On Mar 13, 1:18 pm, Artem wrote:
.... (snipping out a lot to keep this a reasonably length...)

No transmitted. Only received.

I understand, but my point is that an antenna is reciprocal: there is
the same percentage loss in the copper resistance whether receiving or
transmitting.

....

Is the BF981 will be good enough for this?

I would think almost any modern RF mosfet would be fine. You do not
need very good noise figure for HF reception, even with an antenna
with modest efficiency. Just make sure the amplifier input has high
effective shunt resistance at the operating frequency-- greater than
200kohms--to preserve the high Q.
....
Understand. Its uneasy to find high stable capacitor at this range.
But maybe I can find vacuum capacitors or something like this.

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.

Cheers,
Tom