On Fri, 2006-11-03 at 07:13 -0500, Bill Tippett wrote:
> W9OY:
> >It made me think of a radial field in a whole new
> light. It's more about effective coupling into a
> volume of ground and less about absolute numbers and
> such.
>
> Yes, maximize surface area coverage, but no
> need to make the ground screen any finer than 0.025
> wavelengths at the end points (however far out they are).
> In a nutshell, that is what Rudy, W8JI, etc are saying.
> Ground rods are meaningless for RF and I would not be
> using them if they weren't necessary for lightning
> protection of my tower.
>
> This concept makes it very simple to put
> down an effective radial system in any area using
> the minimum effective amount of wire. From page 9-15
> of ON4UN's 4th edition:
>
> ############################################
> In actual practice, when laying radials on an irregular
> lot where the limits are the boundaries of the lot, the most
> practical way to make best use of the wire you have is just
> walk the perimeter of the lot and start a radial from the
> perimeter (inward toward the base of the antenna) every
> 0.015 . (1.3 meters for 80 meters or 2.5 meters for 160) as
> you walk along the perimeter.
> ############################################
>
> Since that was published, the thinking is now that
> 0.025 wl will get you within 0.5 dB of "perfect ground"
> (near field that is). This is equivalent to ~60 1/4 wave
> radials instead of the ~100 used in ON4UN's book.
> This rule-of-thumb is based on actual field strength
> measurements by W8JI, N6LF, etc. Apparently W8JI
> and W7EL (Eznec author) will be publishing a paper
> about their radial system measurements in the future:
>
> http://lists.contesting.com/archives//html/Topband/2006-08/msg00023.html
>
> 73, Bill W4ZV
>
All this furor over a few percent efficiency change has no effect beyond
a couple miles that is discernible in the receiver with its logarithmic
response. Without a precision field strength meter and a very controlled
environment (isolated from all antenna sized objects and reflectors by
more miles than the measuring distance) being sure of the change to 0.5
dB is impractical. And it would help for a consistent measurement to
control the soil moisture in the top 2 meters of depth. It takes a 30%
loss or 50% gain in radiated power to move the receiver s-meter 1/2 S-
unit and unless the signal to noise ratio at the distant receiver is 0
dB, such a small change in signal isn't going to affect copy. And unless
the propagation distance is only a couple miles with no moving
reflectors and absorbers in the neighborhood, propagation attenuation
isn't going to stay constant within 6 or 10 dB or more.
Put it up the best you can with as many radials as will fit the lot
(include the metal fence in the radial network) and go chase DX and
listen to all that electrical noise coming from all directions.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|