Message-ID: <4E4AC760.3000202@kabelmail.de>
Date: Tue, 16 Aug 2011 21:39:12 +0200
From: DK1IS <DK1IS@kabelmail.de>
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To: rsgb_lf_group@blacksheep.org
Subject: LF: Remote Receiving Antenna: UHF-Link
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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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    Dear LF-Group,<br>
    <br>
    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.<br>
    <br>
    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=A0 -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. <br>
    <ul>
      <li>Obviously one has to use a single channel link without any
        multplexing.=A0</li>
      <li>Obviously FM is the right transmission mode - are there
        ISM-bands and link units which offer a signal bandwidth of about
        4 MHz?<br>
      </li>
      <li>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?</li>
      <li>Do you know any offers for such UHF link modules?</li>
      <li>Did you have a similar challenge? How did you manage it?<br>
      </li>
      <li>Any other ideas are welcome!</li>
    </ul>
    73,<br>
    Tom, DK1IS<br>
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