A few Q's:
1) Tuning an antenna with base loaded coil - Inreasing ground helps with OA field strength of course, but adjustable length antenna position (loading coil is fixed) for resonance doesn't seem to change. Without any specific regard to ground system changes, how much will resonance be affected, if at all, by the ground system?...Or am I missing something?
2) I'm working on a multi-wire (all bonded as one) cage monopole design. Will efficiency increase with a conical design with the largest diameter at the top? If so, how much larger should it be at the top than at the bottom for, say, a 3m length? IOW, is there a ratio I can start with?
The reactance of an r-f ground system consisting of conductors buried in the earth adds very little to the reactance present at the base of a ~3-meter Part 15 AM vertical monopole -- so the loading coil reactance needed to resonate the monopole is practically unaffected by that r-f ground.
Here are some numbers for 1640 kHz using 6 evenly spaced, horizontal radials each 6 meters long and buried/centered around the base of a 3 meter long x 0.16" OD vertical monopole. The base of the monopole is located at the surface of the earth, and the physical length of the "ground lead" is zero.
Gnd Syst Config Earth Conductivity Base Z (No Coil)
- Direct Perfect 0.13 -j 4208 ohms
- Radials 30 mS/m dc 15 1.84 -j 4204
- Radials 8 mS/m dc 13 4.2 -j 4204
- Radials 1 mS/m dc 5 25.1 -j 4216
The radiation resistance of the monopole (0.13 ohms) is shown in the perfect ground case above, and is the only part of the antenna base Z that can produce useful radiation.
The loading coil needs to offset the "j" term of the base Z to resonate the system on 1640 kHz, which is nearly a constant for all these conditions.
Changing the length and/or the diameter of the monopole connected to the output of the loadiing coil changes the j term of its base impedance. For example, reducing the length of a 0.16" OD monopole from 3 meters to 2.9 meters changes the base Z to 0.12 -j 4330 ohms (perfect ground).
Increasing the OD of a 3-meter monopole to 6" (constant OD) changes the base Z at 1640 kHz to 0.1 -j 1681 ohms.
The larger OD monopole needs less inductance to resonate it on 1640 kHz, which will reduce the r-f loss in the coil and improve system efficiency slightly. The lower reactances also produce greater r-f bandwidth for this system.
I haven't modeled a conical cage antenna, but hopefully the above comments will give some insight into what might be expected.