TV Antenna Basics

Signal Amplifiers & Preamplifiers

Many people think that connecting an external amplifier to the antenna will improve the performance of the antenna. This is usually wrong. Receivers always have more gain than is necessary.
(The receiver has an Automatic Gain Control circuit, AGC, which will reduce strong signals. The AGC makes all stations the same strength at the demodulator. When you add a preamplifier, the TV receiver lowers its own gain, usually by an equivalent amount.)

Normally the signal to noise ratio will be set by the receiver’s first transistor. But if an external amplifier is added, the first transistor in that amplifier determines the S/N ratio. (Since the external amp will greatly magnify its own noise as well as the signal, the receiver’s noise becomes insignificant.) Since there is no reason to think the external amp’s first transistor is quieter than the receiver’s first transistor, there is generally no benefit to the S/N ratio from an external amplifier.

But an external amplifier will compensate for signal loss in the cable if the amplifier is mounted at the antenna. Without this amplifier, a weak signal, just above the noise level at the antenna, could sink below the noise level due to loss in the cable, and be useless at the receiver.

RG-6 will lose 1 dB of the signal every 5.5m at channel 52. For a DTV channel, 1 dB can be the difference between dropouts every 15 minutes (probably acceptable) and every 30 seconds (unwatchable). This author recommends a mast-mounted amplifier whenever the cable length exceeds 6m. (If you are in a good-signal area or you have no high-numbered UHF channels, you can to an extent ignore this advice.)

The preamplifier should have a gain equal to the loss in the cable (for your highest channel) plus another 10 dB (to keep the receiver’s first transistor out of the picture).
The amplifier can usually exceed this target by another 10 dB without causing trouble.

When figuring the cable loss, be sure to include the loss in any splitters and baluns. If a 2-to-1 splitter were 100% efficient then you would figure a 3 dB loss since each TV gets half of the power. But splitters are usually 80% to 90% efficient.

2-to-1 splitter	3.5-4 dB
3-to-1 splitter	5-6 dB
4-to-1 splitter	7-8 dB
75Ω-to-300Ω balun	0.2-2 dB (a balun is an adapter)

The antenna and the amplifier both have gains measured in dB, and many people add these two numbers (and then maybe subtract the losses) to find the strength of the signal at the receiver. But this sum is worthless. The net gain in front of the amplifier should always be kept separate from the net gain that follows.

You might not need an amplifier if the antenna signal is strong. But an amplifier can never make up for an antenna signal that is too weak.

Receiver noise

Actually there is a reason to think the external amplifier is quieter than the receiver. Long ago designers made an effort to make the TV’s first amplifier stage very quiet. But now 90% of homes use cable or satellite boxes (strong sources) and most of the rest are rural homes using antennas that have mast-mounted amplifiers. So the TV’s noise is rarely a factor. Some TV makers no longer put any effort into making their sets quiet.

Suppose you live in an apartment 25km from the transmitter. Your indoor antenna mostly works, but you are troubled by dropouts and some snow appears on analog channels. Will adding an amplifier right at the TV improve things? Yes, if it is quieter than the TV. Unfortunately TV makers see no reason to publish the noise figures for their receivers. So buying an amplifier for an indoor antenna is a total crapshoot.

Transmission cable

Twinlead (ribbon cable) used to be common for TV antennas. It has its advantages. But due to its unpredictability when positioned near metal or dielectric objects, it has fallen out of favor. (Such objects, even if not touching the cable, cause a portion of the signal to bounce, return to the antenna, and get retransmitted.)

Coaxial cable is recommended. It is fully shielded and not affected by nearby objects. Transmission cable has a feature called its characteristic impedance, which for TV coax should always be 75 ohms. (50-ohm coaxial cable is also common. Avoid that cable.) Although rated in ohms, this has nothing to do with resistance. A resistor converts electric energy into heat. The “75 ohms” of a coaxial cable does not cause heat. Where it comes from is mathematically complicated and beyond our scope here.

But coax also has ordinary resistance (mostly in the center conductor) and thus loses some of the signal, converting it into heat. The amount of this dissipation (loss) depends on the frequency as well as the cable length.

Type:	Center conductor:	Cable diameter:
RG-59	20-23 gauge	0.242 inches
RG-6	18 gauge	0.265 inches
RG-11	14 gauge	0.405 inches

Cable Chart in Db

The above chart is only approximate. There are many cable manufacturers for each type and there is no enforcement of standards. If the mast-mounted amplifier gain exceeds the cable loss then it shouldn’t matter what cable you use. But there are two problems with this:

Some cable has incomplete shielding. This is most common for RG-59, another reason to avoid it.
When the cable run is longer than 200 feet, the low-numbered channels can become too strong relative to the high-numbered channels. In this case, RG-11 or an ultra-low-loss RG-6 is recommended. (These alternatives are expensive.) Alternatively, frequency compensated amplifiers will work.

This author usually recommends RG-6 for all TV antennas.

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