Adapting the tuning range
of the pa0nhc magnetic active receiving antenna 20161221

In the prototype, the tuning range is between 3.48MHz and 3.82MHz.

The tuning pot meter R15 must be a log. type of good quality. Use a large tuning knob for easy scale calibration.

Two 3W zener diodes (D105 and D106) limit the supply voltage for Vr2 to abt. 30V, thereby reducing the dissipation of Vr2.

            The antenna circuit.
        With L1 = 20 uH :
- it needs 103 pF capacitance for resonance at 3.5 MHz.
- it needs   88 pF capacitance for resonance at 3.8 MHz.
The needed capacitance variation is therefore 15 pF.

The tuning voltage in this design is ((14 V to 19 V) - 13 V) = 1V to 6V. 
The capacitances of both varicap diodes in the BB814 then vary between 58 pF and 25 pF. 
As both diodes are connected in (anti-) series, the resulting capacitance variation is between 29 pF and 12.5 pF, that is 16.5 pF variation.

The stray capacitance of L1 + PCB is estimated to 13 pF.
(C11 / C12 / C12a) should therefore resemble (88 pF - 13 pF) = 75 pF.
With 50 pF trimmer C11 adjusted halfway, C12 / C12a should resemble (75 pF - 25 pF) = 50 pF.
Chosen standard values for C12 and C12a are therefore 100 pF NP0 / C0G.

            Adapting the tuning range.
The maximal allowed tuning voltage for a BB814 is 18V. 
To avoid IMD generation, the minimum useable tuning voltage is 1V.

The maximal allowed output voltage of VR2 is therefore (13 + 18) = 31 Vdc.
The  minimal allowed output voltage of VR2 is therefore (13 +   1 ) = 14 Vdc.

The maximal output (or tuning) voltage could be changed by changing the values of R15, R15a and R15b.
The minimal output (or tuning) voltage could be changed by changing the value of R14a (1 kOhm change here = 1Vdc change).. 

Trimmer C11 determines the lowest tuning frequency.

With the here used ferrite rod (Ui=125), tuning to another frequency band (between 100 kHz and 21MHz) is possible by :
- changing the self inductance and number of turns of L1
- changing the values of C12 and C12a.

Trimmer R15b determines the highest tunable frequency, and therefore the tuning "span". 

Further widening the tuning range could be done by :
- soldering two BB814s on top of each other ( in parallel)
- a wider tuning voltage range.
- a larger value of R15, 15a, 15b.