Topband: FCP model

Tom W8JI w8ji at
Wed Aug 1 15:57:22 PDT 2012

Let me throw this out for comments.

I think I found a valid test for the theory the FCP does not radiate, and 
thus does not have ground loss.

My countering statement was it cannot be a counterpoise, and cannot have 
current, without E and H fields. Even if we null farfield radiation (which 
is an electromagnetic field, as opposed to E and H induction fields near the 
conductor) we still must have local fields, or current will not flow out 
along the conductor.  Counterpoises only work because they have fields.

I modeled an FCP with enough spacing to not violate segment rules. I made 
two FCP's at right angles, with one foot spacing, 90 degrees from each 
other. I connected them at the normal feed terminal to form a "dipole" of 
sorts, using one as the counterpoise for the other. I used lossless wire in 
the model.

Resistance at the current maximum in freespace is  0.03 ohms. This very low 
resistance indicates very deep cancellation of farfield radiation. 
Efficiency was 99.2 percent. This indicates a very small model error of some 
type (probably because of close spacing between wires or failure to align 
segments) , because it should be 100%.

I moved the wires over real earth at 45 feet height. Resistance now changed 
to  0.04 ohms and efficiency changed to 20 percent. This indicates 
nearfields are impinging on lossy soil, because that is the only source of 
loss beyond the initial 0.8% error.

I moved the wires to 10 feet, and current maximum feed resistance increased 
to 1 ohm. Efficiency was then 0.4%. This indicates severe ground losses.

Now the points of this are:

1.) 10 feet is too close to the "soil" used in this model. Elevated radials 
at 10 ft are not going to be good if soil acts like the model.

2.) 45 feet could be high enough to be reasonably isolated from wire E and H 
fields in this model.

3.) Cancelling radiation is the farfield has very little to do with local E 
and H field levels that cause loss.

4.) We can't make local fields go away or it will no longer be a 
counterpoise. Those are the fields that allow current to flow out on the 
open ended conductors. For example, nylon rope would be a good non-radiating 
counterpoise with no local E or H fields, unless we rub a furry cat along 
the rope.

5.) We reduce ground loss by spreading the fields out as evenly as possible 
over the largest possible area of lossy media.

What we should not conclude is that fields are distributed the same when an 
antenna is connected. They are not. The E field in particular  will move 
toward the antenna open end.

(That's why we should put counterpoises below the flattop wires, so fields 
are less intense on lossy earth. When I was 12 or 13, I knew to put a 
counterpoise below the flattop wire. :-)

This test does not quantify losses. It does not quantify anything. It only 
shows trends. It shows relatively intense E and H fields surround the wires, 
even though someone 20 miles away might not hear the radiation field.

I think a test like this shows the difference between EM radiation, electric 
induction, and magnetic induction fields.

73 Tom 

More information about the Topband mailing list