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Re: re: QRM 2m, question for sat builders



Hi Drew,

I think I can answer your actual question.

First a little background:

At 435 MHz, the sky temperature is about 45K versus around 300K
at 144 MHz. You can view this as around an 8dB advantage lower
received noise and (all things being equal) a better received S/N

The path loss at 435MHz is about 9dB higher than at 144 MHz so
this is about a 9dB disadvantage in received signal to noise ratio
(again all things being equal.)

Now, the above numbers look almost like a wash so why does it
seem so different? First, let us assume the same Satellite EIRP and
the same SIZE ground station antennas. Note that for the same physical
sized antenna, the antenna at 435 MHz would have around 9dB higher gain.

So, at 435 MHz, if you use a low noise preamp and a directional antenna,
(avoiding most of the earth noise,) the received S/N will be several
dB (at least) higher on 435 MHz.

But (a really really big but!) If you use omni antennas like a 1/4 wave
vertical on each band, not only is the +9 dB better path loss a huge
improvement on 2m, but you don't benefit from the lower sky noise
at 435 MHz because the earth noise is the dominant source of antenna noise.
So with omni antennas, the received S/N is much better on 2m by around 9db.

These numbers are not exact as it depends on the RX noise figure etc.
but I am sure you get the idea.

Did that help?

73,
Tony AA2TX

---

If you use a directional ground station antenna and
can get a nice low antenna temperature
At 06:05 AM 2/15/2005 -0500, Andrew Glasbrenner wrote:

> >The path loss goes up with frequency but antenna gain goes up at the same
> >rate for a constant size antenna. The pointing accuracy must be greater at
> >higher frequencies but for the size antennas that amateurs use, this isn't a
> >limiting factor below 2 GHz.
>
>To bring this back to the original point, the required use of directional, 
>gain antenna systems for receive is one of the factors that makes Mode J 
>more difficult to the beginner. Theoretical this and that aside, my 
>experience on the LEOs has shown that you can receive a 2m satellite 
>downlink from LEO on just about anythng better than a wet noodle, and the 
>same just can not be said for mode J.
>
>73, Drew KO4MA
>----
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>
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----
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