I am planing on side mounting a 2 meter J-Pole part way up on a tower.
How far from the tower should I mount the antenna? I do plan on making
the final adjustment of the feed point with the antenna mounted up a few
feet on the tower. The radiation pattern is not a big concern. In fact
I would prefer that it favor one quadrant.

Thanks for any advice.

Ron - K0QVF
http://www.southslope.net/~ronton/

Ron, K0QVF wrote:
"How far from the tower should I mount the antenna?"

Supposing you want the least effect on the pattern and impedance, place
the antenna as far as possible from the tower.. Satisfaction is obtained
with much closer spacing. The tower is non-resonant. Its reactance will
impede induced current from a radiator which is really close by. If
there is little induced current there is little re-radiated energy to
foul the pattern.

The 3rd edition of Kraus` "Antennas" has a graph on page 350 of gain in
field intensity versus spacing from a flat reflector. At 0.5 wavelength
the gain is 0 dBi. That`s less than the resonant 1/2-wave antenna alone
which has about 2.14 dB gain over an isotropic.

The graph shows a gain of about 2.14 dBi with a spacing of only 0.1
wavelength spacing. So, anything greater than 0.1 wavelength from the
tower should be fine. That`s 20 cm in the 2-meter band, or about 8
inches. More distance means less coupling and should be better.

Best regards, Richard Harrison, KB5WZI

The only way to get 0 dBi maximum gain from a lossless antenna is to
have a perfectly circular pattern in all azimuths and elevations. It's
not possible to have a maximum gain (that is, gain in the best
direction) less than 0 dBi unless loss is present. This is something
everyone with a basic understanding of antennas should know.

So it seemed to me very unlikely that a dipole spaced a half wavelength
from a reflector would have a perfectly circular pattern and, therefore,
it must have gain greater than 0 dBi in some direction. I don't have
Kraus' third edition (yet), but there's a diagram on p. 546 of the
second edition which I suspect is the same as the one Richard is
referring to. The caption under the graph clearly says that the gain at
0.5 wavelength is 0 dB *relative to a half wave dipole in free space*,
or about 2.15 dBi, not 0 dBi.

If the third edition really says that the gain of a half wave antenna
spaced 0.5 wavelength from a reflector is 0 dBi, it's an error and
should be brought to the editor's attention so it can be corrected.

I also believe that while you might draw some possible parallels, you
can't directly apply the characteristics of an antenna in proximity to
an infinite reflecting plane (as in Kraus) to those of an antenna in
proximity to a tower.

Roy Lewallen, W7EL

Richard Harrison wrote:
. . .
The 3rd edition of Kraus` "Antennas" has a graph on page 350 of gain in
field intensity versus spacing from a flat reflector. At 0.5 wavelength
the gain is 0 dBi. That`s less than the resonant 1/2-wave antenna alone
which has about 2.14 dB gain over an isotropic.

The graph shows a gain of about 2.14 dBi with a spacing of only 0.1
wavelength spacing. So, anything greater than 0.1 wavelength from the
tower should be fine. That`s 20 cm in the 2-meter band, or about 8
inches. More distance means less coupling and should be better.

Best regards, Richard Harrison, KB5WZI

Roy Lewallen wrote:

I also believe that while you might draw some possible parallels, you
can't directly apply the characteristics of an antenna in proximity to
an infinite reflecting plane (as in Kraus) to those of an antenna in
proximity to a tower.


Roy is gallantly trying to be non-commercial about this, so someone else
had better say it:

Download the free demo version of any NEC-based antenna modeling
program, and model the effects of the actual tower geometry. It's the
only way to see what's really happening.

You'll get a good first approximation by modeling the tower as its three
very long vertical legs. Since you're only interested in the effects of
the tower on the J-pole's omnidirectional pattern, you can use any kind
of vertical omni to model the antenna itself - a center-fed dipole for
simplicity.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

I agree, although I have joked with Roy, his EZNEC 4.0 is a serious and
"state of the art" application. Although my experience with applications of
this nature is limited (I have not examined EVERY one) it is a well designed
and thought out work. His efforts are notable...

Regards,
John

"Roy Lewallen" wrote in message
...
The only way to get 0 dBi maximum gain from a lossless antenna is to have
a perfectly circular pattern in all azimuths and elevations. It's not
possible to have a maximum gain (that is, gain in the best direction) less
than 0 dBi unless loss is present. This is something everyone with a basic
understanding of antennas should know.

So it seemed to me very unlikely that a dipole spaced a half wavelength
from a reflector would have a perfectly circular pattern and, therefore,
it must have gain greater than 0 dBi in some direction. I don't have
Kraus' third edition (yet), but there's a diagram on p. 546 of the second
edition which I suspect is the same as the one Richard is referring to.
The caption under the graph clearly says that the gain at 0.5 wavelength
is 0 dB *relative to a half wave dipole in free space*, or about 2.15 dBi,
not 0 dBi.

If the third edition really says that the gain of a half wave antenna
spaced 0.5 wavelength from a reflector is 0 dBi, it's an error and should
be brought to the editor's attention so it can be corrected.

I also believe that while you might draw some possible parallels, you
can't directly apply the characteristics of an antenna in proximity to an
infinite reflecting plane (as in Kraus) to those of an antenna in
proximity to a tower.

Roy Lewallen, W7EL

Richard Harrison wrote:
. . .
The 3rd edition of Kraus` "Antennas" has a graph on page 350 of gain in
field intensity versus spacing from a flat reflector. At 0.5 wavelength
the gain is 0 dBi. That`s less than the resonant 1/2-wave antenna alone
which has about 2.14 dB gain over an isotropic.

The graph shows a gain of about 2.14 dBi with a spacing of only 0.1
wavelength spacing. So, anything greater than 0.1 wavelength from the
tower should be fine. That`s 20 cm in the 2-meter band, or about 8
inches. More distance means less coupling and should be better.

Best regards, Richard Harrison, KB5WZI

Roy Lewallen
, W7EL wrote:
"I don`t have Kraus` 3rd edition (yet), but there`s graph on p 546 of
thye second edition which I suspect is the same as the one Richard is
referring to."

I`m sure that`s it. I have Kraus` 1950 edition of "Antennas" and the
identical groph is on page 327 in it.

If you look at the patterns of a 1/2-wavelength antenna at spacings of
1/4, 1/2, and 1/16 wavelengths spacing from a flat reflector nearby,
they are all nearly circular, indicating little distortion in their
unblocked direction.

Best regards, Richard Harrison, KB5WZI

Roy, I looked at the graph and get a different interpretation. Every spacing
except 1/2 wave length spacing shows gain. That being the case the pattern
must be distorted for all cases except .5 wavelength.

I have Kraus 1950 edition

"Richard Harrison" wrote in message
...
Roy Lewallen
, W7EL wrote:
"I don`t have Kraus` 3rd edition (yet), but there`s graph on p 546 of
thye second edition which I suspect is the same as the one Richard is
referring to."

I`m sure that`s it. I have Kraus` 1950 edition of "Antennas" and the
identical groph is on page 327 in it.

If you look at the patterns of a 1/2-wavelength antenna at spacings of
1/4, 1/2, and 1/16 wavelengths spacing from a flat reflector nearby,
they are all nearly circular, indicating little distortion in their
unblocked direction.

Best regards, Richard Harrison, KB5WZI

I have to apologize. I also misinterpreted the graph. The confusing
graph is, in the Second Edition, Fig. 12-4 on p. 546. I've uploaded it
temporarily to http://eznec.com/misc/Kraus2_Fig_12-4.JPG.

The caption says that the gain is relative to a half wave dipole in free
space with the same power input. The numbers on the left side Y axis are
the numerical gain, 0 to 3, relative to a dipole. 0 represents a
numerical gain mulitplier of zero, or no field intensity at all. 1 is
the gain of a half wave dipole in free space (about 2.15 dBi). A value
of 2 represents a gain in field intensity by a factor of 2, or 6 dB
relative to a dipole. The right hand side Y axis labels are the gain in
dBi. Note that 2.1 dBi corresponds approximately with the value of 1.0
on the left side. The bottom horizontal line corresponds to zero field
strength -- a gain of minus infinity dBi -- *not* zero dBi as Richard
said, or zero dB relative to a dipole, which I initially assumed.

What I missed was that the gain is "in direction [phi] = 0", quoting
from the caption. So this isn't a graph of the maximum gain, but the
*gain in one specific direction* -- normal to the reflecting plane. At
0.5 wavelength spacing, the "gain in field intensity" (left set of Y
axis labels) is a *factor* of zero, meaning that the field strength is
zero, or minus infinity dBi. Sure enough, if you model the antenna, or
two elements spaced one wavelength, you find that the pattern has a null
directly broadside to the antenna ([phi] = 0). It has gain in other
directions, but that's not what the graph is showing.

Of course, any lossless antenna has a gain of 0 dBi in some directions.
In the case of the element and reflecting plane, the gain directly
broadside to the antenna has a gain of 0 dBi at spacings of roughly
0.425 and 0.575 wavelengths. There's no particular significance to this
-- the maximum gain is greater in other directions.

These gains and patterns can easily be seen with any modeling program,
including the EZNEC demo, by modeling a dipole over perfect ground. You
can also model two elements fed 180 degrees out of phase at twice the
spacing and no ground and see that the pattern is identical except for
being bidirectional.

Roy Lewallen, W7EL

Fred W4JLE wrote:
Roy, I looked at the graph and get a different interpretation. Every spacing
except 1/2 wave length spacing shows gain. That being the case the pattern
must be distorted for all cases except .5 wavelength.

I have Kraus 1950 edition

"Richard Harrison" wrote in message
...

Roy Lewallen
, W7EL wrote:
"I don`t have Kraus` 3rd edition (yet), but there`s graph on p 546 of
thye second edition which I suspect is the same as the one Richard is
referring to."

I`m sure that`s it. I have Kraus` 1950 edition of "Antennas" and the
identical groph is on page 327 in it.

If you look at the patterns of a 1/2-wavelength antenna at spacings of
1/4, 1/2, and 1/16 wavelengths spacing from a flat reflector nearby,
they are all nearly circular, indicating little distortion in their
unblocked direction.

Best regards, Richard Harrison, KB5WZI

*Chuckle* Yes, they're nearly circular for close spacings (1/8 and 1/16
wavelength), but those aren't "circular patterns". The antenna isn't at
the center of the circle -- it's on the circumference(*). I hope you're
not seriously presenting this as evidence that the gain can be zero dBi.

The patterns and gains are, or should be, identical to those of half of
a W8JK array with twice the spacing. For example, the pattern and gain
of a dipole 1/8 wavelength above a perfect reflector is exactly the same
as half the pattern of a W8JK array made of two of those dipoles spaced
1/4 wavelength. And those are far from an isotropic pattern.

(*) The patterns in the book also are circular because of the scale
factor which was chosen. If some other scale factor were chosen, they
wouldn't be circular. A truly circular pattern (one with the antenna at
the center) is circular regardless of the scale. You can illustrate this
with the EZNEC demo or standard program. Open the dipole1.EZ example
file and click FF Plot to generate a 2D pattern. Notice that the two
lobes are roughly circular in shape (with the antenna at the
circumference, as in Kraus' diagrams). Leaving the 2D plot on the
screen, in the main window Options menu, select 2D Plot Scale and choose
Linear dB. Note how the shape of the two lobes changes. You can get a
wide variety of shapes by changing the scale -- this technique is very
useful to antenna manufacturers to make their beam lobes look extra
narrow. In the main window, change the Plot Type to Elevation. Due to
the orientation of the antenna, you'll now get a plot of the pattern
looking end-on to the dipole. This will be a truly circular pattern.
Click FF Plot to generate the pattern. Change the 2D Plot Scale back to
ARRL Type and note that it remains circular. An isotropic antenna has a
circular pattern like this regardless of the orientation; the 3D plot is
a sphere. And that's the only antenna which can have a free space gain
as low as 0 dBi when there's no loss.

Roy Lewallen, W7EL

Richard Harrison wrote:
Roy Lewallen
, W7EL wrote:
"I don`t have Kraus` 3rd edition (yet), but there`s graph on p 546 of
thye second edition which I suspect is the same as the one Richard is
referring to."

I`m sure that`s it. I have Kraus` 1950 edition of "Antennas" and the
identical groph is on page 327 in it.

If you look at the patterns of a 1/2-wavelength antenna at spacings of
1/4, 1/2, and 1/16 wavelengths spacing from a flat reflector nearby,
they are all nearly circular, indicating little distortion in their
unblocked direction.

Best regards, Richard Harrison, KB5WZI

I believe that all three people (Richard, Roy, and Fred) who have
commented on this topic have misread the figure in Kraus. The gain at
one-half wave spacing from the reflector is *zero*, not zero dB
(relative to a dipole), nor zero dBi.

As expected from the simple mirror image model there is complete far
field cancellation in the phi = 0, theta = 0 direction. That is the only
direction referenced in the figure.

73,
Gene
W4SZ



Roy Lewallen wrote:
The only way to get 0 dBi maximum gain from a lossless antenna is to
have a perfectly circular pattern in all azimuths and elevations. It's
not possible to have a maximum gain (that is, gain in the best
direction) less than 0 dBi unless loss is present. This is something
everyone with a basic understanding of antennas should know.

So it seemed to me very unlikely that a dipole spaced a half wavelength
from a reflector would have a perfectly circular pattern and, therefore,
it must have gain greater than 0 dBi in some direction. I don't have
Kraus' third edition (yet), but there's a diagram on p. 546 of the
second edition which I suspect is the same as the one Richard is
referring to. The caption under the graph clearly says that the gain at
0.5 wavelength is 0 dB *relative to a half wave dipole in free space*,
or about 2.15 dBi, not 0 dBi.

If the third edition really says that the gain of a half wave antenna
spaced 0.5 wavelength from a reflector is 0 dBi, it's an error and
should be brought to the editor's attention so it can be corrected.

I also believe that while you might draw some possible parallels, you
can't directly apply the characteristics of an antenna in proximity to
an infinite reflecting plane (as in Kraus) to those of an antenna in
proximity to a tower.

Roy Lewallen, W7EL

Richard Harrison wrote:

. . .
The 3rd edition of Kraus` "Antennas" has a graph on page 350 of gain in
field intensity versus spacing from a flat reflector. At 0.5 wavelength
the gain is 0 dBi. That`s less than the resonant 1/2-wave antenna alone
which has about 2.14 dB gain over an isotropic.

The graph shows a gain of about 2.14 dBi with a spacing of only 0.1
wavelength spacing. So, anything greater than 0.1 wavelength from the
tower should be fine. That`s 20 cm in the 2-meter band, or about 8
inches. More distance means less coupling and should be better.

Best regards, Richard Harrison, KB5WZI