RE: Record Miles/watt

From: Kent Torell (torell@sicom.com)
Date: Tue Dec 19 1995 - 18:31:50 EST

>By contrast, a pair of 10 dBi horns on 2304 MHz spaced 10 meters apart
>has about 39 dB of path loss. Roughly 786,000,000 miles per watt.

Zack;
You forgot to change milliwatts to watts; I get 1000 times more!

Now, having confused the group thoroughly, an explaination of these numbers
and "non-intuitive" nature of the behavior.

Zack defined the performance based on a reciever sensitivity of -150 dBm.
This is a rough equivalent of a 100 Hz bandwidth receiver with a 4 dB noise
figure. Or, a 25 Hz bandwidth receiver with a 10 dB noise figure.

He then connected this receiver to a 10 dbi antenna (db gain over
isotropic) and placed it 10 meters away from a similar antenna connected to
a transmitter at 2.3 GHz. The theoretical path loss is 39 db from standard
formula:
free space path loss, db = 96.6 + 20log(f,ghz) + 20log(d, mi) - antenna gain

To receive -150 dbm, the transmitter has to put out -111 dbm, or -141 dbw.
10 meters is 0.0062 miles; miles per watt is 0.0062 / (10^(-14.1)) or
782,000,000,000 miles per watt.

What is non-intuitive here is that "miles per watt" implies linear
communications performance with distance; path loss actually increases as
the square of the distance, so it is 4 times harder to go 2 miles than 1
mile.

If we move the receiver 100 meters away, 10 times farther, the path loss
will increase by 20 db to 59 db. The transmitter now has to put out -91
dbm, or -121 dbw. The miles per watt is now only 78,000,000,000 miles per
watt; 10 times worse.

So what? Well, we might guess that the longest miles per watt awards will
go to the people with the lowest power and the shortest distances, i.e.
milliwatts and hundreds of miles. The confounding variable in this
prediction is the ionosphere; the path loss is increased by its reflection,
and as the angle of reflection increases, its loss goes up. So, who knows?
Sounds like an opportunity for more qrp fun!

72, AB7OA

Kent Torell torell@sicom.com 602-483-2867
SICOM 7585 E. Redfield, #202 Scottsdale, AZ 85260

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