Dear Jim, LF,
Thanks for the calculations and explanations.
By now my loop has a unloaded Q of 230. It is still a prototype and i
expect the final Q will be 250.
I took the loop to my trip to Hamburg and tried to copy my own test
signal and the signal of DF6NM, yesterday 16 UTC, from a parking place
near Hannover (abt 360 km). Here a screenshot:
http://dl.dropbox.com/u/19882028/LF/DF6NMesDK7FC.png
The RX sensitivity of my /p RX is still not optimal but it becomes
better each day. I spend a lot of time to optimise it. It is now 10 dB
better than in this picture and will be improved even more. The RX takes
just 22 mA at 12V now. So i hope to get good copy of UK CW signals and
others too, soon!
A test is not far away. BTW we have a fieldday in our local club next
weekend. I will be RX-QRV from there and maybe TX-QRV from Heidelberg
and can steer the PC via the web, maybe....
Tnx es 73, Stefan/DK7FC
PS: I do not think that a 20 dB amplifier will work well with my 706 and
that loop. Even more would be needed i am afraid.
Am 03.07.2011 15:50, schrieb James Moritz:
Dear Stefan, LF Group,
A bit late, but better than nothing...
One more question: Does it make sense (regarding good SNR) to further
try to improve the Q or does it just make frequency adjustment
complicated? If the dynamic range of my soundcard can handle the signal
of DCF39 and DCF49 and DLF and i should have no problem, right? The
background noise should be limited by the band noise only of course. Is
there a straight dependency of gain and Q?
For a given size of loop, the signal power delivered to the optimum
load impedance is proportional to the Q. Under this condition, the
bandwidth is inversely proportional to the Q. At 137kHz, I estimate
the band noise field strength is about 1uV/m in a 300Hz bandwidth in
quiet conditions, For a 1m diameter circular loop, this will give an
induced EMF of 2.3nV. The EMF across the loop terminals at resonance
will be this value multiplied by the Q, 0.4uV. If the loop is matched
to a 50ohm receiver input (a transformer with 3:1 turns ratio would be
about right), the voltage at the RX input will be about 0.07uV. For
receivers which maintain good sensitivity at LF, this would be several
dB above the receiver noise floor, so actually no preamplifier would
be needed. But many receivers do not have good LF sensitivity, so
often a preamp will be needed. The preamp in the "bandpass loops"
article will work well here.
With the loop loaded in this way, the Q will be about 176/2, i.e. 88.
This will give significant filtering of nearby utility and broadcast
signals - e.g. 138.8kHz -4.5dB, 135.4kHz -10dB, 153kHz -25dB. This
would certainly be helpful if dynamic range was a problem.
Cheers, Jim Moritz
73 de M0BMU
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