Hello Clemens,
I measured the Q by coupling to a smaller loop that was connected to my
DDS VFO. First i found the resonance frequency, then i in- and decreased
the frequency until û was 0.707x u max, the standard way. I measured the
voltage across the C.
Soon i will have a higher Q. Looking forward to that.
Tnx es 73, Stefan/DK7FC
Am 16.06.2011 23:15, schrieb Clemens Paul:
Stefan,
just to be sure how you have measured Q.
If you are feeding the loop with a 50 Ohm source and want
to measure the unloaded Q of the loop as per the formula
Q = f0/B you would have to look where the SWR rises to 2.618
(not 5.8).
This translates to a voltage drop at the terminals from e.g. 224mV to
200mV
(-0.969dB) for a 0dBm power source (i.e. power reduction is 20%, not
50%).
73
Clemens
DL4RAJ
.
----- Original Message ----- From: "Stefan Schäfer"
<[email protected]>
To: <[email protected]>
Sent: Wednesday, June 15, 2011 2:40 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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