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

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.

On Tue, 25 Apr 2017 15:39:57 -0400, rickman wrote:

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

Yep. The problem with the alternating coax cable antenna design is
that only every other 1/2 wave section radiates. The result of half
the radiation is half the gain. Or, as you've noticed, the antenna is
twice as long as it might be with phasing elements between 1/2 wave
sections.

For marine VHF it can't be used on shore, so hanging it from
a tree would not work.

I've shoved them into a fiberglass radome filled with urethane foam.
Works nicely hanging from a tower. I guess it might also work hanging
from a tree.

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.

Lots of ways to build a marine VHF antenna. The key is the length.
They come in 3ft, 6ft, 9ft, and 12ft lengths. The shortest are for
top of the mast, where low gain is needed to compensate for pitching
and rolling. The 12ft is for deck mounting, where the effects of
pitch and roll are somewhat less.

Inside the radome is usually a coaxial sleeve dipole with various
numbers of 1/4 wave phasing sections. You start with a coaxial
antenna something like this:
http://www.hamuniverse.com/w4bwsverticalbazooka.html
That's good for about 2dBi gain if the manufacturer bothers to use a
brass sleeve instead of folding the braid over the outside of the coax
cable. For more gain and lower radiation pattern, put another 1/4
wavelength sleeve 1/4 wavelength below the first sleeve and connected
to the coax shield at the top:
http://www.w8ji.com/Image1/Skirt_feed.jpg
I think that page was lifted from an early version of Henry Jasik
"Antenna Engineering Handbook".

The sleeve does not need to be cylindrical such as in the Isopole type
antenna:
https://www.google.com/search?q=isopole+antenann&tbm=isch. You can
keep adding them until you run out of space. However, there's a
catch. As the gain goes up and the vertical radiation pattern goes
down, the usable transmit bandwidth also goes down. Coaxial antennas
are better than most, but you still have to be careful not to add so
many elements that the VSWR is too high at the transmit band edges.

Also, some junk out the
http://www.thehulltruth.com/marine-electronics-forum/343939-here-s-what-shakespeare-5206-vhf-antenna.html
I've seen worse. I won't mention the manufacturer, but there's one
that's nothing more than a length of copper tape stuck to the inside
of a fiberglass radome, with a very narrow band matching system in the
base. Some are just a 5/8 wave antenna with a matching transformer in
the base.

Incidentally, marine antennas are very much like land mobile
commercial antennas. However, there's one difference in the coax
cable. Marine antennas use various schemes to prevent water from
wicking up the braid by capillary action, such as silicone grease
filler in the braid or having the outer jacket reflowed into the
braid. The idea is to not have any air gaps inside the cable that
might fill with water. However, I doubt the kayak will be in the
water long enough for this to be a problem, but if your friend has
money, it wouldn't hurt to spend a few dollars extra for marine grade
coaxial cable.



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

On 25 Apr 2017 20:24:44 GMT, Rob wrote:

I wonder, why is the "2 meter band" not called the "6 1/2 feet band"
in the USA? This alternating between meters and feet is getting a
bit funny.

Chuckle. Yeah, that's a problem. I sometimes irritate the local hams
by referring to various HF bands by the equivalent feet, yards, or
cubits. When asked which is the right way, my usual answer is when
test equipment manufacturers start labeling their equipment in units
of feet or meters, I might consider using those designations. Until
then, since most test equipment is labeled by the frequency, I'll use
Hz, KHz, MHz, GHz, etc.

Incidentally, if you need more confusion, there are the various
microwave frequency bands designed by letters, such as DBS
broadcasting is on K and Ku bands in the US. Then, there are bands
that are designated by their service type or spectrum auction block
numbers:
http://wireless.fcc.gov/auctions/default.htm?job=bandplans


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

On Tue, 25 Apr 2017, rickman wrote:

On 4/25/2017 3:19 PM, Dave Platt wrote:
In article , rickman wrote:

Yesterday I couldn't think of the term for the antenna style they use in
the commercial marine VHF antennas, but I believe it is called
"co-linear" or something like that. It is a bunch of coax sections
connected inner to outer at specific lengths. I have never seen a Ham
recommend using that type. But I guess Hams go more for permanent
installations with ground planes of some type. The co-linear needs no
ground plane I believe.

"Colinear" or "collinear", depending on whom you read. They're
basically a vertically-stacked array of individual radiating sections.

http://www.rason.org/Projects/collant/collant.htm

There are quite a few commercial ham antennas which use this approach
(the "Stationmaster" probably being the best known). They're fairly
popular for use on the 70 cm ham band, and some repeaters and base
stations use them on 2 meters.

This design is generally used when you want a substantial amount of
directional gain, and are willing to pay the price (length) for it.

I don't think this design would be a great choice for a kayak antenna,
because the individual coax sections in the "stack" are a
half-wavelength long (at the coax's velocity factor) and there are
usually quarter-wave sections at the top and bottom. The shortest
2-meter collinear (one half-wave section and two quarter-wave) would
be 2 meters in length - over six feet - and a marine VHF antenna
wouldn't be much shorter.

With a collinear of the type shown in the above link, you'd need to
mast-mount it up some distance - the bottom quarter-wave tube is
RF-hot, and if its bottom end is near water (or anything grounded) it
would tend to de-tune the antenna.

As others have noted, the OP really doesn't need a high-gain antenna.

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

Someone pointed out the classic groundplane antenna, with radials.

But, that's just a variant on the basic dipole, where there are two
elements of the same length. But the form of the groundplane puts the
radiation more where you want it.

For mobile, the whip antennas are often just a variant on the groundplane,
except the body of the car acts instead of the radials.

There are longer antennas, but still single pieces, that provide some
gain, but more important, don't need a ground plane, which of course is
hard to find in a kayak that isn't made of metal. So those are longer
whips, with some matching in place.

The collinear is like stacked dipoles, providing more gain, but needing
more height, and of course the matching stubs stick out the side. They
are fine on a tower, not so useful on a kayak.

Michael

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

On 4/25/2017 7:23 PM, Jeff Liebermann wrote:
On Tue, 25 Apr 2017 15:39:57 -0400, rickman wrote:

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

Yep. The problem with the alternating coax cable antenna design is
that only every other 1/2 wave section radiates. The result of half
the radiation is half the gain. Or, as you've noticed, the antenna is
twice as long as it might be with phasing elements between 1/2 wave
sections.

For marine VHF it can't be used on shore, so hanging it from
a tree would not work.

I've shoved them into a fiberglass radome filled with urethane foam.
Works nicely hanging from a tower. I guess it might also work hanging
from a tree.

My point is it is illegal to use *marine* VHF when on shore.


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.

Lots of ways to build a marine VHF antenna. The key is the length.
They come in 3ft, 6ft, 9ft, and 12ft lengths. The shortest are for
top of the mast, where low gain is needed to compensate for pitching
and rolling. The 12ft is for deck mounting, where the effects of
pitch and roll are somewhat less.

I'm not necessarily looking to build an antenna. Someone had injected
the construction project into the conversation that my friend asked me
about. I did some digging on the interweb and realized I should ask
here. Mostly what I've been told is the same as what I found myself
although the flexible J-pole is new. I was hoping there might be some
more insight into the issues.

Personally I would recommend a commercial antenna unless they want
something that can be rolled up which some of the suggestions could be.


Inside the radome is usually a coaxial sleeve dipole with various
numbers of 1/4 wave phasing sections. You start with a coaxial
antenna something like this:
http://www.hamuniverse.com/w4bwsverticalbazooka.html
That's good for about 2dBi gain if the manufacturer bothers to use a
brass sleeve instead of folding the braid over the outside of the coax
cable. For more gain and lower radiation pattern, put another 1/4
wavelength sleeve 1/4 wavelength below the first sleeve and connected
to the coax shield at the top:
http://www.w8ji.com/Image1/Skirt_feed.jpg
I think that page was lifted from an early version of Henry Jasik
"Antenna Engineering Handbook".

That's interesting. I wish I were building one.


The sleeve does not need to be cylindrical such as in the Isopole type
antenna:
https://www.google.com/search?q=isopole+antenann&tbm=isch. You can
keep adding them until you run out of space. However, there's a
catch. As the gain goes up and the vertical radiation pattern goes
down, the usable transmit bandwidth also goes down. Coaxial antennas
are better than most, but you still have to be careful not to add so
many elements that the VSWR is too high at the transmit band edges.

Also, some junk out the
http://www.thehulltruth.com/marine-electronics-forum/343939-here-s-what-shakespeare-5206-vhf-antenna.html
I've seen worse. I won't mention the manufacturer, but there's one
that's nothing more than a length of copper tape stuck to the inside
of a fiberglass radome, with a very narrow band matching system in the
base. Some are just a 5/8 wave antenna with a matching transformer in
the base.

Incidentally, marine antennas are very much like land mobile
commercial antennas. However, there's one difference in the coax
cable. Marine antennas use various schemes to prevent water from
wicking up the braid by capillary action, such as silicone grease
filler in the braid or having the outer jacket reflowed into the
braid. The idea is to not have any air gaps inside the cable that
might fill with water. However, I doubt the kayak will be in the
water long enough for this to be a problem, but if your friend has
money, it wouldn't hurt to spend a few dollars extra for marine grade
coaxial cable.

Yeah, water is amazingly corrosive, even fresh water.

--

Rick C

On 4/25/2017 7:36 PM, Michael Black wrote:
On Tue, 25 Apr 2017, rickman wrote:

On 4/25/2017 3:19 PM, Dave Platt wrote:
In article , rickman
wrote:

Yesterday I couldn't think of the term for the antenna style they
use in
the commercial marine VHF antennas, but I believe it is called
"co-linear" or something like that. It is a bunch of coax sections
connected inner to outer at specific lengths. I have never seen a Ham
recommend using that type. But I guess Hams go more for permanent
installations with ground planes of some type. The co-linear needs no
ground plane I believe.

"Colinear" or "collinear", depending on whom you read. They're
basically a vertically-stacked array of individual radiating sections.

http://www.rason.org/Projects/collant/collant.htm

There are quite a few commercial ham antennas which use this approach
(the "Stationmaster" probably being the best known). They're fairly
popular for use on the 70 cm ham band, and some repeaters and base
stations use them on 2 meters.

This design is generally used when you want a substantial amount of
directional gain, and are willing to pay the price (length) for it.

I don't think this design would be a great choice for a kayak antenna,
because the individual coax sections in the "stack" are a
half-wavelength long (at the coax's velocity factor) and there are
usually quarter-wave sections at the top and bottom. The shortest
2-meter collinear (one half-wave section and two quarter-wave) would
be 2 meters in length - over six feet - and a marine VHF antenna
wouldn't be much shorter.

With a collinear of the type shown in the above link, you'd need to
mast-mount it up some distance - the bottom quarter-wave tube is
RF-hot, and if its bottom end is near water (or anything grounded) it
would tend to de-tune the antenna.

As others have noted, the OP really doesn't need a high-gain antenna.

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

Someone pointed out the classic groundplane antenna, with radials.

But, that's just a variant on the basic dipole, where there are two
elements of the same length. But the form of the groundplane puts the
radiation more where you want it.

Yes, that has some potential for a portable antenna by making the
radials hinged and letting it all fold up and slide into a tube. I
expect I'm not the first to think of this. I know there are Ham events
where the bring equipment into the field and have contests.


For mobile, the whip antennas are often just a variant on the
groundplane, except the body of the car acts instead of the radials.

There are longer antennas, but still single pieces, that provide some
gain, but more important, don't need a ground plane, which of course is
hard to find in a kayak that isn't made of metal. So those are longer
whips, with some matching in place.

The collinear is like stacked dipoles, providing more gain, but needing
more height, and of course the matching stubs stick out the side. They
are fine on a tower, not so useful on a kayak.

Out the side? I haven't see that yet.

--

Rick C

In article ,
Jeff Liebermann wrote:
On Tue, 25 Apr 2017 15:39:57 -0400, rickman wrote:

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

Yep. The problem with the alternating coax cable antenna design is
that only every other 1/2 wave section radiates. The result of half
the radiation is half the gain. Or, as you've noticed, the antenna is
twice as long as it might be with phasing elements between 1/2 wave
sections.

Jeff,

Do you have a sim or model of that situation, showing that there aren't
significant RF currents on every other half-wave section? Somehow I
can't make sense of how that would happen.

My mental model of the alternating-sections design has been that all
of the sections do radiate... the alternating hookup forces them to
radiate in phase with one another, rather than out of phase (and thus
tending to squint the pattern badly upwards and downwards).

Now, I have heard that the alternating-coax collinear doesn't have as
much gain as an array of separate dipoles hooked up with a phasing
harness... but I've always read that as being explained by the fact
that the upper sections are carrying smaller RF currents than the
lower because some power has been radiated away before the signal
reaches the upper part of the antenna.

So, I'd appreciate enlightenment here!

On Tue, 25 Apr 2017 17:24:37 -0700, (Dave
Platt) wrote:

In article ,
Jeff Liebermann wrote:
On Tue, 25 Apr 2017 15:39:57 -0400, rickman wrote:

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

Yep. The problem with the alternating coax cable antenna design is
that only every other 1/2 wave section radiates. The result of half
the radiation is half the gain. Or, as you've noticed, the antenna is
twice as long as it might be with phasing elements between 1/2 wave
sections.

Do you have a sim or model of that situation, showing that there aren't
significant RF currents on every other half-wave section? Somehow I
can't make sense of how that would happen.

No, I don't. I'll see what I can find but I don't recall ever seeing
such a model.

My mental model of the alternating-sections design has been that all
of the sections do radiate... the alternating hookup forces them to
radiate in phase with one another, rather than out of phase (and thus
tending to squint the pattern badly upwards and downwards).

Now, I have heard that the alternating-coax collinear doesn't have as
much gain as an array of separate dipoles hooked up with a phasing
harness... but I've always read that as being explained by the fact
that the upper sections are carrying smaller RF currents than the
lower because some power has been radiated away before the signal
reaches the upper part of the antenna.

On every other 1/2 wave element, the wire that carries the signal is
inside a shielded and grounded conductor. I don't think it's going to
radiate.

So, I'd appreciate enlightenment here!

Well, I'll see what I can find and do. If necessary, I'll throw
together a model. Modeling coax cables with NEC2 might be difficult
or impossible, but I'll see if I can fake it:
"The Dipole and the Coax"
https://www.antennex.com/w4rnl/col0606/amod100.html
"Neither software core (NEC and MININEC) is capable of physically
modeling conventional coaxial cables. The transmission line
function within NEC creates lossless non-radiating mathematical
models of lines and hence cannot capture common mode radiation.
Therefore, the method used to show common mode radiation is to
place a third leg into the dipole."
and so on... This is probably more than I want to attempt without
some study time.

"Collinear antenna structure"
https://www.google.com/patents/US6771227

"Collinear antenna of the alternating coaxial type"
https://www.google.com/patents/US20040125038

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