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Author Topic: What does Closed Collinear antenna mean?  (Read 2407 times)
MDNITERDER
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Posts: 146




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« on: September 04, 2012, 12:15:05 PM »



 I have a Laird/Antenex CW4405C Wideband UHF Antenna. Some sites say 5/8 over 5/8 but im lost on what this really is, I have never heard of a 5/8 over 5/8 or closed collinear antenna.

 does this mean this antenna is ground plane independent, Is this a good or bad design vs a normal 5/ wave.

 Any ideas anyone?
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W4OP
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« Reply #1 on: September 04, 2012, 12:36:28 PM »

I am not sure about the closed term but a 5/8 over (an inverted) 5/8 offers more gain than, say stacked half wave radiators as it places the 2 current loops further apart.
Unless it is a voltage loop (max) at the base, it requires a ground plane.

Dale W4OP
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MDNITERDER
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Posts: 146




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« Reply #2 on: September 04, 2012, 12:41:42 PM »

I should also state this is a mobile antenna,
http://lairdtech.thomasnet.com/viewitems/vehicular-mobile-radio-antennas/wideband-antennas
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WB6BYU
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« Reply #3 on: September 04, 2012, 12:56:32 PM »

"Closed coil" means that the coil in the middle of the element is enclosed
rather than being open to the air, like this:

http://www.hamradio.com/detail.cfm?pid=H0-000553

Typically with an "open coil" antenna the coil is wound with the same material as the
whip on either side, so there are no joints.  In my experience, this makes it more
reliable.

Compare it with this:

http://www.hamradio.com/detail.cfm?pid=H0-000567


The "5/8 wave over 5/8 wave" means that the antenna has a 5/8 wave lower radiator,
a loading/phasing coil or other assembly of some sort, and a second 5/8 wave radiator
on top of the first.  This gives some gain over a standard 5/8 wave radiator as long as
the whip isn't tilting over too much at speed.  It is a very common design:  the
effectiveness relates more to the mechanics of the implementation and how it is matched
and mounted, since any antenna of about the same length probably uses the same
internal element configuration.
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MDNITERDER
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Posts: 146




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« Reply #4 on: September 04, 2012, 01:09:42 PM »

One thing I have noticed on this antenna, which I was always under the impression does not happen on 5/8 wave is the picket fence affect on reception.

 I also have a thought that this maybe due to my temp mag mount until my nmo mount arrives
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W4OP
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« Reply #5 on: September 04, 2012, 02:01:17 PM »

The lobe at the horizon is compressed over a single 5/8 wave radiator and that may exacerbate picket fencing.

Dale W4OP
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WB6BYU
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« Reply #6 on: September 04, 2012, 02:08:20 PM »

Picket fencing usually doesn't have anything to do with the antenna (unless there
is an intermittent connection somewhere.)  Instead it is due to multi-path
propagation.

For example, if the direct path to a repeater is blocked by a hill and you are picking
up signals via two different paths - say a reflection off a water tower on the North
side of town, and another reflection off a big metal barn on a hill on the South side
of town.  The actual signal at your receiver will depend on how the two signals
combine at your antenna:  when they are in phase, they add and you have good
reception.  When they are out of phase (and of nearly equal magnitude) then the
two signals cancel each other out, and you get a null.  The relative phase of the
signals depends on the differences in the lengths of the two paths.

A shift of 1/2 wavelengths makes the difference between in-phase and out-of-phase,
and that's only 12" on 440.  So when you are in a bad spot, the received signal fades
out and back in again about every two feet of travel.  That's "picket fencing".

Now, it could also be due to an intermittent connection in the antenna, especially the
type with lots of joints along the length.  One way to tell is to park where the signal
is about half strength and hit the antenna a few times with a stick:  if the signal varies,
it may be a contact issue.  When you are in an area with bad picket-fencing, try driving
slowly forward a few feet and see if you can see the signal go up and down on the
meter.  (I do this at traffic lights to find a good place to stop where I can continue
my conversation.)  If you can see the effect in slow motion when the antenna is
not being subject to vibration, then it is due to propagation rather than a bad
contact.
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N4CR
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« Reply #7 on: September 04, 2012, 03:53:02 PM »

The "5/8 wave over 5/8 wave" means that the antenna has a 5/8 wave lower radiator,
a loading/phasing coil or other assembly of some sort, and a second 5/8 wave radiator
on top of the first.

And in order for the second 5/8 wave to be completely in phase with the first 5/8, the loading coil must be 3/8 wave long. The frequency of these measurements being accurate is typically the middle of the advertised specification.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
W4OP
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Posts: 436


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« Reply #8 on: September 04, 2012, 07:02:03 PM »

Picket fencing usually doesn't have anything to do with the antenna (unless there
is an intermittent connection somewhere.)  Instead it is due to multi-path
propagation.

 Hi Dale,
What you say is absolutely true. But I recall a white paper that Motorola did many years ago that suggested a narrower lobe in the elevation pattern may diminish picket fencing as the 2nd signal many times arrived from a higher angle (local building reflection etc). Although I have not modeled the stacked 5/8, I do recall that  a single 5/8 has the desired low angle  lobe but also a wide higher angle lobe. That's all theory and what happens in the mobile environment may not agree well.

I think of  picket fencing when I pull up to a stop light here in the mountans and move forward a bit to get out of what is likely a multipath null on the NPR FM station I listen to.

Dale W4OP
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WB6BYU
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Posts: 13343




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« Reply #9 on: September 04, 2012, 09:41:52 PM »

Quote from: W4OP

...I recall a white paper that Motorola did many years ago that suggested a narrower lobe in the elevation pattern may diminish picket fencing as the 2nd signal many times arrived from a higher angle (local building reflection etc)...



There are lots of practical aspects to mobile antennas that don't always show up in
models.  For example, since maximum radiation from a 5/8 wave antenna is from the
upper section, it is more prone to polarization shift at higher speeds than a 1/4 wave
whip as the antenna tilts backwards.  Some studies on police cars at high speeds
showed that the quarter wave whip was often just as good as a 5/8 wave whip
in the 150 - 170 MHz range because of this.  (It probably is applicable to ham
antennas at lower speeds using more flexible antennas.)

The angle of arrival will depend on the environment:  on UHF in a downtown city with
lots of tall building that might be the case, but in rolling hills I would think not.

Along the same line:
Quote from: N4CR

And in order for the second 5/8 wave to be completely in phase with the first 5/8, the loading coil must be 3/8 wave long.


Actually it would be 1/4 wavelength, so that combined with the top 1/8 wave of
the lower element and the lower 1/8 wave of the upper element it makes 1/2 wavelength.

But this doesn't always work the way a designer would hope:  being at the high impedance
part of the antenna, it often ends up with a self-resonant inductor rather than just a
"loading coil", and achieving the require phase shift without a ground reference is more
difficult than one would imagine by the number of designs that just use a stub and
assume it works.  They tend to work much better when feed in the center rather than at
one end with some sort of phase-shift network.  It would be interesting to see what the
actual vertical radiation pattern is for many antennas


Quote

I think of  picket fencing when I pull up to a stop light here in the mountans and move forward a bit to get out of what is likely a multipath null on the NPR FM station I listen to.



I run into this at a few intersections on the way to work each day:  I stop well back of
the car ahead of me, then creep forwards to get a good signal.  One quirk, however, is
that because a repeater uses different frequencies for uplink and downlink, sometimes
the peak for received signal puts me in a null for the uplink, so I have to split the
difference.

Multi-path also causes the signal strength to vary with modulation on an FM signal, due
to the different deviation frequencies arriving with varying relative phase shifts over the
two paths.  That distorts the audio, and increasing transmitter power doesn't help.


These are all good exercises for the reader following along at home to see how well they
understand why things happen this way.
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