On Mar 13, 7:40 pm, Tom Bruhns wrote:
On Mar 13, 9:35 am, Artem wrote:

Hello all.
I'm looking how to make narrow band active antenna for 7 or 10.1mzh.
My idea: I will use magnetic antenna with one loop. A one-turn loop of
3.14m cupper pipe with diameter 15mm has 2.687uH. With Varicaps of
192pf it will have resonance frequency of 7MHz. I will load this LC
tank directly to Gate one of dual gate MOSFET.http://homepage.eircom.net/%257Eei9g...//homepage.eir...
Should it work? I'll never seen such schematics. Usually people use
transformers. But I will place this transistor directly inside gap in
the loop of cupper pipe. The Gate of MOSFET will add only additional
2pf to Varicaps and it will be very easy to compensate.

Sorry for English.

You should probably download Reg Edwards' RJELoop3 and/or RJELoop1
programs, which will tell you that the conductor loss resistance of
your loop will be about ten times the radiation resistance,

How can calculate radiation resistance? resistance at 7MHz is
skin effect
http://circuitcalculator.com/wordpre...ct-calculator/
0.028mm.
for copper pipe 15mm in diameter:
octave:1 15*3.14*0.028
ans = 1.3188mm^2

0.0155Om/m * 3.14m
ans = 0.048670 Om.
I think it's not bad.

even counting the relatively low Q of varactor diodes, you'll have
quite a bit of loss. On the other hand, even at 7MHz the atmospheric
noise is so high that the loss won't be a significant problem so long
as your amplifier is reasonably low noise. I'd recommend you use a
C0G ceramic or possibly silvered-mica capacitor for most of the tuning
capacitance, to keep the Q as high as possible (the losses as low as
possible).

You recommended did hot use varistors? I'm thinking about
some kind
http://www.toshiba.com/taec/componen...//273/1343.pdf
20 in parallel
--------------------------------------
Ultra low series resistance: rs = 0.20 Ù (typ.)
--------------------------------------
It will be 0.02 Om

Reg suggests the Q for use at 7MHz will be around 2000, so
the bandwidth is quite narrow. If the Q really is that high (polish
the copper and coat it with a protective varnish or paint...), the
parallel resonant impedance will be up around 200k ohms, so it should
be a decent match to your FET amplifier. Make sure any loading at the
gap is well above 200k ohms resistive, to keep from introducing
significant additional loss.

I'm thinking about soldering a box from FR4.

If you want the antenna to also do a
good job rejecting locally generated E field noise, you need to keep
things well balanced.

What you mean about balanced? Differential output? I should think
about it.

But at 7MHz, this is probably of marginal
utility since any noise generators whose noise would be rejected would
have to be very close to the antenna--within a few tens of meters.
-- I've done similar amplifiers for multi-turn loops for LF, down
around 150kHz, using a balanced JFET design directly across the loop,
with good success.

You can find the suggested programs athttp://www.zerobeat.net/G4FGQ/.
And you likely would get some additional replies if you cross-post to
rec.radio.amateur.antenna.

Thank.

Artem.