LF: Re: Re: VLF 9 kcs: preamp and antenna noise

["Clemens Paul" <cpaul@gmx.net>]

Markus,Jim,

 

the point of Jim was (i believe) to take a dummy with approximately the same impedance

of the antenna (at the rx input) to have a more exact reference for comparison

of rx or preamp noise versus antenna noise.

My point which maybe I did not express clearly enough was that three different passive loads

caused the same noise at perseus with a NF of around 28dB.

So any of these extremely different loads can be used as a dummy reference against antenna

noise_at_this_RX_noise_figure with the same result.

Building an extra special dummy would be of no practical use for this NF of 28dB. 

As Jim points out in his second email:

>But unless the noise figure of the preamp/receiver is quite low, this effect

>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.

Do tiny (in wavelength) antennas have even lower efficiency (including

mismatch losses)?

 

73
Clemens
DL4RAJ

 

 

----- Original Message -----

From: Markus Vester

To: rsgb_lf_group@blacksheep.org

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)

From: Clemens Paul

Sent: Sunday, June 06, 2010 8:48 AM

To: rsgb_lf_group@blacksheep.org

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" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
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
>


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