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  #21   Report Post  
Old September 7th 05, 05:58 PM
Ron Murray
 
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| Sorry to disagree, Reg, but it appears you're overlooking an
important
| point--the difference between the efficiency of the radiating system
| itself, versus the efficiency of the ground area external to the
| radiating system.
|
| BL&E shows that when 90 - 120 (actually 113) radials of 0,4 w/l form
| the ground system for a 1/4 wl radiator, the efficiency is 98.7%
| efficient, REGARDLESS OF THE SOIL RESISTIVITY UNDER THE RADIALS.
This
| is shown by obtaining the field strength of 192 mv/meter at 1 mile
for
| 1000 watts delivered to the antenna under the conditions described
| above, compared to 194.5 mv/meter with a perfect ground having an
| efficiency of 100%
|
| It is only the soil resistivity of the ground external to the radial
| system that determines the field stength external to the radial
| system. Consequently, the soil resistivity (or conductivity, if you
| like) is significant only in the areas external to the radial
system.
|
| Walt, W2DU

Walt,

What if the ground outside the radial system was comprised of 30-1000
feet of sand and rock overlying any conductive soil below? Would we
then be able to measure 192 mv/meter at one mile with our 113 radials
of 0.4 w/l?

Ron, WA4IWN


  #22   Report Post  
Old September 7th 05, 08:13 PM
Roy Lewallen
 
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dansawyeror wrote:
All,

A fundamental basic question, which is the primary purpose of radials:

1. is it to create a ground, that is a as close as possible to zero ohm
virtual reference for the 'real' vertical half of the dipole?


There is no such thing as "creating a ground". As for a "virtual
reference", you can declare any point on any conductor a "reference" and
for that matter "ground" that you wish.

2. Or are they to create a real resonant half of a dipole?


There's no need to try to make a dipole. If the radials are on or very
near the ground, their sole purpose is to reduce the amount of loss due
to current returning to the base of the antenna. The current entering
the antenna at the base equals the current flowing into the source
through the ground. This ground current results in I^2 * R loss; radials
reduce the R and therefore the loss.

In the case of a "ground plane" antenna with highly elevated radials,
the radials provide a path for the base current to flow (again, current
out of the source -- into the antenna -- has to equal the current into
it -- from the radials). Because of the physical configuration, it's
sometimes more convenient to build an antenna this way instead of making
a dipole. The radials radiate very little, and the vertical section
radiates twice as much per unit length as a dipole, resulting in the
same overall gain and pattern.

If it is the first then what does the 'efficiency' curve look like for a
shortened, loaded, vertical? That is if the vertical element is loaded
to resonate at 1/5 of a half wave length what does the ground resistance
profile look like for 120 radials at various lengths of 1/20 wave, 1/10
wave and 1/5 wave?


The answer to this depends on the ground conductivity and frequency. But
the radiation resistance of the shortened antenna will be less than that
of a full-height one. Therefore, if the ground resistance is fixed and
determined by the ground system (not completely true -- it does depend
some on the antenna height -- but close enough for discussion), the
efficiency of the short antenna will be less than for a full-height one.
I recommend finding and reading "The W2FMI Ground-Mounted Short
Vertical", by Jerry Sevick, W2FMI in March 1973 QST. He built several
antennas very much like you describe and made extensive measurements.


The question I am really driving at is if mesh is layed down at 100%
coverage about what fraction of a wave length needs to be covered to
create a 2.5, 5, and 10 ohm equivalent ground for the vertical above?


Sorry, I don't know the answer to that one right off the bat. It could
be determined with NEC-4 modeling, but I don't have time to do that. I
suggest that you locate a copy of Brown, Lewis, and Epstein's paper
"Ground Systems as a Factor in Antenna Efficiency", now posted on the
web. You should be able to get a fairly good idea from their
measurements of 113-radial systems.


In the paragraph above is the mesh simulating a ground or is it fact
operating as a ground.


I really don't know what "simulating a ground" and "operating as a
ground" means. But the radial field doesn't act like either real Earth
or a perfect infinite plane, if that's what you mean. If sufficiently
fine, a mesh will act like a solid conductor the size of the mesh.

Roy Lewallen, W7EL
  #23   Report Post  
Old September 7th 05, 09:08 PM
Reg Edwards
 
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"Walter Maxwell" wrote in message
...
On Wed, 7 Sep 2005 14:07:39 +0000 (UTC), "Reg Edwards"
wrote:

MK,

How satisfying it is to read your message, written in plain, easy

to
understand, well-punctuated English, without any undeciferable

coded
abbreviations.

I agree with what you say although I am unfamiliar with exactly how
the FCC fits into the scheme of things.

Amateurs and commercial broadcasters have a common fundamental
requirement. There is a service area to be covered with a given

field
strength. Depending on frequency, requirements then diverge. But

the
design methods used to satisfy requirements are all confined (or
should be) to the principles of engineering economics. Inevitably,

the
Dollar, Pound, Frank, Mark, Rouble and the Yen rule the roost.

Both commercial broadcasters and amateurs do a cost-befit analysis.
The broadcaster takes into account the revenue acruing from selling
the service. The amateur, whether he likes it or not, has to ask
himself what the satisfaction of using the station is worth.

Amateurs' bank accounts are not unlimited.

Field strength at the limits of the service area depends on the

power
efficiency of the radiating system. If engineering economics

dictate
use of a set of buried ground radials then the peformance of the
ground radials must be included. Considering the system as a

whole,
it may be economical NOT to achieve the maximum possible radiating
efficiency. Indeed, the maximum is seldom the target.

If there is an economical choice in the matter, once the location

of
the station is decided, everybody agrees that efficiency depends on
soil resistivity at the site. To estimate efficiency it is

necessary,
at the very least, to make a guess at soil resistivity. Perhaps

just
by looking at the type of weeds growing in it. Or it can be

measured.

Depending on how far it enters into station economics, it is

possible
to numerically estimate efficiency from the number and length of
radials AND FROM SOIL RESISTIVITY.

B.L & E and the FCC don't enter into it.
----
Reg.


Sorry to disagree, Reg, but it appears you're overlooking an

important
point--the difference between the efficiency of the radiating system
itself, versus the efficiency of the ground area external to the
radiating system.

BL&E shows that when 90 - 120 (actually 113) radials of 0,4 w/l form
the ground system for a 1/4 wl radiator, the efficiency is 98.7%
efficient, REGARDLESS OF THE SOIL RESISTIVITY UNDER THE RADIALS.

This
is shown by obtaining the field strength of 192 mv/meter at 1 mile

for
1000 watts delivered to the antenna under the conditions described
above, compared to 194.5 mv/meter with a perfect ground having an
efficiency of 100%

It is only the soil resistivity of the ground external to the radial
system that determines the field stength external to the radial
system. Consequently, the soil resistivity (or conductivity, if you
like) is significant only in the areas external to the radial

system.

Walt, W2DU

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

Walt, just what is it you cannot agree with? You appear to be making
an argument where none exists.

It is obvious there must be a distant point beyond which a large
number of radials will approach 100% efficiency regardless of ground
resistivity. B.L & E and the FCC arbitraliry decided on
1/2-wavelength and 120. Both nice round figures.

I'm sorrry to say you appear unable to agree that for the remaining
99.9% of all possible cases, ie., for cases less than 1/2-wavelength
and fewer than 120 radials, that GROUND RESISTIVITY in the immediate
vicinity of the antenna DOES HAVE A SIGNIFICANT EFFECT ON EFFICIENCY
and it cannot be disregarded.

My only criticism of B.L & E is that they forgot to measure soil
resistivity before leaving the site. And apparently, nobody has ever
bothered to go back and do it for them.

The only mention of their work occurs on this newsgroup. When laying
radials, 99% of amateurs forget B.L & E (if they have ever heard of
them) and the magic number of 120.

Hasan Schiers has recently given a blow-by-blow account of a sensible
way to lay a set of radials with the reasoning behind it.
----
Reg.


  #24   Report Post  
Old September 7th 05, 09:17 PM
Richard Harrison
 
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Reg, G4FGQ wrote:
"The broadcaster takes into account the revenue accruing from selling
the service."

Yes, and he takes into account the ease of compliance with FCC Riles in
the USA.

Although 90 radials is a minimum requirement for the U.S. AM
broadcaster, here is the rule quoted by John Edward Cunningham in his
1977 "The Complete Broadcast Antenna Handbook", from page 311:

"The current FCC Rules specify that the radials should be at least 1/4
wavelength long and that there should be as many as practicable, but in
no case less than 90. The Rules add that a system of 120 radials spaced
every 3-degrees and extending 0.35-0.4 wavelength from the tower is
considered an excellent ground system. In addition, a square ground
screen 24 or 48 on a side is often provided at the base of the tower,
particularly when the tower height is such as to cause a high voltage.

Whenever a less than optimum ground system is used, the FCC requires a
complete field-intensity survey to establish that the effective field at
one mile meets minimum requirements."

Best regards, Richard Harrison, KB5WZI

  #25   Report Post  
Old September 7th 05, 10:08 PM
Roy Lewallen
 
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Reg Edwards wrote:
. . .
My only criticism of B.L & E is that they forgot to measure soil
resistivity before leaving the site. And apparently, nobody has ever
bothered to go back and do it for them.


What would you suggest as a method of measuring the RF ground
resistivity to a depth of 17 - 100 feet (3 skin depths at 3 MHz for the
range of likely resistivities)? Assuming it's very likely that the
resistivity would be quite different at different depths within that
range, how should they have used that information? How would you use
that information if someone "bothered to go back and do it for them"?

Reg, all signs point to your being seriously in need of a holiday. How
about a little trip to New Jersey to show us Yanks how it should have
been done? For less than the price of a couple of bottles of decent
wine, you can buy everything you'll need -- bucket, pocket DVM, trowel
-- right there, so you won't even have to carry any equipment with you.

Roy Lewallen, W7EL


  #26   Report Post  
Old September 7th 05, 11:16 PM
Reg Edwards
 
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Dear Roy,

I'm sorry to say your worthless comments constitute an abject
admission of defeat in an argument which exists only in your
imagination.

By the way, are you still using your S-meter as the North American
Standard of signal strength?
----
Reg.


  #27   Report Post  
Old September 8th 05, 02:40 PM
Reg Edwards
 
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Dan Richardson wrote -
Had BL&E been Englishmen I sure things would be different.G

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

They sure would!

They would have been instructed to go back and finish the job.
----
Reg.


  #28   Report Post  
Old September 8th 05, 05:09 PM
Reg Edwards
 
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Dan Richardson wrote -
Had BL&E been Englishmen I sure things would be different.G

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

They sure would!

They would have been instructed (by their employers) to go back and

finish the job.
----
Reg.


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

I am reminded of the military engineer who was dispatched by Napolion,
an engineer himself, in connection with standardisation of the Metre,
to measure the distance between the Earth's Equator and the North
Pole.

Measurements began, but the further the engineer departed from his
beautiful lady friend in Paris the more difficult it became to make
progress along the route. Eventually, he couldn't withstand the mental
and physical stress. He returned to her Parisian boudoir and resorted
to cooking the books in what time he had to spare.

So, the International Standard of Length, The Metre, held in Paris,
France, carefully guarded by the German occupying forces during WW2,
may or may not be equal to 39.37 English inches.

Actually, the most fundamental physical measurement standard is the
Mass of the Standard Kilogram on which everything else depends. But it
is quite an arbitrary quantity.

I have just finished a bottle of Blossom Hill, Californian, white
wine. Makes a pleasant change to arguing about what 'amateurs' BL&E
might, or might not have done before leaving the site.
----
Reg.


  #29   Report Post  
Old September 8th 05, 05:22 PM
David G. Nagel
 
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Reg Edwards wrote:

Dan Richardson wrote -
Had BL&E been Englishmen I sure things would be different.G

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

They sure would!

They would have been instructed (by their employers) to go back and


finish the job.

----
Reg.



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

I am reminded of the military engineer who was dispatched by Napolion,
an engineer himself, in connection with standardisation of the Metre,
to measure the distance between the Earth's Equator and the North
Pole.

Measurements began, but the further the engineer departed from his
beautiful lady friend in Paris the more difficult it became to make
progress along the route. Eventually, he couldn't withstand the mental
and physical stress. He returned to her Parisian boudoir and resorted
to cooking the books in what time he had to spare.

So, the International Standard of Length, The Metre, held in Paris,
France, carefully guarded by the German occupying forces during WW2,
may or may not be equal to 39.37 English inches.

Actually, the most fundamental physical measurement standard is the
Mass of the Standard Kilogram on which everything else depends. But it
is quite an arbitrary quantity.

I have just finished a bottle of Blossom Hill, Californian, white
wine. Makes a pleasant change to arguing about what 'amateurs' BL&E
might, or might not have done before leaving the site.
----
Reg.


All the above is/maybe true but remember that all the formule work using
the values represented in the METRE and the KILOGRAM. Something must be
correct here....

Dave WD9BDZ
  #30   Report Post  
Old September 8th 05, 05:51 PM
Walter Maxwell
 
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On Thu, 8 Sep 2005 16:09:23 +0000 (UTC), "Reg Edwards"
wrote:


I have just finished a bottle of Blossom Hill, Californian, white
wine. Makes a pleasant change to arguing about what 'amateurs' BL&E
might, or might not have done before leaving the site.
----
Reg.

I hate to say this, Reg, but calling BL&E 'amateurs' is insulting to
me. I knew them well, and worked with B and E in the RCA Laboratories
antenna lab, and they are anything but 'amateurs'. I think you
finished off the Blossom Hill to rapidly.

Perhaps you should take a look at the long list of IRE articles
published by Dr. G.H. Brown, reporting his work that shaped the
present design of all AM BC antennas. Did you ever wonder what
happened to the diamond-shaped AM towers? And why the AM towers
constructed since 1940 have a uniform cross section?

And did you know that Dr. Brown gave John Kraus, W8JK, the idea of
close spaced elements that culminated in the 'W8JK Beam' antenna?

L, (Bob Lewis) is also a fine engineer, although he is also a ham,
W2EBS.

Walt
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