On 7/9/20 8:39 AM, Ignacy Misztal wrote:
N6LF's experiment showed that 4 elevated radials did as well as 60 ground
radials but his soil had good conductivity.
In average soil the elevated radials can be 5 db down - see ON4UN book.
Ignacy NO9E
There's two different things going on with elevated radials..
One is that the radials form the "other half" of a dipole - You could
also do a vertical dipole (with hats, to make it physically shorter, if
you like. Depending on how you want to match it, I would expect the
performance to be quite similar *from a radiated field of the element*
standpoint.
Then, there's the "power absorbed in the soil" issue - an elevated
vertical dipole, or a limited number of above ground horizontal radials
both have the potential problem that the near field of the antenna is
"in the soil" and absorbing power. Highly conductive or highly wet soil
both have a good (high) reflection coefficient, so less of the field
winds up being absorbed.
It will improve the SWR bandwidth (just like having an attenuator in the
line!)
There is a smallish difference between vertical over radials and
elevated vertical dipole because the phase center of the antenna is
lower with the vertical, which changes the angle of incidence for a
reflected ray. There's also increased soil losses for the vertical over
radials, since the high current part of the antenna is closer to the soil.
This is something that is difficult to predict in real life - relatively
few people know enough about the soil properties (particularly the
properties below the surface by more than a meter or so) to model it
accurately.
You can stick a OWL or monopole probe into the soil, but that measures
the properties of the surface. What you really need is something that
can probe 10 meters deep (or several "skin depths") deep. For
reference, at 7MHz, epsilon=13, Sigma = 0.005, Skin depth is about 4 meters.
I've often wondered whether you could get a substantial improvement from
verticals with radials (or elevated verticals) by just having sprinklers
and soaking the top layer of soil real well while operating. Spreading
salt or some ionic substance (fertilizer?) is another option. If the
sigma goes to 0.1 (1/10th that of Seawater) - skin depth is < 1 meter.
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