Hello Tom,
Thanks for your LF test signal during the fieldday of our club in July!
About the link: If the distance to the local QRM is 100m, can't you try
to use an ordinary RG58 cable and try to make a local ground rod, as
discussed last week (so just about 1000000 mails below ;-) ). First i
would try that befor you're doing the effort with the remote link,
although it surely will be an interesting project.
73, Stefan/DK7FC
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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