Antenna Amplifiers For Terrestrial HDTV : What Works & How

The AbilityHDTV Ultra Low Noise Amplifier, Model ULNA, used with a high gain antenna (at least 12 dBi) has been proven to achieve consistent reliable Day/Night reception over distances beyond the earth’s horizon by exploiting the Tropospheric modes of signal propagation. Tropospheric modes can be reliable, as this is relied upon by the United States military for field deployed UHF data link circuits. The key to success from Tropospheric mode propagation is simply achieving adequate signal strengths. If you are located in a deep-rural location and need 100+ mile reception capability, “tropo” is the solution to your reception challenge.

First, the basics of acquiring the signal: The antenna, and the transmission line -

1. The most important component of the reception system is the antenna.

The stronger the signal you have from the antenna the better you can achieve reliable high quality reception. Nothing can make up for degradation of the original signal. Many factors contribute to the reception quality, including:

(a) antenna gain, (b) system Noise Figure, (c) station distance, (d) station transmitting power, (e) obstacles between the station transmitter and the receiving location, (f) height of the transmitter antenna, (g) height of the receiving antenna, (h) reflections from obstacles and moving vehicles that can partially cancel the signal (i) and if station distance requires tropospheric mode, then atmospheric conditions will be more or less favorable depending on many atmospheric variables.

2. Second in importance to the antenna, is the antenna amplifier Noise Figure as this can determine the overall system Noise Figure and consequently, the Signal-to-Noise Ratio (SNR, or S/N).

The FCC reception model supposes a receiver Noise Figure of 8 dB. If the antenna amplifier (“pre-amplifier”) Noise Figure is less than 8dB, the system Noise Figure is lowered to close to that of the amplifier. (See ‘Noise Figure Calculator’ at the bottom of this page)

RF Noise is added to the signal by everything the signal encounters, even including coaxial cable and connectors. As noise increases, or as the signal weakens, the “signal-to-noise ratio”  decreases. When the S/N Ratio falls below the minimum threshold of the receiver’s capability to lock onto the digital signal stream, reception is interrupted.

Best practice dictates that the amplifier be connected as close to the antenna element as possible to avoid introduction of additional RF noise and attenuation of the signal, both of which have the effect of lowering the S/N Ratio. AbilityHDTV amplifiers are attached directly to the antenna connector (model ULNA/EXT) or to the antenna receiving element itself (model ULNA/INT).

The amplifier Noise Figure can be considered as equivalent to subtracting antenna gain for signals that are near the minimum threshold of reception.

The strategy for high quality reception is therefore simple:

Select an antenna that has adequate gain for the station(s) you are trying to receive, after subtracting the amplifier Noise Figure plus all other losses occurring before the amplifier connection. As long as the signal is sufficiently higher than the noise background to meet the minimum S/N Ratio required by the receiver (usually minimum 15 dB), your reception will succeed.

3. FCC broadcast reception models are based on the assumption that ATSC receivers (i.e., HDTV Televisions) have a receiver noise figure between 7 – 10 dB. Employing the AbilityHDTV Ultra Low Noise Amplifier with its ultra-low noise figure (0.45dB ULNA/INT, 0.8dB ULNA/EXT) will thus actually lower the system noise figure since the ATSC receiver noise is now far less than the amplified signal presented to it. In summary, employing a Low Noise Amplifier will actually improve the performance of your television receiver.

Who needs an Amplifier ?

Almost every antenna installation will be significantly improved by having an amplifier installed near the antenna.

The basic reasons why:

A. Overcomes signal loss in the coaxial cable -

All coaxial cable has loss that increases with higher frequencies. RG6 coaxial cable is the standard cable of choice for television antenna connection. While RG6 is quite low-loss at approximately 6dB attenuation per 100-ft at UHF (mid-band) frequencies, assuming an average cable length of 50 feet, this means that your signal will decrease by 3dB while passing through the coaxial cable. 3dB of attenuation equals a loss of 1/2 of the signal power. And where the cable used is longer, say 100 feet, the loss of signal will be about 6dB, or 75% lowering of signal.

Connecting an amplifier near the antenna raises the signal strength so that after the various losses encountered in the distribution system, such as the down-lead coaxial cable, lightning protection devices, any splitters necessary to distribute the signal to various rooms, etc, there will still remain adequate signal available for best attainable reception.

B. Overcomes signal loss caused by (almost inevitable) impedance mismatches -

While large volumes are written about impedance matching, suffice it to say that all but the most carefully engineered antenna and signal transmission systems suffer the degrading effects of impedance mismatch, especially television antenna systems that must cover a wide frequency band. And note, just because the antenna manufacturer states the antenna to be, for instance, 75 ohms, and the coaxial cable (transmission line) is also 75 ohms, there are likely still impedance matching losses because antenna impedance varies significantly over different frequencies. Because of this, close impedance matching becomes more difficult as the band of antenna operation increases. (See the related article: “The High Gain Single Channel Strategy”)

By amplifying the signal at the antenna, greater signal strength arrives at the destination (the television receiver). Remember that once the antenna signal is reduced by loss or noise, it cannot be recovered.

C. Overcomes signal loss introduced by the Antenna Combiner (if used) -

Many installations will employ more than one antenna:

(a) sometimes to employ different antennas for different bands, e.g., VHF-Low, VHF-High, UHF,

(b) to connect multiple antennas of same or different bands that are aimed in different directions so that antenna rotating can be reduced or eliminated.

(c) to “stack” identical antennas to increase the signal

A characteristic of the antenna combiner is that it shares the signal from each port with all other ports. This means that the signal power is “divided” amongst the ports and therefore reduced in strength at the port connected to the coaxial cable down-lead. For a 2-port combiner the reduction is about 3dB (or 1/2, as divided by 2), for a 3-port combiner the reduction is about 4.8dB (divided by 3), and for a 4-port combiner the reduction is about 6dB.

By raising the signal level with the amplifier placed before the signal encounters combiner loss, even after subtracting the combiner loss there is still plenty of signal strength well above the system noise level, a necessity for successful and dependable reception.

Is there anyone that doesn’t need an amplifier ?

As said, most installations will realize better quality reception when using an amplifier, however, there are installations that may function satisfactorily without an amplifier -

In those cases where the broadcasting stations are very nearby, that is 1-20 miles, and the coaxial cable run is relatively short, that is under about 25 feet, and there are no combiners or splitters installed, you will probably be able to achieve quite satisfactory reception quality without installing an amplifier.

The AbilityHDTV Ultra Low Noise Amplifiers (ULNA/INT & ULNA/EXT) were developed for the most demanding reception challenges, such as ‘over-the-horizon’ reception of 60 miles and greater. AbilityHDTV ULNA’s have been demonstrated to achieve consistent 24-hour reception of stations 100 – 140 (and greater) miles distant when used with our higher gain antennas, most successfully with the DataWave RF MaxChannel Series antennas.

For urban/suburban/rural reception, that is distances within 1 – 50 miles, AbilityHDTV also offers several economical good quality amplifiers from Antennacraft.

To learn about how the system Noise Figure is determined by the antenna amplifier, try out this Noise Figure Calculator