GROUND ROLE IN VERTICAL HALF WAVE ANTENNA
g.r...@crf.it
Dear collegues,
Can any one explain me the role played by ground or its surrogates
( radials, rods etc... ) related to an half wave vertical antenna,and
considering also the trasmission line which supply the antenna ?
The question come from some form of vertical half wave antenna,
which works without any radial or ground rods, I've seen these antenna
mounted on wooden roof.
Thank you and 73
IW1CLX
Jim Nance
g.r...@crf.IT wrote in article
<c=IT%a=_%p=CRF%l=CRF.NTLAN...@scrf02.crf.it>...
Cecil Moore
It matters how far away from earth is the maximum radiation point on
the antenna. On a ground-mounted quarterwave vertical, the point of
maximum radiation is at ground level and that's where wire radials
need to be installed. For a half-wave vertical, the point of maximum
radiation is more than a quarter-wavelength off the earth so quarter-
wavelength radials are mostly wasted. A pretty good rule-of-thumb is
to make any radials at least as long as the antenna.
IMO, Quarter-wave radials on a 5/8 WL vertical act mainly as a
counterpoise and not much as a ground plane. If the earth around a
5/8 WL vertical is poor, the radials probably should be much longer
than 0.25 WL.
Take a look at the geometry and assume that radiation hitting the
radials is reflected and radiation hitting the earth is lost to get
an idea of a hypothetical worst case.
73, Cecil, W6RCA, OOTC
James C Sutton
On 16 Apr 97 12:15:55 GMT, g.r...@crf.IT wrote:
>
>Dear colleagues,
>Can any one explain me the role played by ground or its surrogates
>( radials, rods etc... ) related to an half wave vertical antenna,and
>considering also the transmission line which supply the antenna ?
>The question come from some form of vertical half wave antenna,
>which works without any radial or ground rods, I've seen these antenna
>mounted on wooden roof.
>Thank you and 73
>IW1CLX
Think about it this way. There is power/current in your transmission
line and so there has to be current in your antenna. The only way you
can complete the circuit properly is to place a ground system around
the antenna. The current then passes from the antenna through a
capacitive effect between the antenna and the ground system and on
back to originating point.
Yes -- you see HF antennas without ground systems - but their
efficiency is pretty sad - a long way from optimum.
Check into your local am broadcast stations. All will have ground
systems. As I recall - but I may be wrong - the FCC requires
something like 120 radials. If they weren't necessary no one would
spend the money, now would they?
From a practical Ham view about 30 radial is all you need to reach a
point of diminishing returns and 20 is not a bad number to have.
Your feed line provides the return path from the ground/radial system
since the shield of your coax is grounded at the base of the antenna
as are all the radials.
Check the ARRL antenna manual or other similar sources for additional
data.
73/Jim/AC4CZ
Cecil Moore
THamm40820 wrote:
> So, I don't understand the statement "act mainly as a counterpoise but not
> as a ground plane".
Hi Tommy, a counterpoise affects the feedpoint impedance and reflects the
radiation in one direction. A ground plane affects the feedpoint impedance
and reflects the radiation in as many directions as there are radials
(assuming ground level radials).
> 1) a vertical quarter wave cut for 10.11 MHz, therefore 23.15 feet high
> with only one radial connected to the ground side of the feedline. A
> pretty poor antenna, right ?!!
> 2) a 23.15 foot "camping trip" antenna, fed at ground level, and
> vertical, with one counterpoise tuned wire (23.15 feet long) attached to
> the tuner. This, I've been led to believe is a PRETTY GOOD antenna.
Assuming neglible losses in the coax used with number 1, there's not
enough difference to matter. Number 2 is a "PRETTY GOOD antenna" compared
to no antenna at all - so is number 1.
73, Cecil, W6RCA, OOTC
George Baker
James C Sutton wrote:
> Think about it this way. There is power/current in your transmission
> line and so there has to be current in your antenna. The only way you
> can complete the circuit properly is to place a ground system around
> the antenna. The current then passes from the antenna through a
> capacitive effect between the antenna and the ground system and on
> back to originating point.
>
> Yes -- you see HF antennas without ground systems - but their
> efficiency is pretty sad - a long way from optimum.
>
> Check into your local am broadcast stations. All will have ground
> systems. As I recall - but I may be wrong - the FCC requires
> something like 120 radials. If they weren't necessary no one would
> spend the money, now would they?
>
> From a practical Ham view about 30 radial is all you need to reach a
> point of diminishing returns and 20 is not a bad number to have.
>
> Your feed line provides the return path from the ground/radial system
> since the shield of your coax is grounded at the base of the antenna
> as are all the radials.
>
> Check the ARRL antenna manual or other similar sources for additional
> data.
>
> 73/Jim/AC4CZ
Jim, your response sounds spot on for a *quarter-wave* vertical but I
think that the original question referenced a *half-wave* vertical.
Still apply in the same way?
--
73, George
W5YR (original, not vanity)
Fairview, TX
Cecil A. Moore
James C Sutton wrote:
> Yes -- you see HF antennas without ground systems - but their
> efficiency is pretty sad - a long way from optimum.
dipole 100 ft in the air is? I run one on field day stretched between
two trees unreferenced to ground (at least by me). Beats the socks off
any of the single-element ground-mounted verticals no matter how many
radials they have.
73, Cecil, W6RCA, OOTC
THamm40820
Hi,
I think this thread hits on an area I've had some misunderstanding about
for quite some time. I had always assumed that a counterpoise was a
substitute for a ground plane, but one which wasn't in the ground itself.
as a ground plane".
wire tossed up over a tree works OK *IF* a tuned counterpoise wire is
connected to the ground side of the antenna tuner. OK, so, two situations
here:
1) a vertical quarter wave cut for 10.11 MHz, therefore 23.15 feet high
with only one radial connected to the ground side of the feedline. A
pretty poor antenna, right ?!!
2) a 23.15 foot "camping trip" antenna, fed at ground level, and
vertical, with one counterpoise tuned wire (23.15 feet long) attached to
the tuner. This, I've been led to believe is a PRETTY GOOD antenna.
good. I suspect they're both pretty bad, but why do the antenna books
make #1 seem terrible and #2 seem really usable ? Some clarification
would really be appreciated !
Thanks and 73's,
Tommy
KI7KH
DavidOsbo
I'm convinced that many antenna books written by "experts" aren't worth
the paper upon which they're printed. To answer your questions about the
two antennas: They are the same, therefore, I think they are both "pretty
bad" in terms of radiation pattern. This is, when compared to a "good"
antenna, however one wants to define that. But, they are both infinitely
better than NO antenna, and some surprisingly good results have been
obtained using some not-so-good antennas. Just like "beauty is in the eye
of the beholder," many times "a good antenna is in the mind of the user"!
James C Sutton
Tommy -- you have hit on an interesting point re number 2 above and
the meaning of words with relationship to the situation.
From a "good" antenna point of view at a home location this is a poor
way to solve the problem.
HOWEVER if you are on a canoe trip with nothing but trees and a bit of
wire to play with - then it is a "good" antenna. After all - what
else can you do?
73/Jim/AC4CZ
James C Sutton
On Wed, 16 Apr 1997 23:37:15 -0500, George Baker <w5...@swbell.net>
wrote:
>
>James C Sutton wrote:
>
>> Think about it this way. There is power/current in your transmission
>> line and so there has to be current in your antenna. The only way you
>> can complete the circuit properly is to place a ground system around
>> the antenna. The current then passes from the antenna through a
>> capacitive effect between the antenna and the ground system and on
>> back to originating point.
>>
>> Yes -- you see HF antennas without ground systems - but their
>> efficiency is pretty sad - a long way from optimum.
>>
>> Check into your local am broadcast stations. All will have ground
>> systems. As I recall - but I may be wrong - the FCC requires
>> something like 120 radials. If they weren't necessary no one would
>> spend the money, now would they?
>>
>> From a practical Ham view about 30 radial is all you need to reach a
>> point of diminishing returns and 20 is not a bad number to have.
>>
>> Your feed line provides the return path from the ground/radial system
>> since the shield of your coax is grounded at the base of the antenna
>> as are all the radials.
>>
>> Check the ARRL antenna manual or other similar sources for additional
>> data.
>>
>> 73/Jim/AC4CZ
>
>Jim, your response sounds spot on for a *quarter-wave* vertical but I
>think that the original question referenced a *half-wave* vertical.
>Still apply in the same way?
>
George it applies to any vertical antenna. The difference in 1/4 vs
1/2 wave is the location of the current node and the amount of current
coupled between the antenna and the ground through the capacitive
effect. With the current node higher above ground the coupling will be
less since the current and resulting field are at a greater distance
to the ground.
Having said all that - radials are still needed to achieve theoretical
maximum radiation.
James C Sutton
On Wed, 16 Apr 1997 22:28:15 -0700, "Cecil A. Moore"
<cecil...@delphi.com> wrote:
>
>James C Sutton wrote:
>> Yes -- you see HF antennas without ground systems - but their
>> efficiency is pretty sad - a long way from optimum.
>Hi Jim, wonder if you know what the efficiency of a balanced halfwave
>dipole 100 ft in the air is? I run one on field day stretched between
>two trees unreferenced to ground (at least by me). Beats the socks off
>any of the single-element ground-mounted verticals no matter how many
>radials they have.
>
>73, Cecil, W6RCA, OOTC
Cecil - I assume that you are using a 1/2 half wave dipole fed in the
center. If this is true then the ground situation that I described for
a end fed vertical does not apply. I don't know for sure -- but I
would assume the efficiency would be similar to a horizontal center
fed dipole with a different radiation pattern caused by the ground
effect that applies to a horizontal dipole.
Your success may be due to the fact that your antenna is quite
efficient and the verticals lack good ground systems.
I believe the Gap antenna takes such an approach. From what I hear it
is a vertical dipole fed in the middle and is used without a ground
system. The efficiency is unknown to me.
I tried to get some technical information from the company some time
ago but they didn't want to let much out.
73/Jim/AC4CZ
Tom Rauch
On Thu, 17 Apr 1997 14:14:15 GMT, jsu...@erols.com (James C Sutton)
wrote:
>
>On Wed, 16 Apr 1997 22:28:15 -0700, "Cecil A. Moore"
><cecil...@delphi.com> wrote:
>>Hi Jim, wonder if you know what the efficiency of a balanced halfwave
>>dipole 100 ft in the air is? I run one on field day stretched between
>>two trees unreferenced to ground (at least by me). Beats the socks off
>>any of the single-element ground-mounted verticals no matter how many
>>radials they have.
>>
>>73, Cecil, W6RCA, OOTC
Hi Cecil,
That depends on the band. On 160 or 80, my ground mounted 1/4 wl and
shorter antennas "beat the socks" off any dipole I've had at the same
height.
I think that is why 160 operators have all but abandoned the dipole
for DX work, or work beyond a few hundred miles.
In Syvania Ohio I had a 1/4 wl 160 vertical, and also the ability to
use BC stations via remote or direct operation. When I compared a 300
foot vertical 5/8 wl to a 130 ft vertical (both over 120 radials) ,
the shorter vertical always won.
When I compared a dipole at ~350 feet to a 1/4 wl vertical, the dipole
won. Lowering the dipole to 200 feet or so made the 1/4 wl vertical
have the advantage.
Here in Georgia my 35 ft tall top loaded 80 meter vertical ties a 130
ft high dipole for DX.
On 40 meters, my 85 ft high dipole beats my 30 ft vertical by a few dB
except off the dipole ends.
There are many variables, but in ALL cases a good ground system below
the antenna helps. I even use a couterpoise system below my dipoles.
>Your success may be due to the fact that your antenna is quite
>efficient and the verticals lack good ground systems.
>
>I believe the Gap antenna takes such an approach. From what I hear it
>is a vertical dipole fed in the middle and is used without a ground
>system. The efficiency is unknown to me.
The GAsh is a lousy system. They moved the feedpoint up, but the
current distribution remains almost like a conventional radiator.
None of the "ground independent" verticals work nearly as well as a
conventional vertical with a good radial system. I've A-B tested a Gap
and R5 against a Butternut, and the Butternut (with 60 radials) easily
beat the other "ground-less halfwave" antennas. QST found a similar
thing in a GAP review.
Remember the ground system does two things, it provides a second
terminal (in the case of an end fed antenna) and it also minimizes
losses below the antenna from ELECTRIC, MAGNETIC and RADIATION field
coupling. The ground under a half wave antenna is important even
without a physical electrical connection, because the strong electric
field near the element induces ground losses.
It has been my experience, and the experience of hundreds of broadcast
stations, that ground systems are very important with any antenna
mounted near the earth, either with a dipole or vertical.
73 Tom
mike....@ziplog.com
In <3355B4...@delphi.com>, "Cecil A. Moore" <cecil...@delphi.com> writes:
>>Hi Jim, wonder if you know what the efficiency of a balanced halfwave
>dipole 100 ft in the air is? I run one on field day stretched between
>two trees unreferenced to ground (at least by me). Beats the socks off
>any of the single-element ground-mounted verticals no matter how many
>radials they have.
>
>73, Cecil, W6RCA, OOTC
Of course some of us *DO* know that, however.
Some of us don't have 100 foot supports for that at FD, or even *CLOSE*
to that, thus we do chose well installed verticals with ground radials
at our FD operations. The trick is to do a *GOOD* job of installing them.
On 40 CW they can produce rates peaking at 120 contacts an hour and
solid hour after hour average rates which are *FAR* above what a
dipole at 20-30 feet up at the same location will do.
You may also be in a fortunate location for your solution in another
way, Cecil. If your 100 foot high dipole happens to be broadside to
the vast majority of the contest field of possible contacts, you are
also benefiting from the directive effects of that diople. On the other
hand, we in more-or-less south central Texas have to face the fact
that the play comes from three whole quarters around the horn or
more. It is surprising how much the end nulls can hurt you.
I will grant you that a 2 element beam on 40, well up in the air, does
a better job on 40 than a good vertical, as well. That's exemplified
by last year's 2A grand slam winner from down south of us in Magnolia.
However, those efforts are simply beyond the ability of most field day
participants.
Year after year, the rates for a well installed full sized, shunt fed all
grounded vertical with radials bonded directly to the pole, have more
than doubled the contact rate per hour on 40 against any kind of
horizontal wire efforts that BARC has ever been able to get up.
Steady 60+ contact an hour rates on 40 are better, in my humble
opinion, on 40, than most folks can achieve with horizontal wire
efforts. We can prove that year after year after year.
The same effect, only more pronounced, is present on 80 meters,
where the required height for dipole efficiency is there. And yes,
sigh, fewer folk operate 80 than 40, I think...
There is *ONE* situation where a group should also really think
about *LOW* horizontal efforts on Field Day, in my opinion.
During the daylight hours on 40 and 80, so-called cloud warmers
are beneficial, in my experience, in picking up the perhaps otherwise
missed close-in contacts. There is really some wisdom in making
a secondary effort at that, especially if, in your area, there are a
large number of hams that will be on 40 and 80 that are concentrated
over a couple hundred miles from your proposed site. There are, in my
personal opinion, less contacts lost over this fabled 'low angle favoritism'
of verticals, than you might think, in that the vertical angle coverage
of all of the verticals ever likely erected at FD well includes that
needed 30-45 degree launch range. However, brute force from a
very low horizontal wire does seem to have the effect of waking up
your local competition! It at least makes them *THINK* you are doing
a fine job at FD when you blow them off the table in broad daylight!
<reflective laugh>
//-----------------------------
Mike - W5WQN
Mike....@ziplog.com
MIke....@f3000.n117.z1.fidonet.org
Cecil Moore
Tom Rauch wrote:
> On 40 meters, my 85 ft high dipole beats my 30 ft vertical by a few dB
> except off the dipole ends.
Oh yeah, forgot to say I work 40m on field day. A couple of years ago we
actually did A/B comparisons before field day started, dipole VS vertical.
No matter how many radials were added to the vertical, the dipole still beat
it broadside for all signals that we could hear. The vertical was only ever
better off the ends of the dipole.
> The ground under a half wave antenna is important even
> without a physical electrical connection, because the strong electric
> field near the element induces ground losses.
Yes, but doesn't the ground plane under a high dipole simply reflect that
same RF straight up so it doesn't matter whether it gets lost in the ground
or in outer space? Seems to me for a ground plane to help the gain of a 100
ft high dipole for any TOA less than 30 degrees, the ground plane would need
to extend out more than double the antenna height, i.e. more than 200 ft.
Which caused a light to come on upstairs. How about a low dipole with a good
conductive ground plane under it?
73, Cecil, W6RCA, OOTC
Cecil Moore
James C Sutton wrote:
> >How about a low dipole with a good conductive ground plane under it?
>
How about a tin roof as the ground plane? The RF would hit the sloping roof
and have a relatively low angle of radiation? Any way to model this?
73, Cecil, W6RCA, OOTC
mondoray
So...
Take an antenna maker like GAP. They talk about NO RADIALS but then
introduce the term "counterpoise". Eh? What's the difference? Radials
vs. "Counterpoise"?
Ray
N1XNC
Note: I have just spent 6 months in a cave. You are very much smarter
than me. I am very sensitive. I have asked a simple question. Answer it
nicely and I'll up your VISA limit to 50K. Any sarcasm and you are dead
meat. This postscript based on the responses I've observed in this NG.
James C Sutton
On Thu, 17 Apr 1997 09:55:14 -0700, Cecil Moore
<cecil_...@ccm.ch.intel.com> wrote:
>
>Tom Rauch wrote:
>> On 40 meters, my 85 ft high dipole beats my 30 ft vertical by a few dB
>> except off the dipole ends.
>
>Oh yeah, forgot to say I work 40m on field day. A couple of years ago we
>actually did A/B comparisons before field day started, dipole VS vertical.
>No matter how many radials were added to the vertical, the dipole still beat
>it broadside for all signals that we could hear. The vertical was only ever
>better off the ends of the dipole.
There is one factor that no one has mentioned and that is the
difference in antenna pattern between a vertical and horizontal
antenna. For short range opeation - say up to 400-1000 miles a
radiation angle should be between 60-90 degrees. A lower angle will
extend the range well beyond the 1000 mile mark. As most know for real
long distance DX a radiation angle of around 5-10 degrees is required.
And so when comparing antennas it is necessary to include the
distances over which you intend to operate.
Jim/AC4CZ
>
>> The ground under a half wave antenna is important even
>> without a physical electrical connection, because the strong electric
>> field near the element induces ground losses.
>
>Yes, but doesn't the ground plane under a high dipole simply reflect that
>same RF straight up so it doesn't matter whether it gets lost in the ground
>or in outer space? Seems to me for a ground plane to help the gain of a 100
>ft high dipole for any TOA less than 30 degrees, the ground plane would need
>to extend out more than double the antenna height, i.e. more than 200 ft.
>
>Which caused a light to come on upstairs. How about a low dipole with a good
>conductive ground plane under it?
This will provide high radiation angles from say 50 to 90 degrees.
>
Jim/AC4CZ
>73, Cecil, W6RCA, OOTC
Jesse Touhey
>James C Sutton wrote:
>> Yes -- you see HF antennas without ground systems - but their
>> efficiency is pretty sad - a long way from optimum.
>Hi Jim, wonder if you know what the efficiency of a balanced halfwave
>dipole 100 ft in the air is? I run one on field day stretched between
>two trees unreferenced to ground (at least by me). Beats the socks off
>any of the single-element ground-mounted verticals no matter how many
>radials they have.
>
>73, Cecil, W6RCA, OOTC
Hi Cecil, fed properly. with low line loss, a 1/2 wl dipole .5 wl
above ground will have efficiencies in the 90% + region. It will
exhibit more gain at lower angles in it's best directions than any
single element ground mounted vertical system with the exception of a
vertical system mounted on a metal platform over salt water.
For very low take off the vertical system not only requires a
extensive radial system but is dependent on ground conductivity for
low angles. As you know, the horiz dipole will show very little
difference in gain and take off when moving it from poor ground to
salt water conductivity.
The horiz dipole .5wl above ground gets it's gain (over the vertical
system) from the re-reflection of the IN PHASE ground wave combined
with the incident wave. The re-reflected ground wave does not go
straight up at 90 deg. In fact, the dipole appx .5 wl above ground
has hardly any high angle.
Respectfully, Jesse, W6KKT
Tom Rauch
>Tom Rauch wrote:
>> On 40 meters, my 85 ft high dipole beats my 30 ft vertical by a few dB
>> except off the dipole ends.
>
>Oh yeah, forgot to say I work 40m on field day. A couple of years ago we
>actually did A/B comparisons before field day started, dipole VS vertical.
>No matter how many radials were added to the vertical, the dipole still beat
>it broadside for all signals that we could hear. The vertical was only ever
>better off the ends of the dipole.
It's nice to see we have the same experience on 40. 80 meters seems to
be the "breakover band", where the ground mounted vertical becomes
more useful. In a few months, I'll have a 200 foot tall or taller
tower up. Then I can get my 80 meter dipole back up where it works
well. ;-)
>> The ground under a half wave antenna is important even
>> without a physical electrical connection, because the strong electric
>> field near the element induces ground losses.
>
>Yes, but doesn't the ground plane under a high dipole simply reflect that
>same RF straight up so it doesn't matter whether it gets lost in the ground
>or in outer space? Seems to me for a ground plane to help the gain of a 100
>ft high dipole for any TOA less than 30 degrees, the ground plane would need
>to extend out more than double the antenna height, i.e. more than 200 ft.
Think of it this way Cecil, the caharges in the ground below the
antenna still move from dipole exitation. If the media is lossy, the
energy is converted to heat.
With a poor ground below a horizontal antenna... energy that could
have been "reflected" (re-radiated is a better term) up, and used to
cancel high angle radiatio that does no good, is converted into heat.
If you model it on a nice program like EZ-nec, you'll see the effect.
If you do an A-B test with two antennas, one over a good GP and one
not, you'll hear the effect.
>Which caused a light to come on upstairs. How about a low dipole with a good
>conductive ground plane under it?
Beams a good signal straight up, and yet the good GP still does not
hurt the low angle signals!
I had a low dipole over my radials on my 130 ft tower for 75 and 160,
just to work within a few hundred miles. It's like adding an extra
tube or two to the PA !
73 Tom
Tom Rauch
>So...
>Take an antenna maker like GAP. They talk about NO RADIALS but then
>introduce the term "counterpoise". Eh? What's the difference? Radials
>vs. "Counterpoise"?
>
>Ray
>N1XNC
Hi Ray,
I hardly know where to start with an answer to your question, except
to say most antenna ads are full of bunk.
ANY vertical mounted within 1/4 wl or so of earth requires a good
ground system, or the efficiency will stink. A small counterpoise
takes care of only a small part of the problem.
All this talk about "ground independent" antennas is only true if the
antenna is a long distance from ground.
I A-B tested a Gap against a R5 and a Butternut (with 60 70 foot long
radials) , and the Butternut greatly outdid the other antennas.
A ground independent vertical might seem like a good antenna, if
you've never had a real antenna. IMHO, they belong only in situations
wher the user cares less about his signal, and does not or can not
install something else.
My twelve ft mobile antenna beats "ground independent" magic
verticals.
73, Tom
James C Sutton
On Thu, 17 Apr 1997 17:05:59 -0700, Cecil Moore
<cecil_...@ccm.ch.intel.com> wrote:
>
>James C Sutton wrote:
>> >How about a low dipole with a good conductive ground plane under it?
>>
>> This will provide high radiation angles from say 50 to 90 degrees.
>
>How about a tin roof as the ground plane? The RF would hit the sloping roof
>and have a relatively low angle of radiation? Any way to model this?
>
>73, Cecil, W6RCA, OOTC
Cecil - the maximum angle of radiation is primarily based upon the
height of the antenna. Low height and you get high angles of
radiation. Depending upon the frequency your are xmting on you may
have to have a very large tin roof to accommodate your antenna for low
angle radiation. In theory a good idea.
There are several good antenna modeling programs that will detail this
for you. They will provide radiation angles based upon the type of
antenna, height above ground and ground conductivity.
One is sold by Roy Lewallen W7EL - you will note that he has had
several comments re your questions. It is a nice program to work
with.
The other by Brian Breezley K6STI -- is also a good program.
Get some literature from each and compare - it's then up to you.
They both advertise in QST.
73/Jim/AC4CZ
Cecil Moore
James C Sutton wrote:
> One is sold by Roy Lewallen W7EL - you will note that he has had
> several comments re your questions. It is a nice program to work
> with.
Hi Jim, Roy's a nice guy with a nice product. I've used ELNEC and EZNEC
for years. I'll soon need another copy of EZNEC cuz I've just about worn
out the first one. :-) Seriously, for the cost of "throw one up and
see how it works" one can buy EZNEC, learn a bunch about antennas, and
choose the proper ones to match one's boundary conditions.
I just don't know how to model a perfect ground that is limited and not
flat (like a tin roof), like this:
/\
/ \
/ \
/ \
---------------------- ------------------------
73, Cecil, W6RCA, OOTC
Jesse Touhey
On Fri, 18 Apr 1997 14:49:57 GMT, W8J...@worldnet.att.net (Tom Rauch)
wrote:
>>Which caused a light to come on upstairs. How about a low dipole with a good
>>conductive ground plane under it?
>
>Beams a good signal straight up, and yet the good GP still does not
>hurt the low angle signals!
>
>I had a low dipole over my radials on my 130 ft tower for 75 and 160,
>just to work within a few hundred miles. It's like adding an extra
>tube or two to the PA !
>
>73 Tom
Hi Guy's, from the above threads, what am I missing? In my
experience, ground conductivity has little effect on gain at low take
off angle on relatively high horiz dipoles (>100' 80m) . Overall gain
(both high and low angles) will increase less than 1.5db going from
very poor ground conductivity to salt water? As you know, the
vertical system is another story. For efficiency at low take off they
require very extensive ground systems + excellent ground conductivity.
At 100' or more above ground I have never had a 80m single element
vertical system (with extensive radial system) out perform the high
dipole on long dx paths ( in the dipole's best directions). I have
never compared the two antenna's over salt-water, but I'm sure the
vertical would best the dipole at low angle contacts. That high 80m
dipole fed with open-wire feeders is a real barn burner on 40m.
Respectfully, Jesse, W6KKT
Roy Lewallen
In article <3356BA...@ccm.ch.intel.com>,
>>
>
>How about a tin roof as the ground plane? The RF would hit the sloping
roof
>and have a relatively low angle of radiation? Any way to model this?
>
>73, Cecil, W6RCA, OOTC
About the only way to model something like a tin roof with NEC-based
programs is as a wire grid. 0.1 wavelength wire spacing is adequate for
most uses. Wire diameter is usually chosen so that the total surface area
of the wire is the same as for the modeled surface. It can be tedious to
do, but results are generally reasonable. Don't expect extreme accuracy
with this method, though.
NEC's conductive "patches" (not implemented in EZNEC) are suitable only for
closed structures, since the currents are forced to zero at unconnected
patch edges, an unrealistic condition unless the surface is closed.
Roy Lewallen, W7EL
Ralph B. Holland
g.r...@crf.IT wrote:
...
> Can any one explain me the role played by ground or its surrogates
> ( radials, rods etc... ) related to an half wave vertical antenna,and
You may choose to visit my site and explore the Antenna Simulation pages
(Currently for 1/4 wave verticals and 1/2 wave horizontal dipoles).
In general the substitution of an artificial ground system improves
antenna impedance and efficiency when the antenna is sited over poor
ground. The artifical ground system is more effective when if it
elevated above physical (poor) earth.
The effect of ground is at least two-fold:
1. Power fed into an antenna feedpoint is distributed between the
radiated field, and the structure losses. When the antenna uses the
ground as part of the radiating system (ie most antennas and
particularly verticals) it may experience significant ground losses.
Energy is stored in close proximity to the antenna in the volume called
the near-field zone. The near-field zone is often called the reactive
zone, energy is stored in a similar fashion to a resonant circuit which
switches energy from the capacitor back to the inductor and so on. If
this storage area is allowed to intersect with a lossy media then I*I*R
losses will decrease the power available for radiation. This
intersection may be with trees, the shack and indeed with the ground.
2. The ground can provide up to 6dB of improvement of antenna gain - in
some direction(s). If an antenna is sited above a perfect reflector the
field strength at
certain points will be double that of the free space case. Double field
strength
(ie E field or H field) means that the power has been quadrupled (6dB).
If the ground is lossy it is an imperfect reflector. This imperfect
reflection causes a loss of power - i.e. some power penetrates the
ground and basically heats it up.
The power that is reflected by a poor ground, which is dielectric as
well as lossy, begins to undergo changes such as phase shift in
reference to the feedpoint and between the E and H terms, which all
contribute to lowering the effective radiated power from the antenna.
R.f can penetrate several tens of metres into the ground - all this
energy will be lost - the amount that penetrates depends upon the angle
of incidence
and the ground parameters. The concept of direction (or incidence) is
even more complicated in the near field as not all energy fields are
orthogonal to the line of radiation.
The use of an artificial ground or an elevated antenna system is an
attempt to minimise these local losses. An artifical ground system is
not a substitute for a good ground because the antenna improvements are
mainly local; the far field reflections are still disrupted by the poor
ground which extends many wavelengths from your antenna. This is why the
best location is near the sea.
In the case of a half wave vertical the maximum current is not
co-located with the antenna feedpoint nor "ground" attachment. I believe
that it would be necessary to make the radials longer to make them
effective. I have seen in the literature that many 1/2 verticals are not
fed against a substantial ground system. This probably stems from the
belief that since the feedpoint is a high resistance point and requires
high voltage with low current to feed it that the resistance to ground
of some 100 ohms or so is relatively insignificant when compared with
10,000 ohms or so of a resonant 1/2 wave structure. This ignores the
near-field effect where significant losses may be generated and ignores
the far-field effect of poor reflection in the radiative terms.
Modeling would be a very good way to quantify the losses of a half-wave
antenna over poor ground. If I have time I may investigate this effect.
(I have little interest in constructing one of these as my favourite
band is 160m - a half wave vertical antenna on this band would be beyond
my means.)
--
Regards, 73
Ralph Holland, VK1BRH
http://www2.dynamite.com.au/vk1brh
http://www.qsl.net/vk1brh
P.S. You can obtain the modeling code that I use from the net.
Site references can be found from my Antenna Simulation pages.
There are also a few things you need to know so you can email me if you
are interested.
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