Hi Stefan,
based on 470nF and 137kHz the inductance is only 0.47uH (what means XL = 0.4
Ohm). As Q = XL/R for a Q = 8 the loss resistance is 0.05 Ohm.
This seems acceptable to me. Due to the very low XL it will be hard to get a
high Q (eg. for Q = 100 you would need to reduce the losses to 0.004 Ohm.
The low DCF39 signal is probabaly caused by the low Q.
I think the best way to get a better Q is to use a multi turn loop.
About nulling the loop: multipath can be the cause of poor nulling, but I have
experienced that the main reason is that the loop also acts a bit as a
electrical antenna, with a omnidirectional pattern ruins the null's. Best way
to avoid this is electrical shielding of the loop.
73, Rik ON7YD - OR7T
________________________________________
Van: [email protected] [[email protected]]
namens Stefan Schäfer [[email protected]]
Verzonden: woensdag 15 juni 2011 14:40
Aan: [email protected]
Onderwerp: LF: LF RX loop
LF,
Some questions about RX loops for LF.
Yesterday i found a shortcut between my loop ends. Thus i haven't found
a resonance when varying the C. It is a single turn circular loop using
copper tube with 10mm diameter (about 1m). I calculated the L with a
formula of ON7YDs LF info pages and found a C of about 470 nF which
agreed very well. It was easily possible to resonate the loop with an
induced signal by a smaller loop connected to my VFO and measuring the C
volatage with an oscilloscope. I found the Q of the loop is just 8! Is
this normal? Which Q do others achieve, with a similar antenna for 137 kHz?
I'm using PP capacitors which should be well suitable (WIMA MKP10)
http://www.wima.de/EN/WIMA_MKP_10.pdf
It is fascinating to null out s signal, which i have done the first time
now. I can reduce the signal of DCF39 by 25 dB by turning the loop. I
assume there are different paths the signal comes from, this is why i
cannot eliminate it completely, right?
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. The secondary number is now about 25
turns. The resonance frequency is not affected by the load at the output
of the transformer.
Now DCF39 appears at up to 45 dB above noise in 3 Hz which. But it
should be at least 20 dB higher i think. Furthermore the background
noise does not change at all when i connect or disconnect the antenna.
Thus i think it makes sense to build a preamp. I found a suitable
circuit on the pdf of Jim/M0BMUs loop article which looks well for me.
Looking forward to this amp performance!
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!
I'm optimistic to get much proper reception of UK and PA and HB9 CW
signals on 137 soon, when beeing outside the city, steering the LF TX by
the 70cm link :-)
Vy 73, Stefan/DK7FC
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