Jim, Alan, Scott, Daniele,
Thanks for your comments and suggestions!
I will build the preamp and try to get a higher Q. I have some WIMA
FKP-1 caps, 2 kV type in that C range and will buy a bigger copper tube
tomorrow. That makes it even more robust. I would like to have even more
area but the loop must be placed in the car...
My favorite RX is my homemade RX using the SBL-3 mixer. It seems to work
quite well now but still needs some more input level. Since it was
designed to work with the active E field antenna, it has a relatively
high input Z and so i tried to transform the 50 Ohm output of the loop
to a higher Z. Maybe i can make the preamp obsolete then, at least on
the homemade RX. The 706 will definitely need the preamp!
73, Stefan/DK7FC
Am 15.06.2011 23:39, schrieb James Moritz:
Dear Stefan, LF Group,
I did some experiments on single-turn loop Q when designing the
"bandpass loops" (BPloops2.pdf at
https://sites.google.com/site/uk500khz/members-files/files ). With a
1m x 1m square loop made of 15mm copper tubing, I got unloaded Q well
over 100 with C about 400nF at 137k. This was one unusual example of
where the capacitor dominates the loss in a tuned circuit - I got a
much lower Q using a single, lower voltage metallised polypropylene
capacitor than when using 4 x 100n, 1kV capacitors in parallel. I
assume this is because higher voltage = greater electrode area
connected in parallel = less resistance, and more capacitors = less
resistance from the connecting leads and interface to the
metallisation. I have noticed similar effects with high-current
capacitors in QRO PAs. DK7FC's Q of 8 with 470n may be due to this -
it implies Rloss of about 0.3 ohms, which seems much too high for a
few metres of copper tubing, so something is definitely wrong here.
Obviously, it is also very important to have excellent connections
between tubing and capacitors in order to realise milliohms of
resistance. Another possibility is that the loop is inductively
coupled to something that has high losses - you need to keep it a
metre or so away from other conducting objects when doing the
measurements.
I assume there are different paths the signal comes from, this is why
i cannot eliminate it completely, right?
Apart from what others have already said, the null can be degraded by
RF currents induced in nearby conductors acting as parasitic antenna
elements - this includes connecting leads, building structures, cables
etc etc. 25dB is fairly typical I think.
I used a high mu toroid (ferroxube, blue material) to transform the
primary side (=the loop) to 50 Ohm. I have done this by varying the
secondary turn number until i achieved a maximum voltage at a 50 Ohm
load at a given input signal.
Whatever the value of unloaded Q, the "maximum power transfer" theorem
applies, ie. the source resistance of the antenna at resonance should
be transformed to equal the RX input resistance to achieve the maximum
signal power delivered to the receiver - this is what you did
empirically. In this condition, the loaded Q should be half the
unloaded Q , so in Stefan's case tuning should be very flat with a
loaded Q of 4.
Is it important to terminate the loops transformer output with a 50
Ohm load in that case? On an oscilloscope i found that the level of
DCF39 is higher when having no R connected to the output but the
waveform looks much better / cleaner!
In my loop designs, I have made a trade-off by increasing the loading
(reduced loaded Q ), which increases the bandwidth, but reduces the
signal power delivered to the receiver. If you wanted to obtain
maximum loop selectivity, you could reduce loading of the loop, which
would also reduce the power delivered to the RX, so it is your
choice... Whatever you do, the band noise at the output of a loop like
this is only a fraction of a uV, so you either need a RX with a low
noise figure at 137k (very rare!) or a preamp with a low noise figure.
Actually, with the preamp in the BPloops article, simply connecting
the 50ohm input directly across the parallel tuned single turn loop is
quite a good combination (loaded Q about 15 for my loop, a little
higher for Stefan's slightly smaller loop, but only assuming he can
improve the unloaded Q to say 50 or more). The preamp has low enough
noise to easily hear the band noise with these loops, but beware -
some receivers have such bad sensitivity at 137k that further gain
will still be needed.
I am quite pleased with the way the single-turn loops have worked out
- they work just as well as multi-turn loops, but are almost
impossible to de-tune, are less susceptible to unwanted capacitive
coupling, easy to make weatherproof, are mechanically simple and robust.
Cheers, Jim Moritz
73 de M0BMU
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