Hi Stefan,
ok,then you've measured the *loaded* antenna Q because the 50 Ohm source
of your VFO is included in the circuit.
The Q of a LCR circuit is defined Q=f0/B.
That formula was derived with a series LCR circuit fed by a *voltage source*.
So when talking about antenna Q usually the *unloaded* Q is referred to.
But I believe the main problem of our measurement is the scope.
The scope is grounded.
Therefore you upset the balance of the loop when you connect the probe to
the antenna-C.
Then the loop acts no longer as a pure magnetic loop but (at least partially)
as an E-field probe.
The safest way to get reasonable Q measurement results is to make two
small coupling coils made of a few turns of solid wire with a diameter
of a few centimeters (not critical).
Then place these the before and behind the loop.
You need scope and a signal source *with enough power* or
alternatively a spectrum analyzer (which is much more sensitive)
with tracking generator.
Then look for the 3dB points..
Place the coupling coils as far as possible apart to ensure the most
loose possible coupling to the loop.
73
Clemens
DL4RAJ
----- Original Message -----
From: "Stefan Schäfer" <[email protected]>
To: <[email protected]>
Sent: Thursday, June 16, 2011 11:42 PM
Subject: Re: LF: Re: LF RX loop
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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