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

On Mar 14, 10:06 pm, Tom Bruhns wrote:
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.

I mean that I will use you suggestion about balanced amplifier. I'll
use two FETs. I'll place one more tube from middle of the loop to the
gap. And I will use this point as ground point for balanced amplifier.
In this case I will have differential signal at FET Drains and I will
need transformer for put this signal to coaxial cable.

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.

My cable only 10m long. So I this it would be unnecessary. And I will
operate with three signals:
1. Varicaps voltages.
2,3 - voltages at secondary FETs gates.


Cheers,
Tom

On Mar 14, 2:15 pm, Artem wrote:
....

I mean that I will use you suggestion about balanced amplifier. I'll
use two FETs. I'll place one more tube from middle of the loop to the
gap. And I will use this point as ground point for balanced amplifier.
In this case I will have differential signal at FET Drains and I will
need transformer for put this signal to coaxial cable.

The transformer can be physically quite small.


Yes...very good. That should help keep the loop nicely balanced,
especially if you build it all very symmetrical. That one more tube
can also then help support the loop mechanically. Good luck with your
project!

Cheers,
Tom

On Mar 15, 12:28 am, K7ITM wrote:
On Mar 14, 2:15 pm, Artem wrote:
Yes...very good. That should help keep the loop nicely balanced,
especially if you build it all very symmetrical. That one more tube

Hi. I have some problem. I can't receive nothing except noise And self
oscillation frequency.
I have some photos. Could any give me some suggestions?
Antenna:
http://artembond.no-ip.info/apache2-...t/DSC_9427.JPG

Chematics:
http://artembond.no-ip.info/apache2-...t/DSC_9431.JPG

Amplifier:
http://artembond.no-ip.info/apache2-...t/DSC_9426.JPG

In article , Artem wrote:
On Mar 15, 12:28 am, K7ITM wrote:
On Mar 14, 2:15 pm, Artem wrote:
Yes...very good. That should help keep the loop nicely balanced,
especially if you build it all very symmetrical. That one more tube

Hi. I have some problem. I can't receive nothing except noise And self
oscillation frequency.
I have some photos. Could any give me some suggestions?
Antenna:
http://artembond.no-ip.info/apache2-...t/DSC_9427.JPG

Chematics:
http://artembond.no-ip.info/apache2-...t/DSC_9431.JPG

Amplifier:
http://artembond.no-ip.info/apache2-...t/DSC_9426.JPG

I think you need to narrow band those photos.

greg

On Mar 20, 10:07 pm, (GregS) wrote:
In article , Artem wrote:


I think you need to narrow band those photos.

Sorry for hosting.

On Thu, 20 Mar 2008 20:07:43 GMT, in sci.electronics.design
(GregS) wrote:

In article , Artem wrote:
On Mar 15, 12:28 am, K7ITM wrote:
On Mar 14, 2:15 pm, Artem wrote:
Yes...very good. That should help keep the loop nicely balanced,
especially if you build it all very symmetrical. That one more tube

Hi. I have some problem. I can't receive nothing except noise And self
oscillation frequency.
I have some photos. Could any give me some suggestions?
Antenna:
http://artembond.no-ip.info/apache2-...t/DSC_9427.JPG

Chematics:
http://artembond.no-ip.info/apache2-...t/DSC_9431.JPG

Amplifier:
http://artembond.no-ip.info/apache2-...t/DSC_9426.JPG

I think you need to narrow band those photos.

greg

Yes
www.irfanview.com has an excellent bit of software that does just that


martin

On Thu, 20 Mar 2008 12:58:02 -0700 (PDT), Artem
wrote:

Hi. I have some problem. I can't receive nothing except noise And self
oscillation frequency.

Try choking both what are labeled "RF Out" and "Shield Cable."

73's
Richard Clark, KB7QHC

On Mar 21, 1:36 am, Richard Clark wrote:
On Thu, 20 Mar 2008 12:58:02 -0700 (PDT), Artem
wrote:

Hi. I have some problem. I can't receive nothing except noise And self
oscillation frequency.

Try choking both what are labeled "RF Out" and "Shield Cable."

Thank. It's good idea. I'll try.

On Mar 20, 12:58 pm, Artem wrote:
On Mar 15, 12:28 am, K7ITM wrote: On Mar 14, 2:15 pm, Artem wrote:

Yes...very good. That should help keep the loop nicely balanced,

especially if you build it all very symmetrical. That one more tube

Hi. I have some problem. I can't receive nothing except noise And self
oscillation frequency.
I have some photos. Could any give me some suggestions?
Antenna:http://artembond.no-ip.info/apache2-...t/DSC_9427.JPG

Chematics:http://artembond.no-ip.info/apache2-...t/DSC_9431.JPG

Amplifier:http://artembond.no-ip.info/apache2-...t/DSC_9426.JPG

:-) 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.

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? 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. 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.

If other things occur to me, I will post them...

Cheers,
Tom