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Beginners guide to AM Transmitters, AM Antennas & AM Grounding
Let's take a quick look at the four critical areas impacting range in low power AM broadcasting: transmitter, based loaded antenna, a good ground and fine-tuning.
First is the transmitter type.
Check to see what kind of transmitter is used and pay attention to how transmitter output is configured.
Is the transmitter a simple oscillator or is a buffer amplifier present? This is important because a simple oscillator will be very sensitive to small changes in the antenna, ground or surroundings. A simple oscillator will be a bit trickier to tune.
A more sophisticated transmitter will have a final amplifier section that will protect the oscillator from the effects of local environment changes. Either can be made to work but it's important to know which is being used.
Check the schematic and the circuit board to see if there are internal matching circuits (small inductors or a few capacitors) between the transmitter output and the antenna terminal. These low-tech approaches to antenna matching are great for transmitting across a room but not across the street.
Jumper these items out of the circuit when tuning for maximum range.
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Ground rods, ground radials and ground screens are all very helpful. The FCC won't count anything below ground so put some metal under the dirt.
Six radials, ten feet long, will deliver at least 50% of the maximum benefit so start there. A ground connection to the transmitter more than a few inches long can result in an unbalanced dipole and an NOUO for violating the 3 meter rule.
Finally, tune the system by adjusting components. Changing the connecting point (tap) on the coil changes the inductance and can make fairly large-stepped adjustments.
The antenna can be shortened slightly for fine tuning. The filament of a low current lamp will indicate the current level passing through the antenna when connected in series with the antenna and coil. A micro-ammeter can be connected in series with the antenna tuning circuit and the range should be in the low uA.
Better range, better signal penetration and better audio quality can be achieved by first understanding what you are working with and then maximizing performance with extreme fine tuning.