[amsat-bb] Re: Example of S-band interference

Tim Tapio tim at timtapio.com
Fri Sep 8 17:24:13 PDT 2006

Why are so many people (many being a relative number) enjoying S band if this 
is the case?  

I listened to a tape of a guy on S-band, his corner reflector and 
downconverter laying on the roof of his car....

Color me confused....

de Tim, K4SHF

On Friday 08 September 2006 11:17, Robert Bruninga wrote:
> > If my highly pointed S band dish is on my roof and tracking
> the sat
> > in the sky and my nearest neighbor is more than 150 feet from
> my
> > antenna how can my neighbor's telephone, with very limited
> range
> > knock out the signal from Eagle, when I am using my SSB
> preamp?
> The answer is relatively easy to compute.  The simple
> communication equation for power received at a receiver (PR)
> from a distant transmitter (PT) in dB is simply:
> PR = PT + GT + GR -LI - LS
> Where:
> > PR is power received
> > PT is power of the transmitter (lets say 40 dBm (10W)
> > GT is gain ot the TX antenna  (Lets say 8 dB)
> > GR is gain of the RX antenna  (abt 25 dB for a 1m dish)
> > LI is incidental losses (usually less than 3 dB)
> > LS is the space loss due to distance between TX and RX
> Computing space loss is basic physics and boils down to ((4 * Pi
> * R)/ wavelength) squared.
> For a P3E satellite with a Range (R) of about 40,000 km and the
> wavelength of 2.4 Ghz being about .12 meters that loss term is
> about -192 dB.  Add it all up and you get a received signal
> power of about -122 dBm which is just about exactly the minimum
> receive signal for a typical FM receiver.
> Ok, now compare this signal to one from a 10 milliwatt 2.4 GHz
> neighbor's phone about 150 feet away:
> > PT is now 10 dBm
> > GT is at worst  0 dB
> > GR is say - 10 dB (35 dB worse than main lobe)
> > LI is still about 3 dB
> But now the space loss, LS is not 40,000 km away, but only 150
> feet away.  And this computes to be a loss of only - 74 dB.
> Adding it all up gives a power received of about - 77 dBm.
> Notice that this neighbor's wireless phone is a full 45 dB
> STRONGER coming in from the back of the dish than the satellite
> coming in from the front, even though the satellite has 35 dB
> more dish gain in the main lobe.
> For reference, this 45 dB stronger neighbor's phone is
> equivalent to a 32 Killowatt neighbor compared to a 1 Watt
> satellite signal.  Even if you use a 3m dish, then this only
> changes the signal power from the satellite by 10 dB so now you
> have a 10W satellite signal competing with a 32 Kw neighbor.
> Still the neighbor wins...
> Computing it backwards, the neighbor's off-axis signal won't be
> less than the satellite signal until the phone is more than 5
> miles away. (Of course, that is if it was line of sight with
> nothing in the way.  Given that each tree is worth about 10 dB
> at S'band, then you would really maybe need only a small 1 acre
> forest to block his signal sufficiently?
> Bottom line, looking at the numbers, it does not seem to make
> sense to intentionally design a satellite downlink these days
> useable by most of the AMSAT membership that is that susceptible
> to off-the-shelf consumer devices known to cause interference...
> Which is beyond our control to remedy.
> Just my 2 cents.
> Bob, WB4APR
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