On Mar 14, 9:20 am, Artem wrote:
On Mar 13, 11:38 pm, K7ITM wrote:



It should not be that bad. C0G dielectric (also called NPO)
capacitors have a _maximum_ temperature coefficient of 30ppm/C. The
diameter of the loop itself, and therefore its inductance, will change
with temperature in the same range, I believe. C0G capacitors,
especially surface mount type, also have extremely low effective
series resistance. I've found some C0G SMT caps that seem to have
very close to zero temperature coefficient--it varies from lot to lot,
apparently depending on the exact mix of the dielectric. What do you
suppose the temperature coefficient of the capacitance of varactor
diodes is? Note: 100ppm change in capacitance causes 50ppm change in
resonant frequency. That's 350Hz at 7MHz. You probably wouldn't even
notice that. It's only about 10 percent of the 3dB bandwidth of the
antenna.

Thank you very much. About about transformer (for connect coaxial
cable). I can use any ferrite with small permeabilty?

Yes, that should be fine. I suppose a transformer will be a
convenient way to better match the FET amplifier output to a
transmission line. The transformer can be physically quite small.
MiniCircuits and some others (CoilCraft; M/A-Com; ...) sell
appropriate transformers, and of course they are easy to make if you
have an appropriate core. Also you can feed power to the amplifier
through the transmission line. The transformer secondary can return
to a bypass capacitor instead of directly to ground, and your DC feed
appears across that capacitor. The circuit I used for 150kHz loops
used a shunt voltage regulator in the amplifier, and by feeding the
other end with a controlled variable current, I could avoid problems
with uncertain voltage drop in the line plus connectors, and also use
the current through the regulator to control the tuning voltage on the
varicap diodes. So everything was done through the transmission line.

Cheers,
Tom