>will often be masked by internal noise in practice.
My antennas are a 88m longwire and a PA0RDT miniwhip.
When switched to the rx input both give a huge rise in noise with static
crackling,no buzz,
so I'm sure to have a real good SNR here with alphas up to 20dB
over noise at ~12kHz.
As Markus says the noise at VLF is around 140dB kTo, so one could 'afford'
an antenna
loss of 102dB to have a 10dB SNR margin with a NF of 28dB.
That's an antenna efficiency of 0,00000001585.
----- Original Message -----
Sent: Sunday, June 06, 2010 9:38 AM
Subject: LF: Re: VLF 9 kcs: preamp and
antenna noise
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
Eingehende eMail ist virenfrei.
Von AVG überprüft - www.avg.de
Version: 9.0.829 / Virendatenbank: 271.1.1/2920 - Ausgabedatum: 06/05/10
20:25:00
