At 01:21 PM 3/21/98 -0700, Eric wrote:
Snip...
>Seriously, How much of the vegetation will be sticking up above
>your radial plane? Vegetation in the near field is pretty lossy
>stuff too. You might be able to benefit from raising the radial
>plane above the vegetation if possible. It would still need to
>be more dense than four or even six radials though.
>
This is the area where the experimentation needs to be done.
My findings from the elevated G.P. in downtown Spokane included
the fact that there is no R.F. left to couple to the ground or
anything else on the bottom side of 120 elevated radials.
The drill now for amateur purposes and antenna site limitations
is to find out how few elevated radials one can get away with and
still have a fairly impenetrable screen. I think it is a given that
the elevated radials must be above all vegetation and man-made
structures to assure that there is little R.F. remaining on the
bottom of the screen to do any coupling to it. IMO, it is also a given
that the number of elevated radials will be more than 12 and less
than 120 to do the same job as `120 buried radials. When you can
walk around under the radial system and detect no presence of a 1kw
signal being transmitted from the G.P., you have arrived at "radial
nirvana!"
>>I can understand with HF and VHF horizontal Yagis but not for a
>>160M vertical. If rocky hilltops were superior then that is where
>>the BCB towers would be.
When I moved to Honolulu years ago, I asked why all the B.C. stations
antennas were on the beach instead of atop those beautiful 4,000 foot
hills. They said it was because a radial system would be impossible to
construct on such steep terrain, and even if it weren't, a good
portion of the listening audience would be 4,000 feet below the radial
system and hear little or nothing.
>
>Most BCB station operators don't get advertising revenue from
>being able to claim DX listeners (there are exceptions of
>course). So most don't care about a site that is good for
>launching signals beyond their daytime coverage area.
My contention is that a site which maximizes the signal in a
day time coverage area will also be maximized at the night time
DX receiving site, assuming it is a G.P. or vertical.
\
>>I have a very simple question: With my soil AND terrain, what is
>>the lowest TOA I can expect......with any # of radials? I
>>suspect it is around 15 degrees or higher...per ON4UN and the
>>ARRL ANtenna Manual.
It has been well established on this thread and elsewhere that
near-field changes have little to do with TOA. Only the height
of the vertical radiator and the far-field affect TOA.
>Snip...
>
>>>I don't pretend to be able to explain it but I have now been
>>>bitten several times by inferring relative effeciencies from
>>>feedpoint impedance measurements. And I was later sorely
>>>disappointed by actual field intensity measurements which were
>>>done to confirm the work.
>>
>>
>>Over what soil conditions?
>>
>
>Various. Ranging from 3 mS to 15 Ms or so. Mostly in the 3 to 5
>mS range. Conditions around here are VERY spotty and vary over a
>pretty large range. It is one of the things you have to be very
>careful of when doing antenna measurements.
The soil conductivity here in the Dallas area is >30 ms/m...very
good. I started out years ago with 2 buried radials on my shunt
fed tower. Over the years I have added more radials each spring;
now totaling over 35, and the base impedance has never changed!
>>Would you think that my extremely poor soil is the reason the
>>elevated radials appear to work well? With true ground somewhere
>>around 50' + below the surface the coupling effect is
>>minimized. This might tend to explain why BCB sites over
>>excellent ground do not benefit from elevated radials.
The ground conductivity in Spokane is 8ms/m. This might explain
why 120 elevated radials work so good over poor ground.
>A better way to think of it is: "How far down into the earth does
>the skin depth of this poor soil let my fields penetrate?" The
>larger the volume of earth that is interacting with the fields
>around the antenna, the more loss there will be.
And I have learned in Spokane that there is no penetration by the
fields into the poor soil because there IS NO field below the
radials.
(((73)))
Phil, K5PC
"To do is to be".....Sartre
"To be is to do".....Aristotle
"To be or not to be".Shakespere
"Do be do be do".....Sinatra
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