[TowerTalk] 80 meter antenna choice

[jimlux]

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.