Dear Marcus, LF Group,
It seems to me this is quite a large loop, and will generate a large output:
I think across an untuned loop V = 2.1e-8.f.N.A.E
where N = turns, f is frequency in Hz, A is area in m^2, E is f/s in V/m
For your loop, and a f/s of 1uV/m, this gives 0.21uV @ 400k, 0.85uV @ 1600k.,
With a Q of 200, the voltage at resonance will be increased to 42uV @ 400k,
170uV @ 1600k
The loop noise depends on the RX bandwidth, but in 10kHz BW it will be
roughly 6uV @ 400k, 12uV @ 1600k. So for 1uV/m, SNR would be 17dB @ 400k,
23dB @ 1600k. The JFET noise will be negligible. I think most of the time
the atmospheric noise level in 10k BW at MF will be well above 1uV/m, so it
should be more than sensitive enough. Reducing the Q to 100 due to
resistors or varicap will make the output voltage decrease by 6dB and loop
noise decrease by 3dB - still probably OK. Reducing the RX bandwidth will
improve the SNR, of course.
The tuning is a problem - 4:1 tuning range needs 16:1 capacitance
variation, including the stray capacitance of the loop. If L is about 446uH
from the table, this means 350p - 22p. The stray capacitance of the loop
may be greater than 22p, so you may not get the full tuning range. Using a
varicap diode, it is unlikely the minimum- to maximum capacitance ratio
will be high enough. it may be necessary to reduce the number of turns and
use a larger tuning capacitor.
The other problem is that local stations will give very large signals - eg.
10mV/m from an 1MHz MF broadcast station will result in about 1V across the
loop at resonance, which will probably lead to severe intermods from both
the preamp and the varicap. You could connect the preamp across only 1 turn
instead of all 9 - this would still give several uV out for 1uV/m f/s, more
than enough for a decent receiver. In fact, since the loop output is so
large, you could probably eliminate the preamp by using a single turn
coupling loop to a 50ohm RX input. The air-variable tuning capacitor would
also be a good idea for this reason, unless you live a very long way from
any broadcast stations!
Hope this is helpful,
Cheers, Jim Moritz
73 de M0BMU
At 13:35 20/01/2003 -0300, you wrote:
Hi ...
After read an article last year (Wireless World) about loop antennas X DCF39
calibration, I constructed a octogonal RX loop, #16 awg wire, to cover from
400KHz to 1.6MHz.
There´s 9 turns, 5mm spacing, 1.9 meter diameter. About 55 meter of wire.
This is the first version. Later I´ll build another one to 136KHz.
Acording with this article, I should expect a Q=200 ... 220 from such loop.
I considered the NTE618 to tune it and a push-pull pre-amp (J310) with gates
conected to the loop ends. No resistors there. The article consider that for
such loop at this frequency will have something like .20uV of thermal noise.
The resistors will add some extra noise there so I removed them.
But the Q=210 ... and the NTE618 Q is 200.
Is there any gain If I use a dual-gang air variable driven by a step motor +
reduction in place of the NTE618?
|
