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Re: re: FM Sat Channel Spaceing 15 vrs 25k



At 03:56 PM 6/2/2006 -0600, Nate Duehr wrote:
>FT-847 is no longer in production.

- True, that is really too bad. But lots of
AMSAT members already have them.


>I knew there'd be a few replies with a few rigs that can do 50W, but 
>they're not the "typical" radios most people are using.  Especially 
>newcomers.  Many can barely afford a mobile rig.
>
>If we're talking only FM here, a price difference of over $800 is only 
>worth it for those of us crazy enough to buy such dedicated rigs.  If the 
>birds are going to be SSB, then the Kenwood or the Icom for a new 
>satellite operator who either can't or won't buy used rigs, become a 
>technical requirement.
>But if I remember correctly, Bob's posting he said that FM was one of the 
>goals.
>
>The number of models of FM dual-banders available from all manufacturers 
>that do less than 50W on UHF dwarfs this two-radio list.

But hams that buy them do not generally use them for satellites either
whereas hams that buy satellite radios are often interested in operating
on satellites.

>I still say: 50W on UHF FM is NOT common and satellite designers would do 
>better to plan on most radios being far LESS than 50W on UHF for uplinks.


Dear Nate,

I think you are missing Bob's point (and mine too.)

Let's do some math:

For a typical satellite in a 1Km circular orbit, the path
loss to the satellite is about -145 dB.

Let's assume the satellite antenna gain is 0dB and the
ground station is using a 0dB gain omni like a vertical -
not an arrow or other gain antenna. Let's also say the
ground station is running not 50 watts but 5 watts.

This is an TX EIRP of about +37dBm0

This gives a receive signal level at the satellite of
around -108 dBm0.

A cheap hamtronics 70cm receiver like the R305 is spec'd to
provide 12dB SINAD at .15uV or -123 dBm0.

So, with 5 watts into an omni antenna the link margin would
be around 15dB! You can take off -3dB if you want for antenna
polarization mismatch but you are still over 10dB of link margin.

Now , if you have a typical satellite rig with at least 50 watts out,
your link margin is another 10 dB higher. But if you only want to
use a mobile rig at 35 watts, your link margin is only another 8.5 dB
over the QRP rig for 18.5 dB of link margin - and that is with a
0dB gain omni.

Take a 5 watt HT and run it into a little 10dB gain beam and you are
back up to 50 watts EIRP. I really don't think any satellite designers
have to worry very much about whether some ham wants to use a 35w mobile
rig.

73,
Tony AA2TX
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