Post by Deleted on Nov 5, 2017 19:41:24 GMT
A fat monopole offers reduced capacitive reactance and this allows us to reduce system loss resistance while maintaining sufficient VSWR bandwidth. In an earlier post I described a base-loaded fat monopole that has the drawback of requiring very low ground resistance. Center-loading is the way to achieve similar radiation efficiency while having a higher ground resistance. Moving the loading coil from the base to the center raises the required ground loss from 8 to 18 ohms, which is much easier to achieve.
The antenna is a 3 meter monopole composed of a 1" tube topped by a 12" diameter cylinder (wire). Note that the entire antenna can be built as a 12" cylinder. The cylinder lengths above the loading coil are 4, 6, and 9' for the 55, 45, and 5% loading coil versions. The EZNEC model uses a cage structure where the 12" cylinder is a hexagon of thin wires.
Design goal: To build a practical Part 15.219 compliant antenna having maximum radiation efficiency while providing sufficient bandwidth - 20 kHz - for a high fidelity AM signal. Higher radiation efficiency can be had at the expense of bandwidth.
ANTENNAS
Ground mounted
12" diameter
Loading coil Q = 250
System Q = 84, this is calculated as 1700kHz/20kHz = 84
System R = XL/84
Coil R = XL/250
GND R = System R - Coil R, GND R is the minimum ground resistance to have a -3dB bandwidth of 20 kHz or more
Freq = 1700 kHz
-3dB bandwidth = 20 kHz
H
H
H
H
H 12" cyclinder
I loading coil
I 1" tube
I
I
I
The antenna is a 3 meter monopole composed of a 1" tube topped by a 12" diameter cylinder (wire). Note that the entire antenna can be built as a 12" cylinder. The cylinder lengths above the loading coil are 4, 6, and 9' for the 55, 45, and 5% loading coil versions. The EZNEC model uses a cage structure where the 12" cylinder is a hexagon of thin wires.
Design goal: To build a practical Part 15.219 compliant antenna having maximum radiation efficiency while providing sufficient bandwidth - 20 kHz - for a high fidelity AM signal. Higher radiation efficiency can be had at the expense of bandwidth.
ANTENNAS
Ground mounted
12" diameter
Loading coil Q = 250
System Q = 84, this is calculated as 1700kHz/20kHz = 84
System R = XL/84
Coil R = XL/250
GND R = System R - Coil R, GND R is the minimum ground resistance to have a -3dB bandwidth of 20 kHz or more
Freq = 1700 kHz
-3dB bandwidth = 20 kHz
COIL PLACEMENT | COIL L | SYS R | GND R | RAD RESISTANCE | EFFICIENCY |
5% | 131 uH | 16.7 ohms | 11.1 ohms | 0.12 ohms | 0.72% |
45% | 215 uH | 27.3 ohms | 18.1 ohms | 0.28 ohms | 1.03% |
55% | 249 uH | 31.7 | 21.1 ohms | 0.31 ohms | 0.98% |
H
H
H
H
H 12" cyclinder
I loading coil
I 1" tube
I
I
I