On Mar 13, 12:09 pm, Artem wrote:
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 effecthttp://circuitcalculator.com/wordpress/2007/06/18/skin-effect-calcula...
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.

See http://personal.ee.surrey.ac.uk/Pers...les/cloops.htm
for info on calculating the radiation resistance. I believe that will
be helpful. I assume Reg's RJELoop1.exe uses essentially the formula
you'll find there. For your loop, that program says inductance is
2.69uH, conductor loss resistance is 46.2 milliohms, and radiation
resistance is 5.7 milliohms. Although the conductor loss resistance
(essentialy the same as you calculated above is "not bad," you need to
consider it with relation to the very low radiation resistance. A
current in the loop will dissipate far more power in the resistive
loss than in the radiation resistance. You ideally will keep the loss
resistance small compared with the radiation resistance, though for
receiving (because of the very high level of atmospheric noise on HF
and lower frequencies) it matters not nearly so much as for
transmitting. When transmitting, you want your power to go into radio
waves, not heat. When receiving you only need signal greater than
noise, and it is relatively easy to make an amplifier with low enough
noise figure that even an inefficient antenna will result in an
amplifier output whose noise is dominated by the atmospheric noise
received by the antenna.



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 kindhttp://www.toshiba.com/taec/components2/Datasheet_Sync//273/1343.pdf
20 in parallel
--------------------------------------
Ultra low series resistance: rs = 0.20 Ù (typ.)
--------------------------------------
It will be 0.02 Om

So 0.02 ohms sounds like a small amount, but it's almost half as much
as the resistance of the copper loop. This may not be a bad thing,
because the Q is so high that the bandwidth will only be about 3.5kHz
assuming a lossless capacitor, and with the added loss the Q will be
lowered to perhaps 1400, allowing a slightly wider bandwidth. With so
narrow a bandwidth you need to be concerned about the stability of the
varicap diodes' capacitance. Still, I would think a very high Q fixed
capacitor supplying most of the total capacitance would be a good
thing. Use only enough varicap to cover the tuning range you want.
So for example, with 2.69uH inductance, if you want to cover 7.00MHz
to 7.30MHz, you need 192.2pF at the low end and 176.7pF at the high
end, a range of a little less than 16pF. You should be able to do
that easily with two of your suggested varicap diodes, perhaps a
couple of fixed 82pF high Q caps, and a high Q trimmer such as a
piston trimmer to trim the center of the range.

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.

It has more to do with the symmetry of the way the antenna is
mounted. You want to make sure that the capacitance to ground from
each side is as nearly the same as possible. You need to put the gap
in the loop (the feedpoint) either at the top or at the bottom of the
antenna, and for mounting it's often easier to put it at the top.
That way you can clamp onto the middle of the bottom of the loop to
mount to a pole... But if you have a balanced amplifier at the
bottom and bridge the gap symmetrically across the box that amplifier
is mounted in, it should also work well. I recommend to you the
discussion about small loop antennas in King, Mimno and Wing's
"Transmission Lines, Antennas and Waveguides."

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.

Cheers,
Tom