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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