Antenna for Marine VHF
 

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

Antenna for Marine VHF
 

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

Antenna for Marine VHF
 

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

Antenna for Marine VHF
 

rickman wrote:
On 4/25/2017 4:24 PM, Rob wrote:
rickman wrote:
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.

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.

Why is the 70 cm band not the 700 mm band or the 0.07 meter band? Not
sure what issue you have with feet other than it not being familiar
perhaps. As much as I use metric, feet and inches are still ingrained
in my soul. When I look at a flag pole I don't think, geeze, that's 10
meters high! I think 30 feet. It's that simple.

Maybe yes, but it should be obvious that when you are discussing
antennas for the "2 meter band" their dimensions will be nice multiples
of those same 2 meters.

When I think about a "halfwave dipole for 2 meters" I think "1 meter",
not "3 1/4 feet" or "39 inches". So when you want to discuss antenna
sizes I think it would be more convenient to use the same units all
around.

For me, that of course is meters. Scaling like "milli" or "centi"
is natural in the metric system, I don't have to think about that.
(contrary to converting feet to inches or meters to feet)

Antenna for Marine VHF
 

On 4/26/2017 3:55 AM, Rob wrote:
rickman wrote:
On 4/25/2017 4:24 PM, Rob wrote:
rickman wrote:
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.

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.

Why is the 70 cm band not the 700 mm band or the 0.07 meter band? Not
sure what issue you have with feet other than it not being familiar
perhaps. As much as I use metric, feet and inches are still ingrained
in my soul. When I look at a flag pole I don't think, geeze, that's 10
meters high! I think 30 feet. It's that simple.

Maybe yes, but it should be obvious that when you are discussing
antennas for the "2 meter band" their dimensions will be nice multiples
of those same 2 meters.

When I think about a "halfwave dipole for 2 meters" I think "1 meter",
not "3 1/4 feet" or "39 inches". So when you want to discuss antenna
sizes I think it would be more convenient to use the same units all
around.

For me, that of course is meters. Scaling like "milli" or "centi"
is natural in the metric system, I don't have to think about that.
(contrary to converting feet to inches or meters to feet)

So what should I have said rather than 20 feet?

--

Rick C

Antenna for Marine VHF
 

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.

Antenna for Marine VHF
 

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