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Date: Tue, 16 Aug 2011 22:48:40 +0200
From: DK1IS <DK1IS@kabelmail.de>
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Subject: Re: LF: Remote Receiving Antenna: UHF-Link
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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