Home › Forums › temp › Loading coil chart for 10-ft broadcast AM vertical antenna › Radiator OD, Ground Systems and Radiation Efficiency
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[quote=kk7cw]The fatter the antenna in relationship to the diameter the lower the Q-factor, the broader the pass band of the antenna. However, total surface area is still the trump card in improving efficiency.[/quote]
As an example to clarify the above statement about Part 15 AM antenna efficiency, a linear radiator comprised of a 3-meter length of #28 wire by itself has almost the identical radiation efficiency as a 3-meter length of 6″ diameter conductor, when both are resonant and equal r-f currents flow in both of them.
The benefit of the larger diameter radiator is that its input capacitance will be lower, and more constant across the operating bandwidth.
The lower input capacitance means that the loading coil needed to resonate that radiator can have lower losses, leaving more of the available tx power for the radiator, which increases the field it can generate. This, and not the “total surface area” of the radiator is the reason that an antenna system with a larger diameter radiator has slightly better system efficiency.
The more constant capacitance of the larger diameter radiator over the operating bandwidth means that the SWR at the feedpoint stays lower across those frequencies, which in turn allows better power transfer of program modulation into the radiator.
But the biggest and least understood factor affecting the radiation efficiency of the complete antenna system is the r-f resistance of the ground connection. This is not just the DC resistance of the conductor(s) leading from the tx chassis to something buried in the earth (ground rods, etc). It is the resistance in the “ground system” to the r-f currents induced into the earth by radiation from the antenna.
These r-f currents need to be returned to the tx in order to complete the path needed for that current to flow in the radiator. If most of them can’t be collected from the earth, the antenna won’t radiate well.
The r-f currents exist in the earth out to a distance of about 1/2 wavelength from the vertical radiator. Collecting them efficiently requires providing a metallic path for them to return to the antenna system. Broadcast stations typically bury 120 radials of at least 1/4-wavelength each. Such a radial system has a resistance of about 2 ohms to the returning earth currents.
The r-f resistance in the buried ground system path of a typical Part 15 AM antenna system will be considerably higher than 2 ohms.
Here’s how the radiation efficiency of a Part 15 AM antenna system is calculated, using ~typical values.
A: Radiation resistance of 3-meter vertical
at high end of the band = 0.1 ohm
B: Loading coil r-f resistance = 2 ohms
C: Resistance in the ground system (r-f resistance) = 20 ohms
Radiation efficiency of the antenna system = A/(A+B+C) = 0.1/22.1 = 0.45%
So this antenna system will radiate 0.45% of all the power that the tx can supply to its feedpoint. The rest is lost as heat.
The biggest defining element by far in the efficiency of a ground-mounted Part 15 AM antenna system is the loss in its ground system.