Here's my first draft. - http://tekmanx.serveftp.com/~tekmanx/pole.gif

Hi Amos

From memory the Cisco 5.6GHz box had a path prediction sensitivy number
of about -87dBm. ie full speed with that input to the RX. I suspect that
at a lower data rates and extra 10dB might be a good number to use. I
have seen that larger microwave system manufacturers do publish their
specs so look it up for the equipment that is going to be used.

FM rcvs on 2M can often go to about -117dBm. An SSB rcvr on 2m with a
preamp hits thermal noise at about -141dBm. This also needs a good human
ear.

Cheers Bob VK2YQA

Amos Keag wrote:
Receiver sensitivity is unknown. I regularly communicate 20+ miles with
500 milliwatts from a ham radio walkie talkie. [the receiver is located
on top of a mountain] Most communiation grade radios can receive a
signal as small as 0.000000000000002 watts. [one millionth of a volt].
So, you can see why I say 400 milliwatts is plenty. So, the receiver you
use should have a moderately good sensitivity 5 microvolts or smaller.

Telemax wrote:
"And would you guys say my 400 mw radio is overkill for 4 - 10 mile
shot?"

Depends on the whole system. The gains and losses must be totalled to
check for satisfaction.

Industrial microwave I put in almost 50 years ago was limited by the
klystrons available at reasonable cost at the time. As I recall, the
power was 1-watt or +30 dBm. At 6 GHz, the wavelength is only about 1
inch, so plenty of antenna gain was readily available. 6-ft parabolas
were common then and may have been a good cost to benefit trade off.
Their gain was near 38 dB at 6 GHz. We tried to limit our path to about
25 miles for reliability and the cost of radio towers. Here is how our
path might add up:

Free space loss: -140 dB
Antenna Gain (2ea): + 77 dB
Transmit power: + 30 dBm
Misc. loss (2 ea): - 5 dB

Received carrier pwr: - 38 dBm

A receiver sensitivity
for a multichannel RCVR: -80 dBm
would give us a fade margin of: 42 dB.

How would your system add up?

Best regards, Richard Harrison, KB5WZI

Also, I heard 802.11g sucks outdoors. This true? And would you guys say
my 400mw radio is overkill for 4-10mile shot?

Wasn't an article about long-range ham-band radio via 802.11 in the
Tennessee (?) Valley printed in QST a few months ago?

--
--Myron A. Calhoun.
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
PhD EE (retired). "Barbershop" tenor. CDL(PTXS). W0PBV. (785) 539-4448
NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol)

Bob Bob wrote:
"Work out the path loss based on the basic distance formula and plug in
your power losses, and RX sensitivity to determine your margin and max
range."

Add in the antenna gains. You`ll need them to offset path and
transmission losses.

Margins involve choices. Performance and reliability depend on those
choices. I often choose to imagine the earth with only 2/3 its actual
diameter for my path profile. Then I use 0.6 of the first Fresnel zone
for added clearance at the path grazing point. You need 30 or 40 dB
excess signal into the receiver under normal conditions to make the
receiver very quiet and to allow margin for path fades if you need high
performance and freedom from fades.

Path loss is computed from frequency squared times distance squared with
constants as needed for the system you are working with. Someone has
usually worked up a chart of path loss versus distance for the frequency
you are working with. Every time distance doubles, loss increases about
6 dB.

Best regards, Richard Harrison. KB5WZI

Techmanx wrote:
"How high should my tower be for my vertical sector with antenna?"

Assuming line-of-sight is needed, the distance in miles to the horizon
over smooth earth is: the square root of twice the antenna slevation in
feet. You may want to add to the calculated height to allow first
Fresnel zone clearance at the path grazing point.

Best regards, Richard Harrison, KB5WZI