Re: OT: What happens after ditching long distance?

On Fri, 08 Aug 2008 21:22:56 -0400, "Michael A. Terrell"
<mike.terrell@xxxxxxxxxxxxx> wrote:

Tell him to build a passive repeater out of a pair of Yagi antennas
and some coax.

Passive repeaters like that suck. Let's do the math.

Assumption: Typical PCS cell site at 1900MHz and about 1 watt per
carrier. You're at the extreme end of the GSM range (about 25 miles
for the older systems) and want to re-radiate the received signal back
into your house over a distance of about 100ft.

I'll assume two 15dBi yagi's back to back.

Starting at the cell site end:
Tx out: +30dBm
Sector antenna gain: +6dBi
Path loss:
20 log(FreqMHz) + 20 log(Miles) + 36.6
20 log(1900) + 20 log(25) + 36.6 = -130dB
Rx antenna gain: +15dBi (first yagi)
========
Signal at RX in: -79dBm

We now feed that signal into the 2nd yagi and point it
into the house:

"Tx" signal: -79dBm
Coax loss: 0dB
Tx antenna gain: +15dBi (2nd yagi)
Path loss:
20 log(FreqMHz) + 20 log(Miles) + 36.6
20 log(1900) + 20 log(0.02) + 36.6 = -68dB
Rx antenna gain: 0dBi (inside cell phone)
=========
Signal level at Rx input in cell phone:
-132dBm

The GSM standard specifies a minimum receiver sensitivity of -102dBm.
Unfortunately, the passive repeater is 20dB lower in signal level.
This isn't gonna work.

It's actually much worse in the opposite direction because the typical
cell phone only produces perhaps 100mw (+20dBm). Lose another 10dB.
The cell site operators try to compensate for this by using
crygenically cooled cavities and front ends, but that's fairly rare.

The 0dBi gain of the cell phone antenna is rather optimistic. I would
give it about -4dBi or worse, but the official guestimate is commonly
0dBi.

Someone will probably suggest that 25 miles is too far. Fine.... Cut
the range in half and you only improve things with 6dB less path loss.
That's still 14dB short of minimally functional from the cell site to
the phone and up to 24dB short in the other direction.

(Drivel: I hope this makes up for my previously attrocious math in
the never ending global warming thread).

--
Jeff Liebermann jeffl@xxxxxxxxxx
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
.

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