Re: LF: Loop vs. ferrite antenna tests...

[Stefan Schäfer <schaefer@iup.uni-heidelberg.de>]

Hello Daniele,

Am 05.09.2011 14:32, schrieb Daniele Tincani:

 

Hello Stefan, LF,

 

I'm looking for a suitable preamp for my "coming sooner or later" tunable ferrite antenna (hopefully covering as much as possible of LF band with a var. cap. plus some switchable additional cap.). Any suggestions?

Yes. Jim/M0BMU has recently posted a circuit of his ferrite antenna, just a few weeks ago. I is a high-Z amp, just as the PA0RDT design. Probably you can use that if already available, instead of the probe.
What i have done is making a second winding (just 2 turns, placed a bit outside of the center of the rod), matched to 50 Ohm and then use a 20 dB gain preamp. This was/is just a temporary way to connect the antenna to the RX but allows to get a first impression of the usage of that antenna.

73, Stefan/DK7FC

 

Best regards

D.

 

From: Stefan Schäfer <schaefer@iup.uni-heidelberg.de>
To: rsgb_lf_group@blacksheep.org
Sent: Monday, September 5, 2011 2:21 PM
Subject: LF: Loop vs. ferrite antenna tests...

LF,

There was no chance for me to TX in France this year but at least i did some useful comparisons with my single turn loop (1m diameter) and the ferrite antenna.

Markus/DF6NM did some CW beacon transmissions on 136.5 kHz for me. The distance was 428 km (http://no.nonsense.ee/qthmap/?qth=JN59NK&from=jn29od96).
Both antennas were pointing to DF6NM. The whole system was running on batteries and the distance between antennas and netbook was sufficient. Anyway there was a noise pickup, maybe by the local 20 kV line in some 10m distance.

Best S/N for the loop was achieved by using the preamp + a 10 dB attenuator. Without the attenuator, the RX gets overloaded due to DCF39 which is still very strong there. Without the preamp, the noise floor is to close to the soundcard+RX noise it seemed. It was a relatively quiet morning, just some individual cracks / QRN.

The ferrite antenna was used in combination with the same preamp (M0BMU design, 50 Ohm based), but without the attenuator.

The audible performance of the ferrite antenna seems to be even slightly better than with the loop. This is due to the smaller bandwidth (higher Q) of the ferrite antenna that reduces the DCF and HGA levels even more. Markus was well audible on both antennas! The S/N was about 20...23 dB in 3 Hz noise bandwidth.

A comparative spectrogram can be found at http://dl.dropbox.com/u/19882028/LF/DF6NM%20in%20CW%20in%20JN29OD96.png
First one can see the ferrite antenna (slightly lower signal level but about same S/N) recording, then the loop.
SpecLab was using a 10th order butterworth filter centered to 137 kHz, with 2 kHz bandwidth. The noiseblanker was applied as well (9 dB treshold).
An audio file can be found here: http://dl.dropbox.com/u/19882028/LF/DF6NM%20in%20CW%20in%20JN29OD96.WAV
It uses a 500 Hz CW filter at 700 Hz AF. Except the filter and blanker, no further noise reduction is applied.
I personally find the first part of the file better to copy. This is the ferrite antenna!

Tnx Markus/DF6NM for the test.

73, Stefan/DK7FC

PS:
Copied further stations:
DG3MDE : http://dl.dropbox.com/u/19882028/LF/DG3MDE.jpg
DF6NM at 35 dB S/N in 488 mHz: http://dl.dropbox.com/u/19882028/LF/DF6NM_QRSS-3.jpg (wrong locator in the capture)