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Old October 16th 03, 07:47 PM
Serge Stroobandt, ON4BAA
 
Posts: n/a
Default Inverted ground plane antenna: compared with normal GP and low dipole.

Dear Roy, Richard, Art and other readers,

I had a look at that thread (for other readers' convenience included at the
bottom of this message).
Although it is not immediately answering my question whether inverted
groundplane verticals present fewer losses than normal verticals, it
contains some
interesting observations. Again for the sake of the other readers, I took
the liberty to rephrase and summarise these in terms that are more common in
antenna literatu

The electromagnetic field around an antenna exists out of two parts: a near
field and a far field.

Due to maths, the near field rapidly becomes negligible at larger distances,
whereas the far field is the field we use normally to communicate.

Losses in the near and far field can be reduced by reducing ground losses in
the immediate vecinity of the antenna. This can be done through installing
the antenna above "good RF ground", empoying a low impedance radial system
(burried or elevated) for verticals or = 5/8 lambda or choosing a (double
so long!) vertical antenna with a built-in return-path (half square, bobtail
curtain, and half wavelength vertical). NOTE: Both the half square and the
bobtail curtain can be interpreted as an array of 2, respectively 3 inverted
ground plane verticals! (For a brief description see John Devolder, ON4UN,
"Low-Band Dxing," chapter 12)

Losses in the far field are in addition also caused by the ground properties
at larger distances from the antenna, and are therefore less controlable
(apart from choosing a coastal or salt-lake QTH).

Now, my assumption is that by inverting a vertical ground plane, some of the
near field ground losses could be prevented by presenting the high impedance
end of the radiator to the low impedance of the earth immediately
underneath. This tremendous mismatching would cause less coupling of the
antenna's near field into the lossy earth.

Assumptions of the mind are one thing, but eventually the proof of the
pudding is in the eating (and this goes especially to Richard!).
(Art, you see, you don't need to convince me anymore about antenna modelling
;-) However, I do will post a message about a possible bug in AO!)
So I used version 1.0 of Roy's software, called EZNEC to model a couple of
swiftly designed (i.e. non-resonant) antennas.
(Roy, we need to talk about upgrading my version - I will send you a
seperate email for this.)
Here are the results:

Normal 80m (3.650MHz) ground plane vertical
with 4 elevated radials 4.5m above "good" ground (according to Christman
KB8I, see also John Devolder, ON4UN, "Low-Band Dxing," chapter 9)
radiator and radial lengths: 20m
maximum height 24.5m
azimuth 0°: 0.23dBi @ 22° elevation
azimuth 45°: 0.21dBi @ 22° elevation

Inverted 80m ground plane vertical
with 4 elevated radials 24.5m above "good" ground
radiator and radial lengths: 20m
maximum height 24.5m
azimuth 0°: 0.08dBi @ 22° elevation
azimuth 45°: 0.12dBi @ 22° elevation

Result: 0.09-0.15dB in favour of the normal ground plane. The radials up in
the air of the inverted ground plane probably cause some shielding.

The same difference remains when we lower both antennas to 1m above ground:

Normal 80m ground plane vertical
with 4 elevated radials 1m above "good" ground
radiator and radial lengths: 20m
maximum height 21m
azimuth 0°: 0.00dBi @ 24° elevation
azimuth 45°: 0.00dBi @ 24° elevation

Inverted 80m ground plane vertical
with 4 elevated radials 21m above "good" ground
radiator and radial lengths: 20m
maximum height 21m
azimuth 0°: -0.16dBi @ 25° elevation
azimuth 45°: -0.11dBi @ 25° elevation

Being an appartment dweller myself, I once dreamed of putting up an inverted
ground plane antenna using the building as a support for the radiator and
the roof as support and hideawy for the two elevated radials.

Normal 80m ground plane vertical
with 2 elevated radials 1m above "good" ground
radiator and radial lengths: 20m
maximum height 21m
azimuth 0°: -0.86dBi @ 24° elevation
azimuth 45°: -0.66dBi @ 24° elevation
azimuth 90°: -0.45dBi @ 24° elevation

Inverted 80m ground plane vertical
with 2 elevated radials 21m above "good" ground
radiator and radial lengths: 20m
maximum height 21m
azimuth 0°: 0.55dBi @ 29° elevation
azimuth 45°: 0.05dBi @ 26° elevation
azimuth 90°: -0.96dBi @ 21° elevation

Result: Now, the balance turned in favour of the inverted groundplane
vertical in 3 of the 2 azimuth headings, probably because fewer radials for
the inverted GP means less shielding.
Differences for 0°, 45° and 90° azimuth are 1.41dB, 0.71dB and -0.51dB,
respectively.

FIRST TAKE-AWAY MESSAGE: Do not rely on accounts of particular antenna
installation, always model your own antenna installation before jumping to
conclusions.

Ok, but now taking into account Roy's previous thread. How does a low dipole
at the same height of 21m stands out of this?
azimuth 0°: -5.77dBi @ 30° elevation
azimuth 90°: 4.23dBi @ 30° elevation

Result: In its preferred azimuthal direction, a low dipole, even at a height
of about a quarter wavelength, still outperforms any ground plane vertical
antenna. This is an eye-opener, taking into account that low dipoles (a.k.a.
cloud burners) send most of their energy radially away from earth, without
any ionospheric diffraction right into space. Most of the gain of the low
dipole is pattern gain, however. This means that performance in the
perpendicular azimuthal direction is extremely bad with -5.77dBi.

SECOND TAKE-AWAY MESSAGE: Appartment dwellers might find an effective
radiating system in installing a dipole on the roof of their building (may
droop off at the ends) and making it (behind the current balun) remotely
switchable into an inverted ground plane vertical antenna for working the
directions that lie in the dip of the azimuthal dipole pattern. That is, the
current balun should also be employed to make the vertical radiator current
equal to the sum of the currents in the radials.

PS1: I have posted also an exact copy of this message under the subject
"Inverted ground plane antenna: compared with normal GP and low dipole." in
order to attract more readers with this more appropriate title.

PS2: Please, send your comments to the newsgroup under above-mentioned
header. As of tomorrow I will be on a business trip for the next two weeks
and will only be able to respond occasionally.
(Not that I am afraid of you guys!!! :-P)

--
73 de Serge ON4BAA - HB9DWU

propagation & ham info at:
http://salsawaves.com/propagation/




"Roy Lewallen" wrote in message
...
That question has been answered several times in this newsgroup. For one
of the most recent answers, go to groups.google.com and find my posting
on July 21, 2003 in the thread " efficiency of horizontal vs vertical
antennas".

Roy Lewallen, W7EL

Serge Stroobandt, ON4BAA wrote:

This conversation is starting to get interesting :-)

INVERTED GROUNDPLANES:
What about using an inverted 1/4wave groundplane in order to avoid

ground
losses?
Then you have the high impedance point of the antenna close to the lossy
(low impedance) earth.
Due to the severe mismatch less power will couple into the earth.

The low impedance point (feed point) of the antenna is safely high up in

the
(high impedance) air then.


All messages from thread
Message 1 in thread
From: Ron )
Subject: efficiency of horizontal vs vertical antennas


View this article only
Newsgroups: rec.radio.amateur.antenna
Date: 2003-07-15 18:38:29 PST


There doesn't appear to be any ground loss resistance in horizontal antennas
(e.g., dipoles) such as there is in verticals. Does this mean that verticals
are
virtually always less efficient than horizontals due to their ground loss?

RonMessage 2 in thread
From: Roy Lewallen )
Subject: efficiency of horizontal vs vertical antennas


View this article only
Newsgroups: rec.radio.amateur.antenna
Date: 2003-07-15 19:01:01 PST


A vertical dipole, or a base fed half wave vertical, has good efficiency
and no significant ground loss. That is, there's no appreciable loss
from current flowing through the ground to a feedline terminal. For
other verticals, such as a base fed quarter wavelength, ground loss due
to return currents can be reduced to an arbitrarily low value by using a
good enough system of radials.

However, after the wave is launched from the antenna, vertically
polarized signals react differently than horizontally polarized ones
when they strike the ground. Horizontally polarized waves are reflected
with little loss, except ones at high angles. With vertically polarized
waves, it's the low angle ones that take the greatest beating. And since
most vertical antennas tend to concentrate radiation at low angles, a
very sizable fraction of the total radiated power is often lost in the
ground reflection. This mostly takes place well beyond a reasonably
sized radial field, so radials don't help. The only way to reduce this
loss is to see that the reflection takes place from highly conductive
ground, like sea water(*). Otherwise, you lose most of the power that
you want the most.

(*) The effective conductivity of the ground improves as frequency
decreases. So verticals tend to do better at lower frequencies, and
often outperform horizontal dipoles on 160 and 80 meters, even over
average ground.

Roy Lewallen, W7EL

Ron wrote:
There doesn't appear to be any ground loss resistance in horizontal
antennas (e.g., dipoles) such as there is in verticals. Does this mean
that verticals are virtually always less efficient than horizontals due
to their ground loss?

Ron

Message 3 in thread

From: Bob Colenso )
Subject: efficiency of horizontal vs vertical antennas


View this article only
Newsgroups: rec.radio.amateur.antenna
Date: 2003-07-21 15:16:10 PST


But aren't there ground independant verticals, like the Half Square or a
Bob-Tail-Curtain?



U of M
GO BLUE!!!

God, Guns, and Guts
Protect America!!!Message 4 in thread
From: Yuri Blanarovich )
Subject: efficiency of horizontal vs vertical antennas


View this article only
Newsgroups: rec.radio.amateur.antenna
Date: 2003-07-21 15:30:11 PST



But aren't there ground independant verticals, like the Half Square or a
Bob-Tail-Curtain?



U of M
GO BLUE!!!

God, Guns, and Guts
Protect America!!!


Every vertical "needs" the ground for its efficient performance especially
at
the low angles. Vertical dipoles and their elevated cousins are still
"looking"
out at the ground, farther waway and with a bit less of participation.
Wanna see dramatic display of salt water "ground" performance/contribution?
Take your any verticaly polarized antenna and compare its performace between
ground ground and salt water ground. You would see somewhere between 10 - 15
dB
difference.

YuriMessage 5 in thread
From: Roy Lewallen )
Subject: efficiency of horizontal vs vertical antennas


View this article only
Newsgroups: rec.radio.amateur.antenna
Date: 2003-07-21 16:51:50 PST


There are two quite distinct sources of ground loss involved with
vertical antennas. You're talking about one, and Yuri is talking about
the other. Makes for a lively discussion, but it's a lot like the blind
men describing the elephant.

One type of loss is caused when it's necessary to connect one conductor
of the feedline to the Earth. Current returning to this feedline
conductor is equal in magnitude to the current flowing into the antenna
from the other conductor, and it creates a simple I^2 * R loss flowing
through the ground. This loss can be minimized by using a system of
ground radials to decrease the loss resistance R near the base of the
antenna, where the current density is highest. Another solution is to
use a vertical antenna with a high feedpoint resistance. Examples are
the half square, bobtail curtain, and half wavelength vertical. These
antennas require very little feedpoint current, and consequently very
little ground current. They can be very efficient with only a very
simple ground system.

But there's another source of loss, encountered after the signal is
radiated. When a vertically polarized signal strikes the ground, a lot
of its energy is lost to heating of the ground. This is particularly
true at low angles of incidence. The end result is severe attenuation of
low angle radiation. Particularly for low angles, this occurs farther
away from the antenna than a reasonable radial system extends. So you're
stuck with this loss, unless you can physically move your antenna to a
swamp or similar high-conductivity environment. Horizontally polarized
waves react differently.

The demo version of EZNEC will show this quite dramatically. If you
choose the MININEC type ground model, it acts like you have a perfect
radial system. That is, the first source of loss I mentioned is zero.
But the second is still there. You can simulate the effect of ground
system loss simply by adding a resistive "load" at the antenna base.
Compare the patterns of a vertical and horizontal, *to the same scale*,
by superimposing them on a 2D plot, using different qualities of ground.
You'll find it quite educational.

Roy Lewallen, W7EL

Yuri Blanarovich wrote:
But aren't there ground independant verticals, like the Half Square or a
Bob-Tail-Curtain?



U of M
GO BLUE!!!

God, Guns, and Guts
Protect America!!!



Every vertical "needs" the ground for its efficient performance especially

at
the low angles. Vertical dipoles and their elevated cousins are still

"looking"
out at the ground, farther waway and with a bit less of participation.
Wanna see dramatic display of salt water "ground"

performance/contribution?
Take your any verticaly polarized antenna and compare its performace

between
ground ground and salt water ground. You would see somewhere between 10 -

15 dB
difference.

Yuri Message 6 in thread

From: luke )
Subject: efficiency of horizontal vs vertical antennas


View this article only
Newsgroups: rec.radio.amateur.antenna
Date: 2003-07-22 07:12:04 PST


Hi,

Good answers to the question !

Always walk away a little smarter after reading this group !

73 luke

Roy wrote:

There are two quite distinct sources of ground loss involved with
vertical antennas. You're talking about one, and Yuri is talking about
the other. Makes for a lively discussion, but it's a lot like the blind
men describing the elephant.

One type of loss is caused when it's necessary to connect one conductor
of the feedline to the Earth. Current returning to this feedline
conductor is equal in magnitude to the current flowing into the antenna
from the other conductor, and it creates a simple I^2 * R loss flowing
through the ground. This loss can be minimized by using a system of
ground radials to decrease the loss resistance R near the base of the
antenna, where the current density is highest. Another solution is to
use a vertical antenna with a high feedpoint resistance. Examples are
the half square, bobtail curtain, and half wavelength vertical. These
antennas require very little feedpoint current, and consequently very
little ground current. They can be very efficient with only a very
simple ground system.

But there's another source of loss, encountered after the signal is
radiated. When a vertically polarized signal strikes the ground, a lot
of its energy is lost to heating of the ground. This is particularly
true at low angles of incidence. The end result is severe attenuation of
low angle radiation. Particularly for low angles, this occurs farther
away from the antenna than a reasonable radial system extends. So you're
stuck with this loss, unless you can physically move your antenna to a
swamp or similar high-conductivity environment. Horizontally polarized
waves react differently.

The demo version of EZNEC will show this quite dramatically. If you
choose the MININEC type ground model, it acts like you have a perfect
radial system. That is, the first source of loss I mentioned is zero.
But the second is still there. You can simulate the effect of ground
system loss simply by adding a resistive "load" at the antenna base.
Compare the patterns of a vertical and horizontal, *to the same scale*,
by superimposing them on a 2D plot, using different qualities of ground.
You'll find it quite educational.

Roy Lewallen, W7EL

Read the rest of this message... (24 more lines)

Message 7 in thread
From: sideband )
Subject: efficiency of horizontal vs vertical antennas


View this article only
Newsgroups: rec.radio.amateur.antenna
Date: 2003-07-22 19:01:10 PST



Really? Even a vertical dipole?

de AI8W, Chris

Yuri Blanarovich wrote:
But aren't there ground independant verticals, like the Half Square or a
Bob-Tail-Curtain?



U of M
GO BLUE!!!

God, Guns, and Guts
Protect America!!!



Every vertical "needs" the ground for its efficient performance especially

at
the low angles. Vertical dipoles and their elevated cousins are still

"looking"
out at the ground, farther waway and with a bit less of participation.
Wanna see dramatic display of salt water "ground"

performance/contribution?
Take your any verticaly polarized antenna and compare its performace

between
ground ground and salt water ground. You would see somewhere between 10 -

15 dB
difference.

Yuri

Message 8 in thread
From: Yuri Blanarovich )
Subject: efficiency of horizontal vs vertical antennas


View this article only
Newsgroups: rec.radio.amateur.antenna
Date: 2003-07-22 19:23:32 PST



Really? Even a vertical dipole?

de AI8W, Chris


Yesereee!
K2KW and "team vertical" (Force 12) did some tests and measurements across
the
San Francisco Bay and found that using half wave vertical dipole and moving
it
from the salt water/beach boundary (0 dB reference) they would get 3 dB gain
being 1/4 wave away from the edge, - 2dB for 1/2 wave and + 2 dB for 3/4
wave.
Anything that is vertically polarized gets help from better ground,
especially
at the low angles in the pattern.
Claims of advertisers that their wundervertical needs no radials or ground
are
full of SWR!

I am trying to find some practical results of verticals being operated on
the
side of a hill. Anyone out there with experience?

Yuri da vertical fan BUm





  #2   Report Post  
Old October 16th 03, 08:23 PM
Reg Edwards
 
Posts: n/a
Default

Dear Serge,

Yes. But why make such a song and a dance about it.

It's quite obvious a 'ground plane' on the ground, or even shallow-buried,
causes greater losses in the ground than any elevated ground plane. The
greater the elevation the more efficient the antenna becomes. Eventually,
with sufficient height, the efficiency equals that of dipole in free space
whatever the antenna's orientation or form of groundplane it employs.

Have a good holiday.
---
Reg, G4FGQ


  #3   Report Post  
Old October 17th 03, 04:34 AM
Mark Keith
 
Posts: n/a
Default

"Serge Stroobandt, ON4BAA" wrote in message ...
Dear Roy, Richard, Art and other readers,

I had a look at that thread (for other readers' convenience included at the
bottom of this message).
Although it is not immediately answering my question whether inverted
groundplane verticals present fewer losses than normal verticals, it
contains some
interesting observations. Again for the sake of the other readers, I took
the liberty to rephrase and summarise these in terms that are more common in
antenna literatu


Why are you calling them "inverted ground planes"? To me , an inverted
ground plane would be one that is standing on it's end, with the
radials on top.
It's just a ground plane. Period. A ground plane is any vertical that
uses elevated radials to supply the lower part of the antenna. "Not to
be confused with decoupling radials." Adding decoupling radials to a
half wave that is elevated, does not turn it into a "ground plane".
It's still an elevated 1/2 wave with a decoupling section. A ground
mount vertical, is well, a ground mount vertical.

The electromagnetic field around an antenna exists out of two parts: a near
field and a far field.

Due to maths, the near field rapidly becomes negligible at larger distances,
whereas the far field is the field we use normally to communicate.

Losses in the near and far field can be reduced by reducing ground losses in
the immediate vecinity of the antenna.


Reducing ground losses under the antenna will have no effect on ground
losses in the far field. Those are fixed per the existing ground cdx
at the time, and are unchangable by you, unless you move, change
seasons, or can have someone make it rain over the whole area for four
days. Rain dancing" :/ I'm not sure what difference that actually
makes as fresh water is not near as conductive as salt water, but I
think helps a small bit. But, yes, if you reduce losses at the
antenna, you have more to work with out in the far field. So if far
field losses were say 2 db to a certain point when using an angle of 5
degrees, if you reduce losses at the antenna by 2 db, "adding radials"
you will see that extra 2 db at the receiving end over the less
radialed version. But the far field losses in DB will be the same.
"2db".



Losses in the far field are in addition also caused by the ground properties
at larger distances from the antenna, and are therefore less controlable
(apart from choosing a coastal or salt-lake QTH).

Now, my assumption is that by inverting a vertical ground plane, some of the
near field ground losses could be prevented by presenting the high impedance
end of the radiator to the low impedance of the earth immediately
underneath. This tremendous mismatching would cause less coupling of the
antenna's near field into the lossy earth.


True, if you actually inverted a ground plane. But adding radials at
the top would be silly... But, yes, if you had a 1/4 wave vertical,
and fed it from the top, say with a feedline running up through the
middle, the point of max current would then be at the top, and high
voltage at the bottom. You would see less ground loss than a base fed
1/4 wave, but you still couldn't totally ignore ground losses under
the antenna. I would still lay out radials.


Here are the results:

Normal 80m (3.650MHz) ground plane vertical
with 4 elevated radials 4.5m above "good" ground (according to Christman
KB8I, see also John Devolder, ON4UN, "Low-Band Dxing," chapter 9)
radiator and radial lengths: 20m
maximum height 24.5m
azimuth 0°: 0.23dBi @ 22° elevation
azimuth 45°: 0.21dBi @ 22° elevation

Inverted 80m ground plane vertical
with 4 elevated radials 24.5m above "good" ground
radiator and radial lengths: 20m
maximum height 24.5m
azimuth 0°: 0.08dBi @ 22° elevation
azimuth 45°: 0.12dBi @ 22° elevation

Result: 0.09-0.15dB in favour of the normal ground plane. The radials up in
the air of the inverted ground plane probably cause some shielding.


This model is faulty as far as I'm concerned. The lower antenna is too
low to the ground to show low ground losses with only 4 radials. The
higher antenna would kick it's tail. I don't care what the model says.
In fact, the antenna at 75 ft would be VERY effective to DX at night.
Thats just over a 1/4 up. The pattern will be good.

The same difference remains when we lower both antennas to 1m above ground:

Normal 80m ground plane vertical
with 4 elevated radials 1m above "good" ground
radiator and radial lengths: 20m
maximum height 21m
azimuth 0°: 0.00dBi @ 24° elevation
azimuth 45°: 0.00dBi @ 24° elevation

Inverted 80m ground plane vertical
with 4 elevated radials 21m above "good" ground
radiator and radial lengths: 20m
maximum height 21m
azimuth 0°: -0.16dBi @ 25° elevation
azimuth 45°: -0.11dBi @ 25° elevation


Again, this model is faulty, and /or misleading when compared to real
world results. An 80m vertical at 3 ft off the ground, with only 4
radials would be a dog compared to the higher one at 63 ft.
If you had a ground mount with 120 radials, which is usually
considered a "near optimum ground system", you would need 60 radials
at 1/8 wave up to equal the 120 on the ground. At 1/4 wave up, you
would need about 8-10 radials for the same low ground losses. At 1/2
wave up, you only need 3 or 4. Once you get to a 1/2 wave, the ground
losses are very low, and start to compete with the efficiency of a
dipole at that height. But from personal use, I can tell you that even
just 4 radials at 1/4 wave up , will do a good job. But 4 radials at
1/8 wave up, will be a dog in comparison. When thinking of radials for
a ground plane, and the proper number of them to equal a certain
reference, you must think in terms of wavelength for the freq to be
used.


Being an appartment dweller myself, I once dreamed of putting up an inverted
ground plane antenna using the building as a support for the radiator and
the roof as support and hideawy for the two elevated radials.


It will work, but the ground losses will depend on the height above
ground in wavelength for the band in use, and the number of radials
used. Refer to the specs above to equal the "120 radial reference".
BTW, I think Reg squirms in his chair every time I bring up 120
radials. But I still think it's a good reference to compare to for
top results. Nothing I've ever seen in the real world has ever
departed from the "specs" I've given above as far as comparing the
number of elevated radials, to the number of radials used with a
ground mount system.


Ok, but now taking into account Roy's previous thread. How does a low dipole
at the same height of 21m stands out of this?
azimuth 0°: -5.77dBi @ 30° elevation
azimuth 90°: 4.23dBi @ 30° elevation

Result: In its preferred azimuthal direction, a low dipole, even at a height
of about a quarter wavelength, still outperforms any ground plane vertical
antenna.


Depends where you are talking, and the angle being used to make that
path. Sure, if you are on 80m, and work 200 miles away, the dipole
will almost always be better. But not so on long paths. The angles
used on long hauls will be fairly low. When modeling, compare the gain
at say 5-10 degrees off the horizon. But even this is deceptive. I
don't totally agree with the way most programs model and compare
ground mount to elevated verticals. I think most programs understate
the gain of elevated verticals, when used at the lower than 1 wave
heights on the lower bands. The patterns seem ok, but the way they
deal with the decreasing ground losses when elevated, seems a little
suspect to me. I find I have to crank up the ground quality to show
the gains I see in the real world. On the low bands, the increase
when using the ground plane vs dipole would usually be greater in the
real world, than is modeled. I think part of this is because the
programs don't seem to take into account the differences between
losses vs freq, and also take take into account the quirks of
propagation on the lower bands. It's my opinion that if you had two
antennas with the exact equal gain in a certain direction and angle,
and one was horizontal, and one vertical, the vertical would win on
160m long paths.
But a modeling program will not deal with that, so you have to
consider it on your own.


MK
  #4   Report Post  
Old October 17th 03, 09:13 AM
Serge Stroobandt, ON4BAA
 
Posts: n/a
Default


"Mark Keith" wrote in message
om...
"Serge Stroobandt, ON4BAA" wrote in

message ...
Dear Roy, Richard, Art and other readers,

I had a look at that thread (for other readers' convenience included at

the
bottom of this message).
Although it is not immediately answering my question whether inverted
groundplane verticals present fewer losses than normal verticals, it
contains some
interesting observations. Again for the sake of the other readers, I

took
the liberty to rephrase and summarise these in terms that are more

common in
antenna literatu


Why are you calling them "inverted ground planes"? To me , an inverted
ground plane would be one that is standing on it's end, with the
radials on top.
It's just a ground plane. Period. A ground plane is any vertical that
uses elevated radials to supply the lower part of the antenna. "Not to
be confused with decoupling radials." Adding decoupling radials to a
half wave that is elevated, does not turn it into a "ground plane".
It's still an elevated 1/2 wave with a decoupling section. A ground
mount vertical, is well, a ground mount vertical.


Hello Mark,

For all clarity, when I say inverted groundplane, I really do mean a quarter
wave vertical fed at the top, with radials at the top!


--

73 de Serge ON4BAA - HB9DWU
http://salsawaves.com/propagation/


  #5   Report Post  
Old October 17th 03, 09:44 AM
Reg Edwards
 
Posts: n/a
Default

For all clarity, when I say inverted groundplane, I really do mean a
quarter
wave vertical fed at the top, with radials at the top!

============================

You should have said you are working on the underbelly of an aeroplane.
Makes a lot difference where you have your shack.




  #6   Report Post  
Old October 17th 03, 02:29 PM
Craig Buck
 
Posts: n/a
Default

Check out Cebik's article on the center-fed Inverted L
http://www.cebik.com/ltv.html. He loves it and so do I.

--
Radio K4ia
Craig "Buck"
Fredericksburg, VA USA
FISTS 6702 cc 788 Diamond 64


"Serge Stroobandt, ON4BAA" wrote in message
...
Dear Roy, Richard, Art and other readers,

I had a look at that thread (for other readers' convenience included at

the
bottom of this message).
Although it is not immediately answering my question whether inverted
groundplane verticals present fewer losses than normal verticals, it
contains some
interesting observations. Again for the sake of the other readers, I took
the liberty to rephrase and summarise these in terms that are more common

in
antenna literatu

The electromagnetic field around an antenna exists out of two parts: a

near
field and a far field.

Due to maths, the near field rapidly becomes negligible at larger

distances,
whereas the far field is the field we use normally to communicate.

Losses in the near and far field can be reduced by reducing ground losses

in
the immediate vecinity of the antenna. This can be done through installing
the antenna above "good RF ground", empoying a low impedance radial system
(burried or elevated) for verticals or = 5/8 lambda or choosing a

(double
so long!) vertical antenna with a built-in return-path (half square,

bobtail
curtain, and half wavelength vertical). NOTE: Both the half square and the
bobtail curtain can be interpreted as an array of 2, respectively 3

inverted
ground plane verticals! (For a brief description see John Devolder, ON4UN,
"Low-Band Dxing," chapter 12)

Losses in the far field are in addition also caused by the ground

properties
at larger distances from the antenna, and are therefore less controlable
(apart from choosing a coastal or salt-lake QTH).

Now, my assumption is that by inverting a vertical ground plane, some of

the
near field ground losses could be prevented by presenting the high

impedance
end of the radiator to the low impedance of the earth immediately
underneath. This tremendous mismatching would cause less coupling of the
antenna's near field into the lossy earth.

Assumptions of the mind are one thing, but eventually the proof of the
pudding is in the eating (and this goes especially to Richard!).
(Art, you see, you don't need to convince me anymore about antenna

modelling
;-) However, I do will post a message about a possible bug in AO!)
So I used version 1.0 of Roy's software, called EZNEC to model a couple of
swiftly designed (i.e. non-resonant) antennas.
(Roy, we need to talk about upgrading my version - I will send you a
seperate email for this.)
Here are the results:

Normal 80m (3.650MHz) ground plane vertical
with 4 elevated radials 4.5m above "good" ground (according to Christman
KB8I, see also John Devolder, ON4UN, "Low-Band Dxing," chapter 9)
radiator and radial lengths: 20m
maximum height 24.5m
azimuth 0°: 0.23dBi @ 22° elevation
azimuth 45°: 0.21dBi @ 22° elevation

Inverted 80m ground plane vertical
with 4 elevated radials 24.5m above "good" ground
radiator and radial lengths: 20m
maximum height 24.5m
azimuth 0°: 0.08dBi @ 22° elevation
azimuth 45°: 0.12dBi @ 22° elevation

Result: 0.09-0.15dB in favour of the normal ground plane. The radials up

in
the air of the inverted ground plane probably cause some shielding.

The same difference remains when we lower both antennas to 1m above

ground:

Normal 80m ground plane vertical
with 4 elevated radials 1m above "good" ground
radiator and radial lengths: 20m
maximum height 21m
azimuth 0°: 0.00dBi @ 24° elevation
azimuth 45°: 0.00dBi @ 24° elevation

Inverted 80m ground plane vertical
with 4 elevated radials 21m above "good" ground
radiator and radial lengths: 20m
maximum height 21m
azimuth 0°: -0.16dBi @ 25° elevation
azimuth 45°: -0.11dBi @ 25° elevation

Being an appartment dweller myself, I once dreamed of putting up an

inverted
ground plane antenna using the building as a support for the radiator and
the roof as support and hideawy for the two elevated radials.

Normal 80m ground plane vertical
with 2 elevated radials 1m above "good" ground
radiator and radial lengths: 20m
maximum height 21m
azimuth 0°: -0.86dBi @ 24° elevation
azimuth 45°: -0.66dBi @ 24° elevation
azimuth 90°: -0.45dBi @ 24° elevation

Inverted 80m ground plane vertical
with 2 elevated radials 21m above "good" ground
radiator and radial lengths: 20m
maximum height 21m
azimuth 0°: 0.55dBi @ 29° elevation
azimuth 45°: 0.05dBi @ 26° elevation
azimuth 90°: -0.96dBi @ 21° elevation

Result: Now, the balance turned in favour of the inverted groundplane
vertical in 3 of the 2 azimuth headings, probably because fewer radials

for
the inverted GP means less shielding.
Differences for 0°, 45° and 90° azimuth are 1.41dB, 0.71dB and -0.51dB,
respectively.

FIRST TAKE-AWAY MESSAGE: Do not rely on accounts of particular antenna
installation, always model your own antenna installation before jumping to
conclusions.

Ok, but now taking into account Roy's previous thread. How does a low

dipole
at the same height of 21m stands out of this?
azimuth 0°: -5.77dBi @ 30° elevation
azimuth 90°: 4.23dBi @ 30° elevation

Result: In its preferred azimuthal direction, a low dipole, even at a

height
of about a quarter wavelength, still outperforms any ground plane vertical
antenna. This is an eye-opener, taking into account that low dipoles

(a.k.a.
cloud burners) send most of their energy radially away from earth, without
any ionospheric diffraction right into space. Most of the gain of the low
dipole is pattern gain, however. This means that performance in the
perpendicular azimuthal direction is extremely bad with -5.77dBi.

SECOND TAKE-AWAY MESSAGE: Appartment dwellers might find an effective
radiating system in installing a dipole on the roof of their building (may
droop off at the ends) and making it (behind the current balun) remotely
switchable into an inverted ground plane vertical antenna for working the
directions that lie in the dip of the azimuthal dipole pattern. That is,

the
current balun should also be employed to make the vertical radiator

current
equal to the sum of the currents in the radials.

PS1: I have posted also an exact copy of this message under the subject
"Inverted ground plane antenna: compared with normal GP and low dipole."

in
order to attract more readers with this more appropriate title.

PS2: Please, send your comments to the newsgroup under above-mentioned
header. As of tomorrow I will be on a business trip for the next two weeks
and will only be able to respond occasionally.
(Not that I am afraid of you guys!!! :-P)

--
73 de Serge ON4BAA - HB9DWU

propagation & ham info at:
http://salsawaves.com/propagation/




"Roy Lewallen" wrote in message
...
That question has been answered several times in this newsgroup. For one
of the most recent answers, go to groups.google.com and find my posting
on July 21, 2003 in the thread " efficiency of horizontal vs vertical
antennas".

Roy Lewallen, W7EL

Serge Stroobandt, ON4BAA wrote:

This conversation is starting to get interesting :-)

INVERTED GROUNDPLANES:
What about using an inverted 1/4wave groundplane in order to avoid

ground
losses?
Then you have the high impedance point of the antenna close to the

lossy
(low impedance) earth.
Due to the severe mismatch less power will couple into the earth.

The low impedance point (feed point) of the antenna is safely high up

in
the
(high impedance) air then.


All messages from thread
Message 1 in thread
From: Ron )
Subject: efficiency of horizontal vs vertical antennas


View this article only
Newsgroups: rec.radio.amateur.antenna
Date: 2003-07-15 18:38:29 PST


There doesn't appear to be any ground loss resistance in horizontal

antennas
(e.g., dipoles) such as there is in verticals. Does this mean that

verticals
are
virtually always less efficient than horizontals due to their ground loss?

RonMessage 2 in thread
From: Roy Lewallen )
Subject: efficiency of horizontal vs vertical antennas


View this article only
Newsgroups: rec.radio.amateur.antenna
Date: 2003-07-15 19:01:01 PST


A vertical dipole, or a base fed half wave vertical, has good efficiency
and no significant ground loss. That is, there's no appreciable loss
from current flowing through the ground to a feedline terminal. For
other verticals, such as a base fed quarter wavelength, ground loss due
to return currents can be reduced to an arbitrarily low value by using a
good enough system of radials.

However, after the wave is launched from the antenna, vertically
polarized signals react differently than horizontally polarized ones
when they strike the ground. Horizontally polarized waves are reflected
with little loss, except ones at high angles. With vertically polarized
waves, it's the low angle ones that take the greatest beating. And since
most vertical antennas tend to concentrate radiation at low angles, a
very sizable fraction of the total radiated power is often lost in the
ground reflection. This mostly takes place well beyond a reasonably
sized radial field, so radials don't help. The only way to reduce this
loss is to see that the reflection takes place from highly conductive
ground, like sea water(*). Otherwise, you lose most of the power that
you want the most.

(*) The effective conductivity of the ground improves as frequency
decreases. So verticals tend to do better at lower frequencies, and
often outperform horizontal dipoles on 160 and 80 meters, even over
average ground.

Roy Lewallen, W7EL

Ron wrote:
There doesn't appear to be any ground loss resistance in horizontal
antennas (e.g., dipoles) such as there is in verticals. Does this mean
that verticals are virtually always less efficient than horizontals due
to their ground loss?

Ron

Message 3 in thread

From: Bob Colenso )
Subject: efficiency of horizontal vs vertical antennas


View this article only
Newsgroups: rec.radio.amateur.antenna
Date: 2003-07-21 15:16:10 PST


But aren't there ground independant verticals, like the Half Square or a
Bob-Tail-Curtain?



U of M
GO BLUE!!!

God, Guns, and Guts
Protect America!!!Message 4 in thread
From: Yuri Blanarovich )
Subject: efficiency of horizontal vs vertical antennas


View this article only
Newsgroups: rec.radio.amateur.antenna
Date: 2003-07-21 15:30:11 PST



But aren't there ground independant verticals, like the Half Square or a
Bob-Tail-Curtain?



U of M
GO BLUE!!!

God, Guns, and Guts
Protect America!!!


Every vertical "needs" the ground for its efficient performance especially
at
the low angles. Vertical dipoles and their elevated cousins are still
"looking"
out at the ground, farther waway and with a bit less of participation.
Wanna see dramatic display of salt water "ground"

performance/contribution?
Take your any verticaly polarized antenna and compare its performace

between
ground ground and salt water ground. You would see somewhere between 10 -

15
dB
difference.

YuriMessage 5 in thread
From: Roy Lewallen )
Subject: efficiency of horizontal vs vertical antennas


View this article only
Newsgroups: rec.radio.amateur.antenna
Date: 2003-07-21 16:51:50 PST


There are two quite distinct sources of ground loss involved with
vertical antennas. You're talking about one, and Yuri is talking about
the other. Makes for a lively discussion, but it's a lot like the blind
men describing the elephant.

One type of loss is caused when it's necessary to connect one conductor
of the feedline to the Earth. Current returning to this feedline
conductor is equal in magnitude to the current flowing into the antenna
from the other conductor, and it creates a simple I^2 * R loss flowing
through the ground. This loss can be minimized by using a system of
ground radials to decrease the loss resistance R near the base of the
antenna, where the current density is highest. Another solution is to
use a vertical antenna with a high feedpoint resistance. Examples are
the half square, bobtail curtain, and half wavelength vertical. These
antennas require very little feedpoint current, and consequently very
little ground current. They can be very efficient with only a very
simple ground system.

But there's another source of loss, encountered after the signal is
radiated. When a vertically polarized signal strikes the ground, a lot
of its energy is lost to heating of the ground. This is particularly
true at low angles of incidence. The end result is severe attenuation of
low angle radiation. Particularly for low angles, this occurs farther
away from the antenna than a reasonable radial system extends. So you're
stuck with this loss, unless you can physically move your antenna to a
swamp or similar high-conductivity environment. Horizontally polarized
waves react differently.

The demo version of EZNEC will show this quite dramatically. If you
choose the MININEC type ground model, it acts like you have a perfect
radial system. That is, the first source of loss I mentioned is zero.
But the second is still there. You can simulate the effect of ground
system loss simply by adding a resistive "load" at the antenna base.
Compare the patterns of a vertical and horizontal, *to the same scale*,
by superimposing them on a 2D plot, using different qualities of ground.
You'll find it quite educational.

Roy Lewallen, W7EL

Yuri Blanarovich wrote:
But aren't there ground independant verticals, like the Half Square or a
Bob-Tail-Curtain?



U of M
GO BLUE!!!

God, Guns, and Guts
Protect America!!!



Every vertical "needs" the ground for its efficient performance

especially
at
the low angles. Vertical dipoles and their elevated cousins are still

"looking"
out at the ground, farther waway and with a bit less of participation.
Wanna see dramatic display of salt water "ground"

performance/contribution?
Take your any verticaly polarized antenna and compare its performace

between
ground ground and salt water ground. You would see somewhere between

10 -
15 dB
difference.

Yuri Message 6 in thread

From: luke )
Subject: efficiency of horizontal vs vertical antennas


View this article only
Newsgroups: rec.radio.amateur.antenna
Date: 2003-07-22 07:12:04 PST


Hi,

Good answers to the question !

Always walk away a little smarter after reading this group !

73 luke

Roy wrote:

There are two quite distinct sources of ground loss involved with
vertical antennas. You're talking about one, and Yuri is talking about
the other. Makes for a lively discussion, but it's a lot like the blind
men describing the elephant.

One type of loss is caused when it's necessary to connect one conductor
of the feedline to the Earth. Current returning to this feedline
conductor is equal in magnitude to the current flowing into the antenna
from the other conductor, and it creates a simple I^2 * R loss flowing
through the ground. This loss can be minimized by using a system of
ground radials to decrease the loss resistance R near the base of the
antenna, where the current density is highest. Another solution is to
use a vertical antenna with a high feedpoint resistance. Examples are
the half square, bobtail curtain, and half wavelength vertical. These
antennas require very little feedpoint current, and consequently very
little ground current. They can be very efficient with only a very
simple ground system.

But there's another source of loss, encountered after the signal is
radiated. When a vertically polarized signal strikes the ground, a lot
of its energy is lost to heating of the ground. This is particularly
true at low angles of incidence. The end result is severe attenuation of
low angle radiation. Particularly for low angles, this occurs farther
away from the antenna than a reasonable radial system extends. So you're
stuck with this loss, unless you can physically move your antenna to a
swamp or similar high-conductivity environment. Horizontally polarized
waves react differently.

The demo version of EZNEC will show this quite dramatically. If you
choose the MININEC type ground model, it acts like you have a perfect
radial system. That is, the first source of loss I mentioned is zero.
But the second is still there. You can simulate the effect of ground
system loss simply by adding a resistive "load" at the antenna base.
Compare the patterns of a vertical and horizontal, *to the same scale*,
by superimposing them on a 2D plot, using different qualities of ground.
You'll find it quite educational.

Roy Lewallen, W7EL

Read the rest of this message... (24 more lines)

Message 7 in thread
From: sideband )
Subject: efficiency of horizontal vs vertical antennas


View this article only
Newsgroups: rec.radio.amateur.antenna
Date: 2003-07-22 19:01:10 PST



Really? Even a vertical dipole?

de AI8W, Chris

Yuri Blanarovich wrote:
But aren't there ground independant verticals, like the Half Square or a
Bob-Tail-Curtain?



U of M
GO BLUE!!!

God, Guns, and Guts
Protect America!!!



Every vertical "needs" the ground for its efficient performance

especially
at
the low angles. Vertical dipoles and their elevated cousins are still

"looking"
out at the ground, farther waway and with a bit less of participation.
Wanna see dramatic display of salt water "ground"

performance/contribution?
Take your any verticaly polarized antenna and compare its performace

between
ground ground and salt water ground. You would see somewhere between

10 -
15 dB
difference.

Yuri

Message 8 in thread
From: Yuri Blanarovich )
Subject: efficiency of horizontal vs vertical antennas


View this article only
Newsgroups: rec.radio.amateur.antenna
Date: 2003-07-22 19:23:32 PST



Really? Even a vertical dipole?

de AI8W, Chris


Yesereee!
K2KW and "team vertical" (Force 12) did some tests and measurements across
the
San Francisco Bay and found that using half wave vertical dipole and

moving
it
from the salt water/beach boundary (0 dB reference) they would get 3 dB

gain
being 1/4 wave away from the edge, - 2dB for 1/2 wave and + 2 dB for 3/4
wave.
Anything that is vertically polarized gets help from better ground,
especially
at the low angles in the pattern.
Claims of advertisers that their wundervertical needs no radials or ground
are
full of SWR!

I am trying to find some practical results of verticals being operated on
the
side of a hill. Anyone out there with experience?

Yuri da vertical fan BUm






  #7   Report Post  
Old February 12th 11, 06:08 AM
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[/i][/color][/quote]

Those of us who live in Florida and operate mobile can verify what computer modelers have been telling us about vertical antennas. I can drive on the beach in my car with a 1/4 wave mobile antenna and make contacts almost at will in the direction of the salt water. Once I get inland, my signal drops like a rock. One field day, I purposely faked a flat tire at the top of the sunshine skyway bridge, 200 feet over Tampa Bay, salt water in ALL directions! I got in about 2 hours of operating before the state cops made me call a tow truck, LOL. I simply re inflated the tire I intentionally flattened, and went on my way. Now I know why all Hams dream of a home "high on a hill overlooking salt water". As Roy and W4RNL have told us for years, we can do little or nothing about the far field the vertical antenna "sees".
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