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Filters and Stubs

Low Pass Filter

There is some confusion about what filters do. Many think filters absorb unwanted energy, converting unwanted energy to heat inside the filter. Another common thought is filters route unwanted energy to ground, making unwanted signals vanish in the equipment or station ground system.

Filters, unless they contain some form of diplexer, reflect unwanted signals back toward the source. Typical filters (and stubs) create a large impedance mismatch at the harmonic, ideally an open or short, although some systems might work best with a certain value and sign of reactance.

Let's look at one filter design in SPICE.

Inside the filter below:

RL1, 2, and 3 represent inductor losses.

RC1 and 2 represent capacitor losses.

Outside the filter:

L4, RL, and C3 represent a frequency-selective load tuned to 80 meters.

Rs is source resistance.

lkow pass filter spice model






With the 80-meter low pass filter, on 3.6 MHz and 7.2 MHz for peak heat, we have:

Component  3.6 MHz power 7.2 MHz power  3.6 MHz voltage 7.2 MHz voltage
Rs 50W 70W 50 volts 76 volts
C1 30 milliwatts 27 milliwatts 70 volts 35 volts
C2 30 milliwatts 1 milliwatts 70 volts 6 volts
L1 170 milliwatts 220 milliwatts    
L2 340 milliwatts 18 milliwatts    
L3 170 milliwatts >>1 milliwatts    
RL 50 watts >70 mW 50 volts ~0 V


3.6 MHz low pass filter, with capacitor input






Component  3.6 MHz power 7.2 MHz power  3.6 MHz voltage 7.2 MHz voltage
Rs 50W 132W 50 volts 58 volts
C1 15 milliwatts 80 milliwatts 50 volts 58 volts
C2 30 milliwatts 5 milliwatts 50 volts 10 volts
C3 15 milliwatts 0.5 milliwatts 50 volts 5 volts
L1 340 milliwatts 80 milliwatts    
L2 340 milliwatts 4 milliwatts    
RL 50 watts 150 mW 50 volts ~0 V


The data above shows filters reflect harmonics back toward the source, through mismatch. Filters generally do not absorb unwanted signals, nor do they route harmonics to ground.

As with stubs, filters normally have sweet and sour spots along transmission lines. The sweet spot is normally NOT directly at the output port of an amplifier for high-frequency band harmonics for both filters and stubs, and is more often very close to the antenna if it is placed near anything, yet we normally place filters right at the amplifier.  I believe the idea a stub or filter needs to be near the transmitter comes from early-day TVI filters.


Power Mains Filter