[amsat-bb] Re: Example of S-band interference
John B. Stephensen
kd6ozh at comcast.net
Sat Sep 9 23:28:10 PDT 2006
Be careful about comparing HEOs and LEOs. HEOs a 40,000 km have an
additional 34 dB of path loss compared to LEOs at 800 km.
----- Original Message -----
From: "Tim Tapio" <tim at timtapio.com>
To: <bruninga at usna.edu>; <amsat-bb at amsat.org>
Sent: Saturday, September 09, 2006 00:24 UTC
Subject: [amsat-bb] Re: Example of S-band interference
> Why are so many people (many being a relative number) enjoying S band if
> 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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