amidon BN73-202. Out of curiosity, I measured the PSV of such a transformer. Under load 450 ohms, there was no noticeable reflection, PSV 1:1. Under load 470 PSV was in ohms 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 up and mechanical work. Thanks to the help of Šaňa OM3TY, I managed to secure the Amidon two-hole cores, who recommended the company RLX. TNX!

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