[Laser] AW: Laser Digest, Vol 38, Issue 3

[Dieter Palme]

> -----Ursprüngliche Nachricht-----
> Von: laser-bounces at mailman.qth.net 
> [mailto:laser-bounces at mailman.qth.net] Im Auftrag von 
> laser-request at mailman.qth.net
> Gesendet: Montag, 20. August 2007 10:02
> An: laser at mailman.qth.net
> Betreff: Laser Digest, Vol 38, Issue 3
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> Today's Topics:
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>    1. 21 mile daytime laser contact durin ARRL 10GHz & Up contest
>       (Kerry Banke)
> 
> 
> ----------------------------------------------------------------------
> 
> Message: 1
> Date: Sun, 19 Aug 2007 15:43:13 -0700 (PDT)
> From: Kerry Banke <kbanke at sbcglobal.net>
> Subject: [Laser] 21 mile daytime laser contact durin ARRL 10GHz & Up
> 	contest
> To: laser at mailman.qth.net
> Message-ID: <426017.68564.qm at web83211.mail.mud.yahoo.com>
> Content-Type: text/plain; charset=iso-8859-1
> 
>  Yesterday Lee, KD0IF and I made a mid day laser contact over 
> a distance of 21 miles near San Diego  as part of the ARRL 
> National  10 GHz and Up contest. Last year we did a 7 mile 
> daytime contact but experienced very heavy scintillation 
> which we have found to be typical for warm weather in the San 
> diego area.  This year we added narrow wavelength optical 
> bandpass filters ahead of our optical receivers and are using 
> a fast AGC provided in the Spectrum Lab software.  This was 
> our first two way  daytime communications  tests with the 
> filters. The scintillation on the raw signal appears to be 
> very minimal compared to our contact last year. We don't know 
> if the narrow optical filters have provided this improvement 
> or if we just had unusually  good conditions so we need to do 
> more daylight tests with and without filters for comparison. 
> The filters are at 920 nm with a 30 nm half power bandwidth. 
> When operating at high power with  1W  collimated to  a 4"  
> dia beam, the laser
>  transmitters delivered  signals that  were a stable 50 dB 
> above the noise in a 1 Hz BW at 21 miles.   Communications 
> was done using PSK31 with a center frequency of 755 Hz.  
>   Does anyone know if adding the  narrow optical filters 
> should reduce scintillation? 

Definitely NO. The narrow filter reduces the background power and
increases the SNR. The fluctuation causes in different temperature of
small regions of the air. Hotter air has an smaller refraction index as
cooler and so is your beam diffracted from the straight line to yours
receiver. Only a bigger beam in diameter or more beams can reduce this
problem. In this case you get an averaging over many different ways. The
best way to reduce this fluctuation is the use of a big adaptive optic
(hi).

73 de Dieter dl7udp

Thanks also to Greg, K6QPV for 
> assisting with the contact.
>    - Kerry N6IZW - 
> 
> 
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> End of Laser Digest, Vol 38, Issue 3
> ************************************
>