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

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

-- 
73 es gd dx de pat g4gvw
 qth nr felixstowe uk
(east coast, county of suffolk)