Am 16.08.2011 22:17, schrieb g4gvw:
Hi Tom,
While it may not be practical at your location, an "optical link" does
not HAVE to rely only on optical "cable" link but might also comprise a
projected light beam in free space. A distance of 100Metres need not be
prohibitive. Just a thought!
73
Hello Pat,
that's interesting! I've already had similar thoughts, especially since
there is a house which can be seen as well from my garage as from my
balcony and it's standing in the right angel for (at least diffuse)
reflection. 4 MHz bandwidth should give no problems for LEDs and
photodiodes but what about the 100 dB dynamic range? Could it work even
in AM? That would be the simplest solution.
What do others think about this?
Tu es 73,
Tom, DK1IS
On Tue, 2011-08-16 at 21:39 +0200, DK1IS wrote:
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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