TV Antenna Terms Glossary - G to P

Impedance

A resistor limits current flow. Resistance is defined by Ohm’s Law: voltage = resistance * current. Likewise, reactance limits current flow. Reactance is somewhat like resistance, but it describes what coils and capacitors do. Reactance is sort of a phase-shifted resistance, and the reactance of a device changes with frequency.

In a circuit that has both resistance and reactance, the term impedance is generally used. Impedance equals resistance plus reactance.

Resistance and reactance are so different from each other that they cannot actually be added together. So impedance is a two-part number in which the resistance and reactance are kept separate. It so happens that resistance and reactance obey the arithmetic of complex numbers (numbers with real and imaginary parts). Thus impedance is commonly expressed as a complex number: Resistance is the real part, reactance is the imaginary part. Be aware that electrical people often use “j” instead of “i” to designate the imaginary part.

Thus “impedance” is a generalized version of the concept of “resistance”, and it can be used in place of the word resistance when there is no reactance. (It is not wrong to do so, but it can confuse some.)

Adding more confusion, the word “impedance” is sometimes a one-part number in which the resistance and reactance have been “combined”, but this is less general and less common. This number expresses the net magnitude of the impedance, but the phase information has been thrown away. You must figure out from the context which version of “impedance” is being used.

Transmission lines have a property called their characteristic impedance. See Transmission lines. See “Terminators for 75-ohm lines”. The characteristic impedance is usually 75 ohms (actually 75+0i) for coaxial lines and 300 ohms (actually 300+0i) for twin-lead.

Antennas have terminal impedance. Matching this impedance to the line is important.

Inductors (coils and transformers)

When a wire carries current, there is energy stored in the magnetic field around the wire. Coiling the wire magnifies this effect. Energy entering or leaving this field affects the circuit. Inductance is a measure of the coil’s effect on the circuit.

The effect of inductance is proportional to frequency. At TV frequencies, even the minuscule inductance of a straight piece of wire becomes important.

A transmission line (coaxial or twin-lead) is a geometry in which the inductance and capacitance cancel each other out, allowing an unimpeded energy transfer for long distances.

Interference

Common sources of interference include:

Adjacent channel interference (a very strong station one channel up or down). R1
Co-channel interference (two weak stations on the same channel). R1
Multi-path interference (usually caused by the direct path being blocked). R1, R2
A very close transmitter (a neighborhood FM station, police station, taxi company, etc.). R1, R3
An industrial noise source (a factory, a clinic, a malfunctioning power transformer). R1
Household appliances. R4
Light dimmers. R4
Fluorescent lights. R4

Remedies:
R1 – See “Nulls in radiation pattern”.
R2 – See “Multi-path interference”.
R3 – See Overload.
R4 – Try one of these:

Try fixing or replacing the device.
Try replacing the device with a device containing some RF filtering.
Try putting an RF noise filter on the power cord of the bad appliance.
Try putting an RF noise filter on the power cord of your TV.
If the TV and the appliance are on the same house breaker, move one to a different breaker.

Appliance noise

Household motors and fluorescent lights often produce noise of the “120 sparks per second” variety. If you tune to an analog station (especially channels 2-6) you may see intense sparkles that are somewhat confined to a broad horizontal band. If so, you must find the appliance and fix it or replace it. Identifying the appliance is sometimes difficult. You might have to shut off the house breakers one at a time, watching to see when the sparkles go away. If every breaker but the TV is off and every appliance on that breaker but the TV is off and the sparkles remain then the noise source is either in a neighbor’s home or is a bad transformer on a utility company pole. (If you can walk around with a portable AM radio tuned to an unexplained buzzy hum, you might be able to further isolate the offending device.) If the source is in your neighbor’s home, brush up on your diplomatic skills. If it is the utility’s transformer, call them. They are obligated to fix it.

Ionosphere

The ionosphere is a charged layer of the atmosphere that can reflect radio waves. Actually it is a series of layers from 48-440km in elevation. It enables communication beyond the horizon for a band of frequencies, and with multiple reflections can reach half way around the globe. What those frequencies are depends on the time of day, the time of year, and time within the 11-year sunspot cycle.

These “skip” frequencies rarely go above 30 MHz and normally play no part in TV reception. On rare occasions they will go above 50 MHz, allowing the signals of channels 2, 3, and 4 to travel far, allowing distant analog channels to be watched. But this also allows atmospheric noise to travel far, so getting a digital lock on a distant DTV channel is unlikely.

Isotropic antenna

This is an antenna with equal radiation (reception) in every direction.

Nobody actually wants one of these. It is mainly a concept for comparison. For example, if an antenna is rated at 7 dBi, that means it is 7 dB better than an isotropic antenna.

Pages: Previous «« 1 2 3 4 5 »» Next

Back to Top