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Author Topic: VHF/UHF ground plane vs "J" pole  (Read 3101 times)
K2OWK
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Posts: 1061




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« on: December 17, 2012, 12:36:38 AM »

Hello, Which is the better antenna for use outdoors the "J" pole or the ground plane on 2 meters/70 CM, both antennas at the same hight.

Thanks,

K2OWK
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KQ6Q
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Posts: 976




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« Reply #1 on: December 17, 2012, 01:47:12 AM »

More info needed! How do you plan to mount the antenna ? TV mast, tree, other structure. Are you going to build it yourself, or do you have a particular commercial unit in mind ? What are your worst weather conditions (wind, ice, snow)? Have you considered a Discone ?
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VU2NAN
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Posts: 245




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« Reply #2 on: December 17, 2012, 05:55:56 AM »

Hi OM Barry,

It would be a good experience to try them both!

73,

Nandu.
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NK7Z
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Posts: 790


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« Reply #3 on: December 17, 2012, 08:33:26 AM »

Hi,

Anyone have any experience in mounting a J-pole on the side of a tower?  How far out does it need to be for 2 Meters?

73's
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Thanks,
Dave
For reviews and setups see: http://www.nk7z.net
WB6BYU
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Posts: 13244




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« Reply #4 on: December 17, 2012, 10:42:58 AM »

Quote from: K2OWK
...Which is the better antenna for use outdoors the "J" pole or the ground plane on 2 meters/70 CM...


Both antennas are essentially a half wave vertical radiator, so from a theoretical standpoint
the performance will be nearly equal.

Both antennas are prone to common mode currents, which means that they may work
considerably worse than expected.  (Such currents on the feedline and/or mast can
raise the angle of radiation and cancel some or all of your signal at the horizon.)  So
the biggest difference will depend on the exact construction of the two antennas and
how well they are decoupled from the feedline and/or mast.  My guess is that it probably
is easier to provide sufficient decoupling for a ground plane than for a J-pole, at least
for the common designs found on the internet.  But sometimes it isn't a trivial task,
and may require a second set of radials 1/4 wave below the first.

Beyond that it will depend on the construction materials:  copper will turn green, but
soldered connections won't corrode as badly as with clamped aluminum joints.  (Copper
to aluminum joints are particularly bad.)  Insulators need to be able to withstand
exposure to the sun, and in some cases that may limit the usable lifespan.  The mechanical
properties of the materials are also important:  #14 wire used for radials won't last as
long in the wind as 3/8" aluminum rod.  (It also depends on how many large birds such
as owls or hawks are likely to perch on it.)  The coax connection should be protected
from the rain and snow as well as being waterproofed.


In the end the details of the specific implementation will make much more difference
in performance (especially over time) than the choice between the two designs.

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K4SAV
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Posts: 1840




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« Reply #5 on: December 17, 2012, 11:40:39 AM »

My guess is that it probably is easier to provide sufficient decoupling for a ground plane than for a J-pole, at least for the common designs found on the internet.  But sometimes it isn't a trivial task, and may require a second set of radials 1/4 wave below the first.

Exactly what I did with the last 2 meter ground plane I built.  I ran an EZNEC analysis of a quarter wavelength ground plane with 4 sloping radials, with the antenna at 30 ft, and found that for "unlucky" lengths of feedline I could loose at much as 6 dB of low angle gain due to common mode currents on the coax.  I added a second set of radials 1/4 wavelength down from the first set and the problem went away.

A J-pole solution would be similar, but it wouldn't look much like a J-pole with two sets of radials.

Jerry, K4SAV
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VE7TIT
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Posts: 62




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« Reply #6 on: December 18, 2012, 07:12:10 AM »

Quote
copper will turn green, but soldered connections won't corrode as badly as with clamped aluminum joints.

Corrosion? Paint it, amigo! Wink
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HAMFESTS
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Posts: 34




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« Reply #7 on: December 18, 2012, 01:43:17 PM »

Exactly what I did with the last 2 meter ground plane I built.  I ran an EZNEC analysis of a quarter wavelength ground plane with 4 sloping radials, with the antenna at 30 ft, and found that for "unlucky" lengths of feedline I could loose at much as 6 dB of low angle gain due to common mode currents on the coax.  I added a second set of radials 1/4 wavelength down from the first set and the problem went away.

Jerry, K4SAV


Jerry,

Wouldn't putting ferrite beads near the feedpoint on the coax do the same thing as the second set of radials 1/4 wavelength down from the first set?
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K4SAV
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Posts: 1840




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« Reply #8 on: December 18, 2012, 05:54:47 PM »

Wouldn't putting ferrite beads near the feedpoint on the coax do the same thing as the second set of radials 1/4 wavelength down from the first set?

That might work, but you will have to put the chokes on the mast as well as the coax (unless you have a non-conductive mast, and that might be bad for lightning protection).
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HAMFESTS
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Posts: 34




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« Reply #9 on: December 19, 2012, 10:00:01 AM »

Jerry,

Thanks for the reply. When the weather gets warm again I hope to do some antenna work.

73
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KB4QAA
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Posts: 2371




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« Reply #10 on: December 19, 2012, 10:18:32 AM »

I would use a 1/4L ground plane every time.  They are simpler to construct, broader bandwidth, easier to tune (if required at all), and LESS likely to have common mode problems.  A few beads can be added to the coax if desired.

Oh, it's also ever so slightly better gain than a Jpole!

The world has gone mad over Jpoles, and OCF antennas. 
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KA7NIQ
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Posts: 258


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« Reply #11 on: January 02, 2013, 06:32:03 PM »

I would use a 1/4L ground plane every time.  They are simpler to construct, broader bandwidth, easier to tune (if required at all), and LESS likely to have common mode problems.  A few beads can be added to the coax if desired.

Oh, it's also ever so slightly better gain than a Jpole!

The world has gone mad over Jpoles, and OCF antennas. 
You are SO Right Bill!
W8JI taught me about common mode, and what it can and will do to the performance of an antenna. Before learning from W8JI, I thought common mode was "no big deal", and other then a hot mike, nothing to worry about.
W8JI also told me about a neat little MFJ current probe, that allows you to see and measure it!

I see no "majic" in J Poles either, and you could not give me one. I will take a 1/4 wave ground plane every time.
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