What is the gain of a dipole over an imaginary isotropic antenna? I
believe I read 7.x somewhere. Whatever it was, it was greater than
anything I expected.

Thanks
Buck

--
73 for now
Buck
N4PGW

Buck wrote:
What is the gain of a dipole over an imaginary isotropic antenna? I
believe I read 7.x somewhere. Whatever it was, it was greater than
anything I expected.

In *free space* the gain of a dipole over isotropic is about 2.14 dB.
Over ground, the reflections from the ground add another few dB.
dBi is *always* referenced to free space. An isotropic over perfect
ground would probably have a gain of about 3 dBi assuming no destructive
interference. A dipole over perfect ground has a gain of about 7.5 dBi.
Rule of thumb: A dipole over average ground has a gain of about 6 dBi.
--
73, Cecil http://www.qsl.net/w5dxp


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Buck--
See: http://www.maxstream.net/helpdesk/article-27
Gain of a dipole is 2.15 dB over an Isotropic - but
qualify that as in a direction perpendicular to the
direction a dipole points! a deep null in signal
occures in the directions that a dipole POINTS.
If vertical polarized , there is a deep null of the top,
and bottom of the dipole- maximum radiation around the
circumfrence , perpendicular to the dipole. (kinda like
a donut) . The only time you will see bigger numbers
is do to sales hype, or- because someone has included
ground reflections into the gain situation ! At least
a couple of antennas on E-BAY, and other places have used
this when "advertiseing" their antennas- If an antenna
pattern looks more like a punk rocker's hair-do, then the
goodyear blimp, or for a dipole/longwire, like a cigar,
the pattern shown has considerable ground reflections,
with undesireable angles of radiation. (at least that
the best way I can describe the situation, someone)
do better??). Jim NN7K

Buck wrote:
What is the gain of a dipole over an imaginary isotropic antenna? I
believe I read 7.x somewhere. Whatever it was, it was greater than
anything I expected.

Thanks
Buck

On Sun, 03 Apr 2005 17:15:03 -0500, Cecil Moore
wrote:

Buck wrote:
What is the gain of a dipole over an imaginary isotropic antenna? I
believe I read 7.x somewhere. Whatever it was, it was greater than
anything I expected.

In *free space* the gain of a dipole over isotropic is about 2.14 dB.
Over ground, the reflections from the ground add another few dB.
dBi is *always* referenced to free space. An isotropic over perfect
ground would probably have a gain of about 3 dBi assuming no destructive
interference. A dipole over perfect ground has a gain of about 7.5 dBi.
Rule of thumb: A dipole over average ground has a gain of about 6 dBi.


I was trying to calculate the ERP of his HT into a vertical dipole and
accounting for the loss of RG-58A coax of 25 feet. It seemed his ERP
was outrageous compared to what I would have expected. I calculated
it with the gain of the dipole over an isotropic as referenced in the
ARRL handbook. The only figure in there was the 7.5 dBi gain.

I saw a harendous (sp) number for his ERP so I decided to stop the
explanation until I could confirm this. I figured I was doing
something wrong.

His HT is about 5 watts output.

Thanks
Buck
--
73 for now
Buck
N4PGW

"Jim - NN7K" wrote in message
m...
Buck--
See: http://www.maxstream.net/helpdesk/article-27
Gain of a dipole is 2.15 dB over an Isotropic - but
qualify that as in a direction perpendicular to the
direction a dipole points! a deep null in signal
occures in the directions that a dipole POINTS.
If vertical polarized , there is a deep null of the top,
and bottom of the dipole- maximum radiation around the
circumfrence , perpendicular to the dipole. (kinda like
a donut) . The only time you will see bigger numbers
is do to sales hype, or- because someone has included
ground reflections into the gain situation ! At least
a couple of antennas on E-BAY, and other places have used
this when "advertiseing" their antennas- If an antenna
pattern looks more like a punk rocker's hair-do, then the
goodyear blimp, or for a dipole/longwire, like a cigar,
the pattern shown has considerable ground reflections,
with undesireable angles of radiation. (at least that
the best way I can describe the situation, someone)
do better??). Jim NN7K

Buck wrote:
What is the gain of a dipole over an imaginary isotropic antenna? I
believe I read 7.x somewhere. Whatever it was, it was greater than
anything I expected. Thanks
Buck


Antenna performance figures are affected by many variables, so performance
claims can be very misleading. To try to keep everybody on the same "level
field", the proper way to define performance is to relate gain to an
isotropic reference in free-space conditions. This is nice physics, but
certainly not real life. Extremely few people can place their 20-meter
dipole in free-space. Most humans string their dipole from tree to tree, or
buy as much metal tower as possible. Those trees, other buildings, tower
metal, guy wires, coax cables, other antennas and the proximity of the
ground all modify the antenna performance in difficult to predict ways. Some
might argue that free-space conditions can never exist in real life, so
antenna testing in their back yard is better than free-space projections.
This would be true, if everybody's back yard was electrically identical to
yours. Thus, until everybody has a standardized back yard, free-space,
isotropically referenced data is your best guide.


Ed
WB6WSN

Imagine that you put 100 watts into an isotropic radiator in free space
and measure its field strength at some distance. Then you create a
ground plane like the ones used by most modeling programs, perfectly
flat and infinite in extent, and make it perfectly conductive. Now
distribute that 100 watts evenly in all directions, above the ground
plane -- imagine a sort of hemi-isotropic radiator which does this.
Measure the field strength at the same distance from the radiator as
before. You'll find that the field strength is 3 dB higher, and the
power density twice as great, as for the isotropic source in free space.
It's simply because you're distributing the same amount of power in half
the amount of space.

That illustrates the first principle, that putting an antenna over
ground gives you an automatic 3 dB increase in average field strength
compared to free space. If you compare the field strength of an antenna
over ground with a free space isotropic source (i.e., give its gain in
dBi), you'll find it has a gain of 3 dBi when averaged in all
directions. (EZNEC's "Average Gain" calculation automatically takes this
into account and normalizes to 0 dB when a ground is present; NEC-2's
average gain calculation doesn't.)

You can also pick up about 3 dB additional gain because of the increased
directivity you get when putting an antenna over ground. You get the
same pattern you'd have if there were another antenna below the ground
(an "image" antenna) and with out of phase current relative to the
actual antenna -- it's like having a two antenna beam. (Pattern gain
won't usually be exactly 3 dB because of the impedance change of the
antenna caused by interaction with the ground -- when the ground is
perfect, it's exactly the same as mutual coupling to the "image"
antenna.) So typically a dipole over ground has a gain somewhere between
about 3 and 6 dBi -- around 4 or 5 is common for most typical grounds
and heights.

Comparing the gain of an antenna to that of a dipole over ground
requires calculating or measuring the gain of the dipole over ground --
you can't just assume some gain. The most universally used way is to get
both gains in dBi, since dBi is universally defined and understood, then
subtract to find the difference.

Roy Lewallen, W7EL

Cecil Moore wrote:

In *free space* the gain of a dipole over isotropic is about 2.14 dB.
Over ground, the reflections from the ground add another few dB.
dBi is *always* referenced to free space. An isotropic over perfect
ground would probably have a gain of about 3 dBi assuming no destructive
interference. A dipole over perfect ground has a gain of about 7.5 dBi.
Rule of thumb: A dipole over average ground has a gain of about 6 dBi.

"Roy Lewallen" wrote in message
...
Imagine that you put 100 watts into an isotropic radiator in free space
and measure its field strength at some distance. Then you create a ground
plane like the ones used by most modeling programs, perfectly flat and
infinite in extent, and make it perfectly conductive. Now distribute that
100 watts evenly in all directions, above the ground plane -- imagine a
sort of hemi-isotropic radiator which does this. Measure the field
strength at the same distance from the radiator as before.

find that the field strength is 3 dB higher, and the
power density twice as great,

That's interesting! That part regarding change of "density"
How does one determine the density changes in various parts of a radiation
pattern?
Does the Laws of Partial Pressures apply?
Does high density radiation penetrate the earths layers more than low
density
R.F. radiation before it returns to earth? If R.F. has density then it must
have mass
thus gravity can return radiation to earth at odd places on earth. Grin
Regards
Art:



as for the isotropic source in free space.
It's simply because you're distributing the same amount of power in half
the amount of space.

That illustrates the first principle, that putting an antenna over ground
gives you an automatic 3 dB increase in average field strength compared to
free space. If you compare the field strength of an antenna over ground
with a free space isotropic source (i.e., give its gain in dBi), you'll
find it has a gain of 3 dBi when averaged in all directions. (EZNEC's
"Average Gain" calculation automatically takes this into account and
normalizes to 0 dB when a ground is present; NEC-2's average gain
calculation doesn't.)

You can also pick up about 3 dB additional gain because of the increased
directivity you get when putting an antenna over ground. You get the same
pattern you'd have if there were another antenna below the ground (an
"image" antenna) and with out of phase current relative to the actual
antenna -- it's like having a two antenna beam. (Pattern gain won't
usually be exactly 3 dB because of the impedance change of the antenna
caused by interaction with the ground -- when the ground is perfect, it's
exactly the same as mutual coupling to the "image" antenna.) So typically
a dipole over ground has a gain somewhere between about 3 and 6 dBi --
around 4 or 5 is common for most typical grounds and heights.

Comparing the gain of an antenna to that of a dipole over ground requires
calculating or measuring the gain of the dipole over ground --
you can't just assume some gain. The most universally used way is to get
both gains in dBi, since dBi is universally defined and understood, then
subtract to find the difference.

Roy Lewallen, W7EL

Cecil Moore wrote:

In *free space* the gain of a dipole over isotropic is about 2.14 dB.
Over ground, the reflections from the ground add another few dB.
dBi is *always* referenced to free space. An isotropic over perfect
ground would probably have a gain of about 3 dBi assuming no destructive
interference. A dipole over perfect ground has a gain of about 7.5 dBi.
Rule of thumb: A dipole over average ground has a gain of about 6 dBi.

On Wed, 06 Apr 2005 00:37:34 GMT, "
wrote:


"Roy Lewallen" wrote in message
...
Imagine that you put 100 watts into an isotropic radiator in free space
and measure its field strength at some distance. Then you create a ground
plane like the ones used by most modeling programs, perfectly flat and
infinite in extent, and make it perfectly conductive. Now distribute that
100 watts evenly in all directions, above the ground plane -- imagine a
sort of hemi-isotropic radiator which does this. Measure the field
strength at the same distance from the radiator as before.

find that the field strength is 3 dB higher, and the
power density twice as great,

That's interesting! That part regarding change of "density"
How does one determine the density changes in various parts of a radiation
pattern?

Jeeze. You use an rf densiometer of course.

Wes Stewart wrote:
On Wed, 06 Apr 2005 00:37:34 GMT, "
wrote:

That's interesting! That part regarding change of "density"
How does one determine the density changes in various parts of a radiation
pattern?


Jeeze. You use an rf densiometer of course.

An alternate method is to put some of the RF into a water-tight and
RF-tight box -- a great application for one of those ammo boxes left
over from survivalist exercises or just plinkin'. Drop the box into a
tub of water and see how much the water level rises. Then open the box
and let the RF out. (Be sure to wear safety glasses. A shielded cod
piece isn't a bad idea either.) Repeat the measurement. You can then
calculate the power density from the difference between the amounts the
water level rises.

Roy Lewallen, W7EL
(a little late this year, but hey, better late than never)

On Wed, 06 Apr 2005 10:24:58 -0700, Roy Lewallen
wrote:

Wes Stewart wrote:
On Wed, 06 Apr 2005 00:37:34 GMT, "
wrote:

That's interesting! That part regarding change of "density"
How does one determine the density changes in various parts of a radiation
pattern?


Jeeze. You use an rf densiometer of course.

An alternate method is to put some of the RF into a water-tight and
RF-tight box -- a great application for one of those ammo boxes left
over from survivalist exercises or just plinkin'. Drop the box into a
tub of water and see how much the water level rises. Then open the box
and let the RF out. (Be sure to wear safety glasses. A shielded cod
piece isn't a bad idea either.) Repeat the measurement. You can then
calculate the power density from the difference between the amounts the
water level rises.

A takeoff on this method would be to leave the rf in the box and
measure the temperature rise of the water. This eliminates the need
for the cod piece.

I'm really surprised that "The 'ole Metrologist", Richard Clark,
didn't already jump in with this.