In the third post about increasing range with low power AM transmitters we will look at transmitter modulation and how we can use it to increase the range of our transmitter.
AM transmitter modulation is a simple and low tech way to superimpose an audio signal onto a radio wave but one disadvantage is inefficiency. By increasing the audio power available to modulate the carrier (our individual chosen broadcast frequency), we can increase the distance over which our station can be picked up by a receiver. The three factors that will help low power AM broadcasters the most are input signal level, impedance matching and signal conditioning.
Because AM transmitters are inefficient, we can increase modulation by increasing the signal strength or Voltage peak to peak (Vpp) of the signals modulating our carrier wave. Amplifying our audio input doesn’t actually make the radio signal from our transmitter travel farther, instead it makes our signal useful for a greater portion of the range we have. As our signal fades as distance from the transmitter increases and the modulation needed by the receiver fades as well. Because an AM transmitter is inefficient, the modulation fades at a faster rate than the carrier.
Impedance matching is the other half of the audio signal strength equation. There is an inverse relationship between signal strength and impedance. Low level signals require a high impedance connection, high level signals work best with a low impedance connection.
Matching the signal strength of the output of the audio source to the input impedance of the modulation stage of the transmitter (the audio input connection) will assure the maximum transfer of power, deliver flat audio response and eliminate distortion.
The audio input impedance of a low power AM transmitter is seldom labeled and in the case of a homemade unit might not be known so some experimentation might be in order.
It’s always best to deliver an audio signal that is the best match for the impedance of the transmitter, through the use of preamplifiers and similar equipment. If this isn’t practical or if the low power broadcaster is experimenting with different configurations, there are some shortcuts available.
The three most common shortcuts are to use a simple LM386 chip amplifier to boost the audio; use a signal attenuation circuit to reduce signals that are too strong and using an impedance matching transformer to change the input impedance of the transmitter. In each case the audio will be distorted by the shortcut process. The LM 386 will elevate midrange and an attenuation circuit will cut off the highs and lows, having the same effect. Matching transformers can offer minimal distortion but only at the top of the price range.
Low power broadcasters can use signal conditioning to increase modulation with bandwidth filters, limiters and compressors. Bandwidth filters remove frequencies above and below the transmitters audio reproduction capability that won’t be broadcast anyway but contribute to overmodulation. Remove those frequencies and you can push more audio power onto the carrier.
Limiters serve a similar function by holding back those occasional audio peaks present in all audio content. Limiting again allows the broadcaster to put a higher level audio signal into the transmitter by putting a top limit on the highest audio peaks, again reducing overmodulation distortion with maximum audio signal strength.
Compressors help low power broadcasters by bringing up the level of the lower strength audio portions of the programming so that a higher audio level is delivered regardless of the loudness or quietness of the original content helping the receiver deliver consistently strong audio.
Using input signal level, impedance matching and signal conditioning are easy, relatively inexpensive and simple ways to improve signal quality and range for low power AM broadcasters.
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