On Mar 21, 3:08 am, Artem wrote:
On Mar 21, 2:56 am, K7ITM wrote:

On Mar 20, 12:58 pm, Artem wrote:

:-) I saw the comment about "narrow-banding" the images. They were
perhaps a little more than we needed, but it was nice to have
something we could actually see. They did not take very long to
download here, but someone with a slow connection may have troubles.

I'm move images to ImageShackhttp://img524.imageshack.us/my.php?image=dsc9427ib7.jpghttp://img262.imageshack.us/my.php?image=dsc9431tv9.jpg



One comment: usually you do not need much voltage gain. It is enough
to get power gain with the FETs. That is, the received signal voltage
across the gap of the loop, as resonated by the capacitors, should be
high enough to be used with a good receiver. The problem is that the
impedance is very high there. But that same high impedance makes for
easy oscillation. From what you posted, it sounds like maybe you have
identified an oscillation. If the AGC voltage is low enough, does the
oscillation stop?

Yes. It's stop. my main reason for this antenna is make very narrow
antenna for city. I can't receive to my receiver almost nothing by big
nose from computers, lamps and other things. But looks like this
antenna did not help. Its amplify narrow-band noise. And even add more
noise when oscillated.

The amplifier I built used two stages, an FET input

stage and a buffer stage, and it had very low voltage gain--I am
remembering about 3:1 or only 10dB, and maybe only 1:1 or 0dB
including the output transformer, but quite a bit of power gain since
it transformed the high loop impedance down to 50 ohms for the
feedline.

Also, there should be no need for the RF chokes from the gate-1 to
source, if the loop is grounded at the bottom.

Source is not grounded for DC. For better transistors matching and
overcurrent protection.

If the loop is grounded at the bottom, the loop plus the RF chokes will short out the
source-to-ground resistor. Maybe there is not a need to raise the
source voltage above DC ground potential anyway. Also, it may help to
NOT bypass the sources to ground, to allow some negative feedback.
That may help stabilize the amplifier.

Sources is not grounded.

Yes, unfortunately noise that is generated more than one or two
wavelengths away from your antenna will be almost entirely
electromagnetic by the time it reaches your antenna. Antennas do not
differentiate between "electrically generated" and "magnetically
generated" noise, when you are far enough that the electromagnetic
field dominates over any near-field electric or magnetic field. The
balanced small loop is good for rejecting electric-field noise only if
(1) the noise is generated close to the antenna and (2) the antenna is
close to the ground (so the electric field is guaranteed to be nearly
vertical) -- -- where "close" means relative to a wavelength. So the
small balanced loop is especially good for LF and VLF work.

Perhaps someone else will have suggestions about what else you might
try.

Cheers,
Tom