[TowerTalk] Snow and rain attenuation

[Michael Tope]

----- Original Message ----- 
From: "Jim Brown" <jim at audiosystemsgroup.com>
To: "Tower Talk List" <towertalk at contesting.com>
Sent: Monday, October 31, 2005 3:50 PM
Subject: Re: [TowerTalk] Snow and rain attenuation


> On Mon, 31 Oct 2005 16:32:50 -0800, Bob Nielsen wrote:
>
>>There are frequencies where there is a resonance with water molecules
>>which causes a quite high attenuation, even without rain or snow.
>
> My DirecTV receiver will die briefly during very heavy rain. I don't know
> what frequencies they're using, but it isn't a very big dish, so there
> probably isn't a lot of gain margin. Remember -- it's a digital "cliff
> effect" -- that is, the signal fades gradually, but you don't notice it
> until the level drops so much that the system starts losing so many bits
> that it can no longer do error correction (or assemble enough bits to make
> a picture).
>
> Jim K9YC
>

The DBS birds use Ku-band which is typically 14.0 to 14.5 GHz on
the uplink and 11.7 to 12.2 GHz on the downlink. Rain fade is a real
issue at these frequencies and typically requires something on the order
of a 10dB link margin depending on the rain statistics for a particular
area in order to achieve reliability that is on par with wired systems. At
2.5 GHz its definitely not an issue. Not sure about 5.8 GHz, but the
charts that AA6EG posted suggest that it is still pretty mild at
5.8 GHz as compared to Ku-band. This is consistent with what I
recall being told by a former colleague regarding C-band satellite links
when I worked in the VSAT business.

Mike, W4EF.............................................................