Paweł Pawłowicz <p...@w...up.wroc [kropka] pl> wrote:

> Ciągle w kółko to samo. Wsadź opornik za L2 i za nim kondensator do
> masy.

Ok, zapomniałem napisać -- próbowałem. Nie pomogło. Z rezystorem, ale bez
kondensatora, oscylacje pojawiły się na samym początku i wygasły, ale mam
wrażenie, że sam whip wtedy też przestał działać. Z dodatkowym
elektrolitem oscylacje wróciły.

Poruszyłem temat też na liście rsgb-lf-group i dostałem właśnie taką
odpowiedź, będę chciał spróbować:

--------

Dear Adam, LF Group,

Looking at the PA0RDT antenna circuit, and your amplifier/DC bias circuit,
I guess the problem may be occuring due to resonance of the DC biassing
components of the PA0RDT amplifier - a resonant circuit formed by the
470uH inductors, the output capacitor C4 and the decoupling capacitors C1,
C3  (the component references on your PA0RDT pcb) will be driven by the
output of the amplifier, resulting in considerable voltage gain at the
resonant frequency to the +ve supply rail of the amplifier. This resonance
would normally be damped by the low resistance of the RX input impedance,
but the op-amp circuit has a relatively high impedance. The resonant
voltage could then be coupled to the input of the amplifier via the FET
gate bias resistors and parasitic capacitances. Not sure what capacitor
and inductor values you have used, but using the 100n and 470u values that
are shown on various schematics on the Internet, the resonant frequency
does come out around 30kHz.

As to how to fix it, I would suggest:

- Increasing the decoupling capacitance in the PA0RDT amplifier; connect
say 10uF in parallel with C1/C3 on your PA0RDT pcb

- Terminate the coax at the receiver end, e.g. connect 50ohms in series
with 1uF between "RF" and "GND" on your amplifier schematic

- Connect a small resistor, e.g. 10ohms in series with C4 on your PA0RDT
pcb - having this in series with the amplifier output will also damp the
resonance - it would reduce the possibility of instability at higher
frequencies too.

Hope this is helpful - let us know what happens!

--
https://www.youtube.com/watch?v=9lSzL1DqQn0