|
Clemens,
I believe what Jim was suggesting is a
comparison of preamp noise output with the antenna connected versus a
passive load, rather than between different passive load impedances. The
intention is to demonstrate that the receiver noise contribution is neglegible
against external noise, which is on the order of
+140 dB kTo minus antenna losses. Thus a very good preamp noise figure
is beneficial only with very short nonresonant antennas,
or tiny loops.
The challenge for a VLF frontend would
mainly lie in decoupling the antenna as much as possible from local
interference sources. One difficulty is that meaningful comparisons
can only be done at times of low external noise. Currently there seems
to a short window with low QRN during most mornings, but this is
probably the worst season of the year.
Best 73,
Markus (DF6NM)
Sent: Sunday, June 06, 2010 8:48 AM
Subject: LF: Re: Re: 9 kcs
Jim, >Since the RX or preamp noise level can be > strongly
dependent on the source impedance, I can not confirm this with my RX,a
Perseus SDR. When I switch on 9kHz or any other qrg between 50 Ohm, open and
short, I always get the same reading within a tenth of dB,e.g -118,4dBm RMS
at a BW of 732Hz (arbitrarily chosen) The reading also stays constant
between the three sourcees when I do the same test with preamp ot attenuator
on. Maybe your observation is true for RX/preamp inputs with considerable
reactance in the input impedance. 73 Clemens DL4RAJ -----
Original Message ----- From: "James Moritz" < [email protected]> To:
< [email protected]> Sent:
Sunday, June 06, 2010 12:29 AM Subject: LF: Re: 9 kcs ... > A better test of receiver performance at 9kHz is to compare the noise
level > with and without the antenna. Since the RX or preamp noise level
can be > strongly dependent on the source impedance, it is best to replace
the actual > antenna with a "dummy antenna" with similar impedance when
making this > comparison. For example, for my loop antenna, I have a
change-over switch > that connects the preamp input to a small choke with
about the same > inductance and resistance as the actual antenna. This
enables quick > comparisons to be made between noise levels at different
times and > locations, and is also a good check on the local QRM level.
Obviously, you > want the antenna noise level to be at least several dB
greater than that > with the dummy antenna, which is usually easily
achieved. > > The general band signal and noise levels are being
regularly monitored at > several locations - you can see the info at http://abelian.org/. But you > already know
that... > > Cheers, Jim Moritz > 73 de
M0BMU > -------------------------------------------------------------------------------- Eingehende
eMail ist virenfrei. Von AVG überprüft - www.avg.deVersion: 9.0.829 / Virendatenbank:
271.1.1/2919 - Ausgabedatum: 06/05/10 08:25:00 | 