On 4/25/2017 5:43 PM, Dave Platt wrote:
In article , rickman wrote:

I think the real problem is this antenna for 2 meter operation is 20
feet long!

Yup. Great for side-mounting on a tower, so-so for top-mounting on a
tower or mast (it'll sway in the breeze and this can wear out the
solder junctions), not so great for vehicle mounting, and downright
impractical for a hand-held :-)

For marine VHF it can't be used on shore, so hanging it from
a tree would not work. When you say using a single half wave section
wouldn't be much different from a marine VHF antenna, what type of
antenna would a marine VHF antenna be? I thought they used a colinear
design.

For "shortened" antennas, what you'll often find is that they're
"loaded" antennas. You can significantly shorten a radiating element,
and still have resonance, by adding an inductor of some sort - the
shortened element has capacitive reactance, and the inductor's
reactance (equal magnitude, opposite sign) cancels it out.

One way to do this is to wind the element itself into a spiral... this
adds inductance and lets you use a longer piece of wire
(i.e. something close to a quarter-wave).

Most "rubber duck" antennas are of this sort - they often have an
inductor in the base (right by the radio), and the whip consists of a
wire wound spiral-fashion around some sort of insulating core. If you
buy a hand-held marine-VHF radio, that's very likely what you'll get.

An example:

http://www.gandermountain.com/modper...ntenna&i=91538

Note they call it "Heliflex" - probably shorthand for "helically wound
flexible". It's only 1' long.

http://www.gandermountain.com/modper...ntenna&i=91513

This one is 6', or almost 2 meters. It might contain some sort of
center-fed dipole (fed up through the center of the lower element), or
it might be sort of the equivalent of a J-pole (half-wave or 5/8-wave
radiator with a matching stub or coil at the bottom).

Interesting. Any idea what the specs mean?

Gain 3dBi
"Marine Gain" 6dB

I know what dBi is, but what is Marine Gain? Is there some reference
antenna they use such as the rubber ducky?

--

Rick C

In article , rickman wrote:

Interesting. Any idea what the specs mean?

Gain 3dBi
"Marine Gain" 6dB

I know what dBi is, but what is Marine Gain? Is there some reference
antenna they use such as the rubber ducky?

That's probably taking into account an assumed 3 dB of additional
gain, due to the fact that the antenna is located a short distance
above the water surface. Salt water is an excellent reflector of RF
energy. At certain angles and distances, the primary wave from the
antenna and the reflected wave from the water surface will be in-phase
with one another, and will reinforce, doubling the strength of the
received signal.

In other places the two signals will largely or entirely cancel out.
There Ain't No Such Thing As A Free Lunch.

On 4/25/2017 6:11 PM, Dave Platt wrote:
In article , rickman wrote:

Interesting. Any idea what the specs mean?

Gain 3dBi
"Marine Gain" 6dB

I know what dBi is, but what is Marine Gain? Is there some reference
antenna they use such as the rubber ducky?

That's probably taking into account an assumed 3 dB of additional
gain, due to the fact that the antenna is located a short distance
above the water surface. Salt water is an excellent reflector of RF
energy. At certain angles and distances, the primary wave from the
antenna and the reflected wave from the water surface will be in-phase
with one another, and will reinforce, doubling the strength of the
received signal.

In other places the two signals will largely or entirely cancel out.
There Ain't No Such Thing As A Free Lunch.

I'd be very surprised at that. My understanding is that the water gets
in the way by obstructing the fresnel zone. But then as I read more
about the fresnel zone I see this is really about reflections rather
than the main wave propagation. So whether reflections off the water
are good depends on the geometry of the antennas and water surface, no?

Then there is the difference between salt and fresh water.

--

Rick C

On Tue, 25 Apr 2017 15:11:46 -0700, (Dave
Platt) wrote:

In article , rickman wrote:

Interesting. Any idea what the specs mean?

Gain 3dBi
"Marine Gain" 6dB

I know what dBi is, but what is Marine Gain? Is there some reference
antenna they use such as the rubber ducky?

That's probably taking into account an assumed 3 dB of additional
gain, due to the fact that the antenna is located a short distance
above the water surface. Salt water is an excellent reflector of RF
energy. At certain angles and distances, the primary wave from the
antenna and the reflected wave from the water surface will be in-phase
with one another, and will reinforce, doubling the strength of the
received signal.

In other places the two signals will largely or entirely cancel out.
There Ain't No Such Thing As A Free Lunch.

Yep, that's it. I did a study of antenna patterns for a simple
vertical dipole over ideal ground at various altitudes:
http://802.11junk.com/jeffl/antennas/vertical-dipole/index.html
Despite that official antenna gain of a dipole being 2.15dBi, at
various altitudes, the gain is quite a bit higher. At 5 wavelengths
or more, the maximum gain is about 7dBi.
http://802.11junk.com/jeffl/antennas/vertical-dipole/slides/vertical-dipole-5-0-wavelengths.html
However, the beamwidth of the lobe that has 7dBi gain is so narrow as
to be useless. The slightest antenna tilt will put the other end of
the path into lower gain lobe, or into a null.

Animated version:
http://802.11junk.com/jeffl/antennas/vertical-dipole/slides/animated-v-dipole.html

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

In article , says...



Interesting. Any idea what the specs mean?

Gain 3dBi
"Marine Gain" 6dB

I know what dBi is, but what is Marine Gain? Is there some reference
antenna they use such as the rubber ducky?


Forget about the advertised gain of an antenna. Most of the time they
seem to be made up numbers.

Just start with a 1/2 wavelength long antenna and call it 0 Db. Every
time the length doubles from the one before it you can call it 3 dB or 3
dB over a dipole. If the advertised number is way off of this, then you
can just call it a made up number.