Distance between outer ends of buried radials
 

I remember reading somewhere that one way to determine the optimum
number of buried radials for a vertical antenna is to use a specific
value for the distance between the outer ends. (I'm not sure if this
is the straight-line distance, or the length of a chord, but there
wouldn't be much difference.) But I can't remember the value.
Something like .05 wavelength rings a bell, but I'm not sure. Does
anyone know what the value is and where I might have read it?

Thanks -

Steve W9HJW

I seem to recall .1 (that's point one) wavelength. Can't remember the
source though. I used to have a memory but can't remember where I left
it.

Paul AB0SI


"Steve Beyers" wrote in message
om...
I remember reading somewhere that one way to determine the optimum
number of buried radials for a vertical antenna is to use a specific
value for the distance between the outer ends. (I'm not sure if this
is the straight-line distance, or the length of a chord, but there
wouldn't be much difference.) But I can't remember the value.
Something like .05 wavelength rings a bell, but I'm not sure. Does
anyone know what the value is and where I might have read it?

Thanks -

Steve W9HJW

Steve, W9HJW wrote:
"Something like .05 wavelength (between ends of buried radials) rings a
bell, but I`m not sure."

That would depend on the length and number of radials. But, the .05
number may be significant in the case of 120 radials. 120 radials is an
FCC standard. Spacing is 3-degrees.

In a right triangle, the side opposite an angle is equal to the tanget
times the adjacent side length. For a 3-degree angle, difference between
the adjacent side and hypotenuse lengths is insignificant.

The tangent of 3-degrees is .05. The distance between radials spaced by
3-degrees is therefore very nearly .05 times the length of the radial.
That`s my guess as to the source of Steve`s .05 number.

Best regards, Richard Harrison, KB5WZI

What effect would it cause if you ran a wire around the perimeter of the
radials - connecting them together ?
And I guess you could do it again at the half-way point so it resembles a
spider-web.
like a big fence just underground.... could you then reduce the length of
the radials??
I may just try this on a manageabe scale - say 10 meters

"Richard Harrison" wrote in message
...
Steve, W9HJW wrote:
"Something like .05 wavelength (between ends of buried radials) rings a
bell, but I`m not sure."

That would depend on the length and number of radials. But, the .05
number may be significant in the case of 120 radials. 120 radials is an
FCC standard. Spacing is 3-degrees.

In a right triangle, the side opposite an angle is equal to the tanget
times the adjacent side length. For a 3-degree angle, difference between
the adjacent side and hypotenuse lengths is insignificant.

The tangent of 3-degrees is .05. The distance between radials spaced by
3-degrees is therefore very nearly .05 times the length of the radial.
That`s my guess as to the source of Steve`s .05 number.

Best regards, Richard Harrison, KB5WZI



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On Tue, 20 Jul 2004 23:26:42 -0400, "Hal Rosser"
wrote:

What effect would it cause if you ran a wire around the perimeter of the
radials - connecting them together ?
And I guess you could do it again at the half-way point so it resembles a
spider-web.
like a big fence just underground.... could you then reduce the length of
the radials??
I may just try this on a manageabe scale - say 10 meters


Hi Hal,

You would then think of the problem in terms of how far must ground
current traverse through soil to find a conductor? This is a loose
analogy because the current traveling through any patch of ground is
infinitesimal. Rather, it is the efficiency by which the radials
shield the antenna from the "bulk" of ground beneath it. Hence,
thinking of it in terms of 3D and proximity yields a better conceptual
model - further, it allows you to understand why elevated verticals
with fewer radials have an equal capacity for "getting out," or so
reports offer.

73's
Richard Clark, KB7QHC

Richard Harrison wrote:
Steve, W9HJW wrote:
"Something like .05 wavelength (between ends of buried radials) rings a
bell, but I`m not sure."

That would depend on the length and number of radials. But, the .05
number may be significant in the case of 120 radials. 120 radials is an
FCC standard. Spacing is 3-degrees.

In a right triangle, the side opposite an angle is equal to the tanget
times the adjacent side length. For a 3-degree angle, difference between
the adjacent side and hypotenuse lengths is insignificant.

The tangent of 3-degrees is .05. The distance between radials spaced by
3-degrees is therefore very nearly .05 times the length of the radial.
That`s my guess as to the source of Steve`s .05 number.

It also works, roughly, for shorter radials.

Jerry Sevick, W2FMI, published data on the effects of shorter radials on
feedpoint impedance, and came to the conclusion that if you use shorter
radials, there is little advantage in using very large numbers. If you
re-plot Sevick's data, it turns out that, for a given length of radial,
you only need about enough to ensure that the ends are about 0.05
wavelengths apart. Adding more radials of the same length will bring
little further reduction in feedpoint impedance.

However, this is a "soft" limit, so others may interpret the same data
to say that the limiting tip-to-tip spacing is different, even as little
as 0.02 wavelengths.

The "tip-to-tip rule" is a useful guideline, but it isn't helpful to
argue over the detail, because feed impedance isn't the only important
criterion. Longer radials will also reduce losses due to ground
reflection, and improve your low-angle signal in particular... but in
order to make a difference at very low angles, the radials need to be
very long indeed.

All the "tip-to-tip rule" is saying is: if the radials are as long as
you can make them, and you already used enough to make the tips
0.05...0.02 wl apart, then you've done all you can. Go look for
something else to improve.



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

Hal Rosser wrote:
"What effect would it cause if you ran a wire around the perimeter of
the radials - connecting them together."

That would enable and enhance current travel in circles around the
antenna base of no value to the radiated signal but a loss sink for
precious power. Ed Laport in "Radio Antenna Engineering" warns against
this "hysteresis current" product in several places and refers to it as
a "former practice". Ed had the benefit of working with Brown, Lewis,
and Epstein at RCA and used their data liberaly in his book.

Best regards, Richard Harrison, KB5WZI

Ian White, G3SEK wrote:
"It also works, roughly for shorter radials."

Yes. .05 times the length along radials spaced at 3-degrees gives the
approximate distance distance between them at any point. It doesn`t have
to be between tips, just the same distance from the tower on both
adjacent radials.

Ed Laport says more and longer radials is better, but there is a point
of diminishing returns which occurs at 1/2-wavelength.

I`ve seen other reports that radial wire is better used when cut into
more and shorter radials.

The FCC`s 120 radials of 1/4-wavelength has been efficient for medium
waves in thousands of instances in all types of soil. It`s overkill in
highly conductive soil, but regulations must be somewhat uniform for
satisfactory enforcement. FCC`s rule is based on B,L,& E`s research. As
I recall, B, L, & E worked at 3 MHz.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
Ian White, G3SEK wrote:
"It also works, roughly for shorter radials."

Yes. .05 times the length along radials spaced at 3-degrees gives the
approximate distance distance between them at any point.

The original poster was asking about the guideline of 0.05 wavelengths
distance between the tips of the radials, independent of the length.
When I replied "It works...", that's what I meant by "it".


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

Richard Harrison wrote:
. . .
The FCC`s 120 radials of 1/4-wavelength has been efficient for medium
waves in thousands of instances in all types of soil. It`s overkill in
highly conductive soil, but regulations must be somewhat uniform for
satisfactory enforcement. FCC`s rule is based on B,L,& E`s research. As
I recall, B, L, & E worked at 3 MHz.

We should always be careful in applying commercial broadcast standards
to ham radio. For a broadcaster, the gain of a few percent in radiated
field represents a measurable increase in the audience size, hence a
significant increase in annual revenue, so it's worth a considerable
one-time cost. For most amateurs, it represents only a fraction of a dB
of signal strength, so isn't worthwhile.

Roy Lewallen, W7EL