Hi James . . .
It seems to me this is quite a large loop, and will generate a large
output:
Yes, it´s large ... at least the bigest one I ever did. And it´s a
prototype. I´m using PVC covered wire ... not soft enough to have evenly
"nice" turns spacing.
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
I think I´ll built a smaller one, just to make some "real" FSM and then
generate a table with the levels obtained with the bigger one. I have an SLM
from G&W which have a balanced 6K / unbalanced 75ohm input Z.
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 thermal noise in my tables are simply the "coil" thermal noise @ 250Hz
BW. I used the article formula´s.
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 loop turns are spaced by 5mm. I used G4FGQ´s software to estimate stray
C ...
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.
Ooopps ... this is true ... air-insulated C is winning ...
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.
I already did this way. I couldn´t resist to test it with an air-variable C
and no pre-amp so I added a 1 turn loop inside the main one, about 3 inches
between coils.
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!
I guess I´ll keep this 1 turn solution ... the local BCs - nearest is about
5Km away - are pushing Kachina´s S Meter from 9+40 to 9+70 with internal
pre-amp off.
I´ll start the 136KHz prototype and compare it´s performance using 153KHz BC
and DCF39 as signal sources.
Thanks on the comments;
73
Marcus
PY3CRX/PY2PLL
http://py.qsl.br
Active from "DC" to Laser ...
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