Hi Stefan a few points from a quick read your message .........
1st The null depth......I think there could be two reasons why it isnt
deeper. One is that the vertical dimenion of the loop could act an an efield
aerial and inject e-field induced voltage if the feed is not well balanced.
Yours looks balanced, but for the best the transformer should have a bifilar
primary (Hi-z side) with the centre tap grounded this should remove any
e-field effect I think..
2nd depending on how far you are from Burg you may be getting some skywave.
The vertical loop will null signal appearing at 0degrees but not signal
maybe coming down at 40 -50degrees from the D-layer. ??
I think the WIMA caps you use are polypropylene and should be
quite good at that frequency. The easiest way is to measure Q is to plot the
3dB
bandwidth with a loosley coupled signal. You dont say whether you are
series or parallel tuning the loop. Since you are seeing a higher impedance
than 50 ohms I assume parallel. The inductance of the unloaded 50 ohm
winding should have a reactance of about 250ohms (because you are using a
very narrow band and dont want extended HF response) I presume you have then
wound turns on the loop side to give the biggest output at the 50ohm side of
the transformer. And yes you will get twice the signal voltage if it is
unmatched.
A small loop will collect a small signal but often a better S/N, but may
need some amplification. because the aerial noise does not exceed the
receiver noise. It you were to go to a bigger loop (2m diameter or so) you
may well find the aerial noise then exceeds the receiver noise, and you can
avoid the amplifier.
Alan
G3NYK
----- Original Message -----
From: "Stefan Schäfer" <[email protected]>
To: <[email protected]>
Sent: Wednesday, June 15, 2011 1:40 PM
Subject: 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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