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 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

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

On 4/25/2017 10:12 PM, Jeff Liebermann wrote:
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.

Huh, what? Doesn't the shield carry the signal as well? Every "other"
section is exactly the same as the non-other sections. Which ones
radiate and which other-ones don't? Maybe I've got the wrong image in
my mind. I thought each section was coax but they connect wire to
shield both ways at each junction.

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


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



--

Rick C

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

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).

Sheesh. I'm an idiot. I just noticed that I already have a model of
an alternating coax collinear on my own web pile:
http://802.11junk.com/jeffl/antennas/CoaxVert/index.html
It's not my design. I vaguely recall asking some questions about this
antenna, and someone sent it to me. Notice in the Geometry that every
other 1/2 wave elements do the radiating. The comments are useful:

CM Coaxial Vertical Antenna, converted with 4nec2 on 28-Nov-08 22:18
CM This "Franklin" array model was created by Linley Gumm,
CM K7HFD. Coaxial cable is modeled as a combination of
CM transmission line model, to represent the inside of the
CM coax, and a wire to represent the outside. The technique is
CM described in the EZNEC manual. See "Coaxial Cable,
CM Modeling" in the index.

Notice that if you replace the non-radiating 1/2 wave coax delay line
sections with a 1/4 wave wire stubs (or end shorted ladder line), the
antenna is now about half its previous length, with no loss in gain,
and probably little change in pattern.

Also:
https://ukradioscanning.com/viewtopic.php?f=5&t=3028
Note that the drawing shows that all elements radiation, but the
nearby comments say "(on outer conductor for radiation)" which means
that only those segments that have exposed outer shield conductors
connected to the feed coax center conductor, do the radiating. That's
every other 1/2 wave segment.




--
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 22:38:56 -0700, Jeff Liebermann
wrote:

Sheesh. I'm an idiot. I just noticed that I already have a model of
an alternating coax collinear on my own web pile:
http://802.11junk.com/jeffl/antennas/CoaxVert/index.html

I fixed a few things, made some better images, and renamed the folder:
http://www.11junk.com/jeffl/antennas/CoaxVertical/index.html

Looks like the 1/4 wave section at the top is missing. I'll fix it
tomorrow. It's late, I'm tired, etc...

--
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 ,
Jeff Liebermann wrote:
On Tue, 25 Apr 2017 22:38:56 -0700, Jeff Liebermann
wrote:

Sheesh. I'm an idiot. I just noticed that I already have a model of
an alternating coax collinear on my own web pile:
http://802.11junk.com/jeffl/antennas/CoaxVert/index.html

I fixed a few things, made some better images, and renamed the folder:
http://www.11junk.com/jeffl/antennas/CoaxVertical/index.html

Looks like the 1/4 wave section at the top is missing. I'll fix it
tomorrow. It's late, I'm tired, etc...

Thanks! I'll pull down this model, read through it so I understand
it, and run a few sims myself.

On Wed, 26 Apr 2017 11:23:23 -0700, (Dave
Platt) wrote:

In article ,
Jeff Liebermann wrote:
On Tue, 25 Apr 2017 22:38:56 -0700, Jeff Liebermann
wrote:

Sheesh. I'm an idiot. I just noticed that I already have a model of
an alternating coax collinear on my own web pile:
http://802.11junk.com/jeffl/antennas/CoaxVert/index.html

I fixed a few things, made some better images, and renamed the folder:
http://www.11junk.com/jeffl/antennas/CoaxVertical/index.html

Looks like the 1/4 wave section at the top is missing. I'll fix it
tomorrow. It's late, I'm tired, etc...

Thanks! I'll pull down this model, read through it so I understand
it, and run a few sims myself.

Great. Maybe you can explain it to me.

Here's a rec.radio.amateur.antenna 24 article thread on the topic from
Nov 2008:
https://groups.google.com/forum/#!msg/rec.radio.amateur.antenna/DREJnRznluQ/58Z0gIimqdwJ

This is where I totally blew it when I incorrectly declared that most
of the RF comes out of lowest element, and very little out the top.
https://groups.google.com/d/msg/rec.radio.amateur.antenna/DREJnRznluQ/bZyCgwa0JvwJ
That was like saying that in a series string of identical light bulbs,
the lowest lights would be brighter. Argh.

Corrections and comments by Roy Lewallen (W7EL) including the original
model of the alternating coax vertical antenna:
https://groups.google.com/d/msg/rec.radio.amateur.antenna/DREJnRznluQ/LOwnb-eZmjMJ
I converted it to 4NEC2 format. See:
http://eznec.com/misc/rraa/
for original model in EZNEC format. This model does NOT run in the
Demo version of EZNEC 5.0 because it contains 80 segments and the demo
program only allows 20.
https://www.eznec.com

Good luck.


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