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Date: Mon, 10 Mar 2014 14:48:22 +0100
From: =?ISO-8859-1?Q?Stefan_Sch=E4fer?= <schaefer@iup.uni-heidelberg.de>
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Subject: LF: Bluetooth instead?
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Hi all,

I want to share my recent results of experiments trying to arrange a 
wireless AF connection, which can be used for a VLF antenna or any other 
RF spectrum such as 630m and 2200m bands. The goal is always to decouple 
local QRM sources from a RX antenna installed in a quiet(er) location, 
to improve the RX capability of the amateur radio station in noisy 
environments.

It looks like there are some cheap and useful devices on the market, 
using bluetooth technique. Last night i did some first tests!

First i simply wanted to transfer music from my smartphone to my 
netbook, using the bluetooth function (never used that before). This 
worked very well in the first attempt! There was no audible sound 
quality loss. The sound was played on the speaker of the netbook. But, 
more precise, it is not the speaker but the standard audio device! :-) 
That means, you can define virtual audio cable 1 as the standard output 
device! And here we are, we can feed the audio signal to SpecLab, 
without additional software except VAC which runs on many LF/MF/VLF PCs 
anyway. This allowed me to watch the spectrum and see if the signal is 
distorted or drifting or anything else. Again there was no audible and 
visible indication of a significant quality loss.

Next i wanted to get an impression about the possible dynamic range of 
this wireless bridge. I uploaded a VLF recording (the original one from 
4X1RF, including my 8970 Hz signal :-) ) as a wav file to the smartphone 
and played it via the bluetooth link. The signal level in this recording 
is quite low and depends on the volume setting of the smartphone. Here i 
saw that the background noise of the system, i.e. the dynamic range, is 
120 dB, with some noise lines/peaks peaking to -100 dB. This looked very 
fine! Also the full 0...24 kHz spectrum was transferred via the 
bluetooth link and it was possible to lock SpecLab to DHO38 which was 
included in the recording! Next, this allowed me to watch the frequency 
drift of the system. Looks quite normal for a crystal, i.e. useful for 
our purposes.

Now i want to try bluetooth for VLF and LF reception and i want to do 
some distance tests. 2 km distance, using a directional antenna, this 
would be very fine.

At amazon, there are relatively cheap bluetooth transmitters with an 
external antenna, e.g. 
http://www.amazon.de/B-Speech-Stereo-Bluetooth-Receiver-Transmitter/dp/B00CB3KGYE/ref=sr_1_1?ie=UTF8&qid=1394458092&sr=8-1&keywords=B-Speech+RTX1+Stereo+Bluetooth+Receiver+und+Transmitter

These do even have an external antenna and will allow to use a high gain 
antenna for higher distances and help to avoid collisions with other 
signal sources. Since my netbook has an internal bluetooth adapter and 
since i now know that the received signal can be fed to SpecLab, it is 
worth to buy the transmitter and to do further tests. This one seems to 
be one of the stronger versions with 100 mW RF power. Hopefully the 
power consumption is acceptable...

I will report about the progress...

73, Stefan/DK7FC