Hello Tom,
Yes I use a UHF link, but "only" for VLF, single channel, covering
500 ... 20000 Hz. The dynamic range is somewhat limited as you can
see on the VLF stream's detail page:
http://abelian.org/vlf/detail.php?stream=vlf6&page=0
But if strongs sferics are occasionally clipped (in the transmitter)
doesn't hurt that much for this application.
One important thing was getting rid of the stereo encoder / decoder,
which I had used in the first setup (it was a cheap "HiFi" wireless
headphone in the 864 MHz band, which didn't deserve its name. Analog
stereo wideband FM, but noisy as hell). Getting rid of the
stereo-stuff (tap the rx after the FM discriminator) gave a better
performance, and no aliasing effect caused by the strong Navy
transmitters in the vincinity of the 19 kHz pilot (there is no
'sharp lowpass' before the stereo modulator which would have cured
some of the problems).
The currently used stream is still analog, using a modified FM
wideband TX (which was once a cheap AM transmitter), and a wideband
RX. Today I'd go for a digital system, something with a 16-bit ADC
on the TX, and 16-bit DAC on the RX side. Not sure if the "bluetooth
HiFi wireless headphones" sold today would be up to the job. For
example, the Sennheiser RS 170 (digital, without compression, but
datasheet not very detailed).
73,
Wolf .
Am 16.08.2011 21:39, schrieb DK1IS:
Dear LF-Group,
from time to time UHF-links are mentioned to carry radio signals
from a remote receiving antenna positioned in a low QRM-region to
the main station. At least Wolf, DL4YHF, seems to have such an
arrangement. Having a high local QRM level at the site of my main
station I would like to try an active receiving antenna (e.g.
mini-whip) about 100 meters apart on the flat roof of my garage
standig in an open area without individual QRM sources. Available
power there is 12 V DC from an 120 Ah accu fed by solar cells for
activating the radio controlled garage door. Now the question is
how to carry the received signals to the shack. Due to the
specifics of the site cables (electric or opto) are impracticable,
so I`m thinking about an UHF-link.
For a preliminary test I purchased one of the well-known cheap
2.4-GHz-audio-video-links for wireless connection between tv sets
and their periphery. They always offer a video channel and two
audio channels for stereo signals. First I analysed the link with
signal generator and selective level meter on the workbench. The
video channel has a flat response between 3 kHz and about 6 MHz
with good linearity in the range from -50 dBm to 0 dBm
input/output. Without TX input the RX noisefloor is about -94 dBm
at 24 Hz bandwidth which should be overcome with a preamplifier at
the TX input. The audio channels work between 0.2 kHz and about 20
kHz with a strong preemphasis, they are fairly linear between -50
dBm and -10 dBm with a noisefloor of about -80 dBm at 24 Hz
bandwidth. In my imagination I alredy saw a mini-whip with the
video channel from LF to 80 meters and two crossed loops for vlf
with the audio stereo channels on my garage ... but a second test
with real band signals at the station RX showed the desaster: due
to obviously muliplexing the three channels for transmission there
were bad QRM spectra about 500 kHz with a lot of sidebands and
intermodulation. Strange that home entertainment sets can accept
this but for ham radio it`s absolutely impossible. So my question
is how to do it better.
- Obviously one has to use a single channel link without any
multplexing.
- Obviously FM is the right transmission mode - are there
ISM-bands and link units which offer a signal bandwidth of
about 4 MHz?
- Could you imagine to feed the whole spectrum of an active
antenna with certainly more than 100 dB dynamic range and a
bandwidth of 4 MHz via such a link or should one insert band
pass filters for the frequencies of main interest to reduce
the dynamic range being needed?
- Do you know any offers for such UHF link modules?
- Did you have a similar challenge? How did you manage it?
- Any other ideas are welcome!
73,
Tom, DK1IS
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