On 4/23/2017 2:31 PM, Jeff Liebermann wrote:
On Sun, 23 Apr 2017 12:00:09 -0400, rickman wrote:

Will 5 watts be heard that far away?

Let's see what the Fiis equation says, assuming line of sight, perfect
conditions, and no obstructions:
http://www.proxim.com/products/knowledge-center/calculations/calculations-system-operating-margin-som

Distance = 20 statute miles
Operating frequency is about 156MHz
Rubber ducky antenna gain is about -3dB
Coax cable loss is zero.
5 watts tx power is +37dBm
25 watts tx power is +44dBm
Rx sensitivity is 0.18uV = -122dBm/12dB SINAD

Plugging into the above calculator, I get for 5 watts:
106.5 dB path loss
-75.5 dbm rx signal strength
46.5 dB fade margin

In other words, with this arrangement, your receive signal is 46.5dB
stronger than the minimum level (12dB SINAD) necessary to hear a
fairly weak and noisy signal. You should theoretically have no
problems being heard at 20 miles.

However, that's theory, not practice. At 156 MHz, the Fresnel zone is
rather large. Quite a bit of signal is lost bouncing off the water
surface, or being absorbed. There are waves that get in the way. The
curvature of the earth raises the wave height at mid span, resulting
in more blockage. My experience is about 5 miles maximum for reliable
communications between two handhelds over water. Icom sorta suggests
3 to 8 miles:
https://www.westmarine.com/WestAdvisor/Selecting-a-VHF-Handheld-Radio
Please read the "What are the limits of range and power?" near the
bottom of the page.

Propagation over water is also full of oddities and anomalies:
"The propagation of VHF and UHF radio waves over sea paths "
https://pdfs.semanticscholar.org/a6ca/1fe1ca92e96a7e7150217b265816891b102e.pdf
I've been in situations where I can see the person or station that I'm
trying to communicate but because of inversion layers and surface
reflections, the signal was weak, variable, or gone.

You mentioned that it's not over salt water. In that case, land
topography has a huge effect on propagation. If your friend is in a
river canyon, raising the antenna a few feet isn't going to do
anything useful. For such situations, land the kayak, climb the walls
of the canyon, and try the radio where there's fewer obstacles.

I said the particular case I was being asked about was not over salt
water. I didn't say it was on a river. The particular case is for use
on the Great Lakes. Other uses I might have at other times would likely
be on salt water such as the Chesapeake Bay and tributaries or the
Atlantic ocean.

Your info is helpful. Thanks.

--

Rick C