Discussion in 'Boat Electronics' started by James Hebert, Jan 8, 2005.

3. ### BobGuest

off the top of my head, looking at the article, i think there's an
error. he says EACH antenna needs to be 12.4 feet above the surface,
but i think the SUM of the antenna heights needs to be this...IOW each
antenna needs to be 6.2 feet high.

4. ### James HebertGuest

If two vessels are ten miles apart, they will each
need antennas 12.4 feet high in order for their
radio horizons to be in view (line-of-sight) of
each other.

The case presented shows how much margin there
is in a typical circuit. There are many poor
which can barely talk to the marina office
from its dock own gas dock, but this does
not constitute a negation of laws of physics.

5. ### BobGuest

why is that? the equation sez 12.4 feet for antenna height. does that
mean 1 antenna could be on the ground and the other at 12.4 feet?

6. ### MeGuest

this is where the Practical and empirical evidence shows that
the math isn't showing what really is hapopeneing.

I have two 1 watt Vhf handhelds, with rubber antennas.
I can talk 16 miles over water with these two radios.
Both myself and my wife are less than 6.5 ft tall.
We are both standing at sealevel. (water lapping at our feet)
Now how does you MATH explain this empirical DATA?

Do my handhelds have receive sensitivity lower than atmospheric
noise? Maybe the Laws of Physics cease to apply north of 58 degrees?
Since this is a perfectly viable Path 24/7 and we have used it
daily for the last 15 years, what is your explanation?

Me who actually does know the answer.........

7. ### EricGuest

Sealevel is not a word. If you mean sea level, that does not equate to
"water lapping at our feet". If you are claiming a 16 mile range, direct,
when you are both at 6.5 feet above sea level, I say your full of crap. If
this is a riddle, then I say your both at a seperate body of water with a
valley between, or your communicating via a repeater.

Eric

8. ### Gary SchaferGuest

It means that the horizon is halfway between. If one antenna was at
sea level and the other at 12.4 feet you would only talk half the
distance.

Regards
Gary

9. ### MeGuest

Yes, Meindert (the engineer) come up with the answer. Vhf does indeed
extend beyond Line of Sight, in all cases, and your math does not take
this into account. The straight that I live on, is 16 Statute Miles
wide, and my wife stands on one side, at the end of a road, at a boat
ramp, and I stand on the other side, in front of my cabin, on the beach.
Both our feet, are being lapped at by the water, at exactly Sea Level,
at that moment, and YES, we do communicate with one watt, Motorola
handheld radios with rubber antennas, and have for the last 15 years.

You made a BAD Assumption, when you stated that VHF (156Mhz) RF
Propigation was only Line of Sight. This is the downfall of your whole
construction. When you have as many years as I have in RF
Communications, you should, by then, have a better understanding of the
basic Laws of Physics, that underpin the technology that your espousing.

Me who knows the difference between Theory and Practice....

10. ### Gary SchaferGuest

The old saying by the Motorola salesman selling low band radios to the
farmers was "100 watts goes 100 miles".
It would do that on many days but you dare not design a system around
those figures.

Regards
Gary

11. ### Doug DotsonGuest

Salesman's name was P.T. Barnum as I recall

12. ### Gary SchaferGuest

By the way, the difference between radio line of sight and optical
line of sight at 5 miles is only about .7 miles at that distance.

The factor to multiply the height of the antenna by is about 1.33 for
radio over optical. The difference is not great.

There are phenomenon's such as ducting that take place on a regular
basis over some radio paths. Usually not over 24 hours though.

I can assure you that I can show you many places where hand held to
hand held will not produce anywhere near 16 miles of range even over
water but on rare occasions.

Regards
Gary

14. ### Jack PainterGuest

James, don't confuse 1w-rubber-duck man (anonymous "me" contributor) with
logic. Heck, after reading his story, I may pull down my antenna and replace
it with a rubber-duck. After all, I only get 20-25 miles reliable range to
surface craft from a 60' amsl antenna w/25w! This might have something to do
with small craft's antenna rocking through an arc of 60 degrees at the
entrance to the Chesapeake Bay!

Tropospheric Ducting is a real problem with VHF-Marine. While Bruce has some
interesting stories to tell about making use of that up North, we normally
find it a real hindrance to good communications in the mid-Atlantic. Having
five or more CG Groups trying to answer the same mayday, and hearing traffic
from a hundred or more miles up and down the coast is not a good thing for

see http://home.cogeco.ca/~dxinfo/tropo.html for ducting forecasts

Jack Painter
Virginia Beach, Virginia

15. ### BobGuest

yes, as a ham i once talked from allentown, PA to n. carolina on VHF
FM in the ham bands.

While Bruce has some
yes, we CG radio operators try to avoid that situation but it's
sometimes inevitable. what's even worse is that we sometimes don't
respond to a mayday, assuming it's in some other CG AOR.

16. ### engsolGuest

My turn for a radio sea story.
border (Kaktovik), when we had a three day RF 'event'. Whereas
we generally could communicate with over-the-pole aircraft at
ranges of 200 miles, we suddnely couldn't talk to them until they
the fuel tanks of the next DEW site west...but we couldn't see aircraft
50 miles away. Talk about refraction!
Just for the record, I'm an RF guy too...I worked for a few years doing
path studies, with all the measuring gear to confirm theory. The end results?
On average, the math worked...but there were days it didn't. The more
marginal the predictions (in terms of range), the more it varied from day-to-day.
The folks paricipating in this thread suggesting that the physical/optical model
of the earth is not the same as the RF model are right on. 4/3K ring a bell?
Cranky tonight Norm B