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Author Topic: a 9 mdb gain jpole?  (Read 4970 times)

Posts: 4

« on: August 23, 2008, 11:23:49 PM »

someone stumbles on to this thru another forum...

take a look...theres just no way a jpole can achieve 9 db gain.  this guy claims they can...and hes an extra class ham!  he outta be ashamed of himself!

another ham, quite erroneously, i might add, took these specs and is seeling these for 11 meters!

both should know better!

Posts: 17044

« Reply #1 on: August 24, 2008, 12:09:29 PM »

Well this probably isn't the worst mis-information that I've seen on the
web, certainly, and it looks like there is some good information in the
article as well.  So I'd suggest that, instead of simply  crying, "this is
wrong", we set about to correct the erroneous assumptions that underly
this claim:  this will give readers a better idea of how to evaluate such
claims in the future.

And you might want to write to the author as well and point out the
correction to them (and the underlying theory).  I've done this before
and the author was grateful for the correction of a long-held assumption
that wasn't correct.

For starters, the gain of a dipole is NOT 3db over a quarter wave
ground plane.  The exact value depends on how you define each
antenna:  whether the ground plane is really over an infinite ground,
and whether the dipole is over the same ground as the ground plane.
But if we build a typical VHF ground plane antenna at a reasonable
height using sloping radials and compare it to a  half wave dipole at
the same height, the gain difference is very small.  In fact, if the ground
plane radials are bent downwards to where they are parallel to the
transmission line, it BECOMES a dipole, therefore both antennas have
the same gain.  With flat radials the maximum difference is about 1dB,
and with the typical slant to get a 50 ohm input impedance the
difference is about 0.5 dB or so.  (Since an isotropic antenna is just
over 2dB down from a dipole, it is unlikely to have any efficient antenna
that is 3dB down from a dipole!)

The next error is the 3dB improvement when adding a 1/2 wave section.
First, this only applies with sufficient spacing between the two elements,
which isn't the case here.  (Two colinear dipoles end-to-end give about
2dB gain over a single one rather than 3dB.  On 2m you need about
2' spacing between the ends of the elements to get the 3dB gain.)

And this 3dB improvement (with optimum spacing) applies to DOUBLING
the antenna.  Since adding the third half wave section is no longer
doubling the existing antenna, it doesn't give as much gain as adding
the second one did.  So it only adds a bit more gain to the 2dB over a
dipole from the first two elements, for a total of roughly 3dB over a
dipole, or perhaps 3.5 dB over a ground plane antenna with sloping
radials at the same average height.

Or about the same as an other 8', 2 x 5/8 wave antenna like the
Ringo Range, IsoPole, etc.

That isn't to say that the antenna won't work, of course, just that one
can't expect the stated gain out of the antenna in normal conditions.

Posts: 10


« Reply #2 on: September 09, 2008, 11:22:27 AM »

Once these two hams built thier antennas and tested them, do you think they would still say they get 9db gain knowing other may get ahold af the finished antenna and test them themselves?

Something may not look right in their theory, but what about their finished product?  What if they stll show 9+ db gain?

Are we saying there is now way this can happen?

I ask all these questions because I know the second ham who bult an 11 meter J-Pole off the first hams theory, but then spent another year in perfecting it.  Then came out and tested the finished product and was getting over 9db gain when he was done.

Is this not possible?

Posts: 17044

« Reply #3 on: September 09, 2008, 03:02:49 PM »

It is possible that one antenna was 9dB better than the
other, but is is not possible that such an antenna can have
9dB of gain over a dipole (or other half wave vertical
antenna) in the same location.

Note that when dealing with dB, you have to specify a
reference:  an antenna doesn't just have "9 dB gain", but
rather "9 dB gain over some reference antenna".  Now, by
using a lossy antenna for a reference (as some manufacturers
do) you can inflate the gain figures. Or you can compare
the gain of the antenna over ground with an isotropic
antenna in free space and claim it is all due to the
antenna (when in fact much of the gain is due to
the reflecting ground underneath.) Or you can simply let
the marketing department invent numbers that have no basis
in engineering principles.  All of these are ways to
improve the gain figures of an antenna, but they don't
mean that the antenna actually will show the reported
gain in the real world.

But, back to the antenna at hand, the gain of a collinear
antenna (such as the one in question) is a function of
the length, presuming that the sections are phased and
fed in an appropriate manner.  Two half wave elements
end-to-end have a gain of about 2 dB over a dipole in
the same location (presuming that the bottom element is
high enough off the ground.)  

Let's give it some numbers for 11 meters:

quarter wave ground plane with sloping radials = 8.5 foot
radiator = -0.5 dBd (slightly less than a dipole)

half wave antenna = 17 feet = 0dBd (no gain over a dipole,
because it IS a dipole.)

two x half wave = 34 feet = 1.8 dBd (the antenna in question.)

2 x 5/8 wave = 43 feet = 3 dBd

4 x half wave with optimum spacing between sections =
100+ feet = 6dB compared to a dipole at the same height.

So using a half wave dipole as a comparison, I don't see
that the antenna in question can have more than about 2dB
of gain.  You can claim other numbers for it if you are
clear what type of antenna you are using for a reference.  
But you can't get 9dBd of gain in an omnidirectional
antenna less than 200' tall on 11 meters - at least not
in accordance with the laws of physics.

Now I'm always happy to consider new things.  If you think
the antenna really is much better than this, we can
simulate it using EZNEC and ever build a model and test
it against a reference antenna.  But unless there is
something horribly wrong with there reference antenna or
the measurement process, I strongly doubt it will show
the gain reported for it.

Posts: 17044

« Reply #4 on: September 09, 2008, 03:36:32 PM »

OK, I need to make a couple of corrections.  I was remembering
the antenna as 2 x 1/2 wave when it really was 3 x 1/2 wave.
That means the overall height of the radiator on 11m would
be around 50 feet.  (The matching stub doesn't count for
this because it doesn't radiate.)

This puts the total gain a bit more than 3dB over a dipole
at the same height.

You do, however, have to take the mounting height into
account.  With the base of the matching stub at ground
level the top will be around 60 feet.  Compared to a
mobile whip on a car or a standard J-pole with its base
mounted on the ground, this antenna may have more than
the calculated gain because the average height is greater.
On the other hand, if you mounted your reference half
wave antenna on a mast with the same top height, this
antenna would have less measured gain in the real world
than the calculated number because the half wave has
a greater average height:  all the radiation takes place
above 43 feet above ground, while the longer antenna
has more radiation at lower heights above ground.  As
a rough guideline, such a stack of elements will only
show the theoretical gain in practice when the base of
the radiator is at least half the height of the top.  So
if you elevated the bottom of the radiator (which is the
top of the matching stub) to 50 feet and compared it to
a dipole at the same average height (center at about
75 feet) you should see the expected 3dB or so gain.

This all goes to show that gain figures are slippery.
You have to be clear what your reference is, and the
relative location of each with respect to ground.  It is
possible that an antenna such as the one described could
have a gain of 9dB compared to some antenna at a much
lower height, but that the gain difference disappears
when the reference antenna is mounted on a mast with
the tops at the same height.

Posts: 10


« Reply #5 on: September 09, 2008, 06:28:05 PM »

For what I know, the 11 meter J-Pole was compaired to a know beam.  The known Beam did 16DB gain.  The J-Pole at the same hight, 100 feet away gave 1 S unit less than the beam to several stations about 20 miles away.  Then an IMax 2000 was used and placed were the J-Pole was, and it gave 2 S units less.  Taking that, the IMax was doing about 3 DB while the J-Pole was doing about 9db.  Does this sound right?

1326 - Gary

Posts: 578

« Reply #6 on: September 20, 2008, 09:19:55 AM »

Just forget it, your stuck in the dream CB world and not the real testing and measurement world. In the antenna testing world we reference to a 1/2 wave dipole or in the automotive world, a 1/4 wave above a very good ground plane. The J Poles I have measured were within a few tenths of the reference 1/2 wave dipole and nowhere close to 9.0 db better you state.

Posts: 10


« Reply #7 on: September 20, 2008, 03:15:34 PM »

Its simple, the real world is a finished tested product.  The fantasy world is all the foolish talking and arguing about it in forums on the internet.  Believe what you and others write.  I will stick to what I have seen & heard in real life.  I was told by a friend of mine that "No matter how much you think you know about antennas, there is still so much more to learn."

Build & learn I say.  Theories will always just be a theory until you make it reality!


Posts: 5639

« Reply #8 on: September 21, 2008, 12:19:29 PM »

There's a (CB) sucker born every minute, when it comes to outlandish gain claims for antennas.


Lon - W3LK
Naugatuck, Connecticut

A smoking section in a restaurant makes as much sense as a peeing section in a swimming pool.

Posts: 10


« Reply #9 on: September 22, 2008, 04:27:26 PM »

I am a professional radio operator and not a CBer as some may call it.  Test one of KE4DFB's antennas out for yourself.  There is a few dozen of his particular J-Pole out there.  Take what you find and size it up with your theory.  Then tell us all what you "think".  The way some of you act to those who talk on 11 meters sure disappoints me.  Now I see why they call you Amateurs.


Posts: 5639

« Reply #10 on: September 23, 2008, 08:31:43 AM »

<< I am a professional radio operator and not a CBer as some may call it.>>

Call yourself whatever you want; it does not change the facts. Neither does it change the laws of physics as applied to antennas. There is no such thing as a J-pole with a 9 dB gain -  unless maybe one compares it against a rubber duck HT antenna.


Lon - W3LK
Naugatuck, Connecticut

A smoking section in a restaurant makes as much sense as a peeing section in a swimming pool.

Posts: 35

« Reply #11 on: March 26, 2009, 11:49:09 AM »

Quote: For what I know, the 11 meter J-Pole was compaired to a know beam. The known Beam did 16DB gain. The J-Pole at the same hight, 100 feet away gave 1 S unit less than the beam.  end of Quote

First of all, a beam with 16db gain on 11m? my my, gain over what, a cow pile?

But then comes the stated way of measurement, 1Sunit less then the beam? Wow very scientific testing. One S-unit less means what? Unless the tests where done on an antenna range with known condition and certified equipment those tests mean NOTHING. One S-unit in one radio is maybe 1db maybe 3db maybe 10db. None of the  radios have calibrated S-meters they are all more or less just indicators.  Did the stations that did the test verified the meter function with a calibrated step attenuator and inserted verifiable attenuation until the S-meter difference was read?  Next, who verified the gain of the Beam?

All you have stated is that you have an antenna that indicated one S-unit less on an uncalibrated field strength indicator in comparison to another antenna that has a gain that is reverenced to some unknown level using an unknown method of measurement at a distance of ...what 20 miles I guess it was? Wow very scientific....  No wonder some of those outlandish clams have persisted in the CB... ahem professional radio operators world.

I deal with real antennas every day, antennas that cost between $25000 and $100000 and need helicopters to be mounted on towers. You may want to buy or sell your antennas in your "professional" world but let me know to whom, because darn it, I will sell them the bottle of snake oil they just have to have to make them work...Cheap, 500 bucks buys you a gallon of virgin olive snake antenna performance enhancing oil.



Posts: 791

« Reply #12 on: March 26, 2009, 02:32:21 PM »

With the method of testing, it is clear that these people donot know about distant reciever 'agc' actions altering the S meter reading.
Know nothing about the fact that pattern shape is where any gains or losses come from.
Nothing about the standards of compairison.
Typical CB style thinking ham or not. Makes little difference.
Just think if these people were giving a paper at a conference. They would be laughed out of the room for being to scientific.@#&
BTW what is 9 mdb? 9 million d bees,  9 mega d-bees or what!
Then bring this crap to a ham forum and expect to greeted as some kind of antenna break-through.

Posts: 14329

« Reply #13 on: March 27, 2009, 05:15:03 PM »

I think you made a couple of poor assumptions in your measurements:

1) You assumed that the reference yagi really has 16dB gain over a dipole

2) You assumed that your receiver S-Meter is accurately calibrated at 6dB per S-Unit.

Bob  AA4PB
Garrisonville, VA

Posts: 2415

« Reply #14 on: March 28, 2009, 11:37:43 PM »

J pole type antennas achieve 9dbd (and more) all the time.  They do so with an ARRAY of them in phase.

So if a 9db set up on two meters is about 20 feet tall, That would make a similar set up on 11 meters about only 140 or so feet tall!

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