In recent columns, I've discussed options for TV-transmitting antennas. This month, I'll look at the other end of the path – outdoor TV-receiving antennas. I'll describe desirable attributes for DTV reception, look at some of the types of antennas available and tell you how you can build a simple UHF receiving antenna.
(click thumbnail)Consumer VHF/UHF TV Antenna
As with transmitting antennas, the main characteristics of receiving antennas are pattern, gain/directionality, bandwidth/VSWR and size. Before DTV, gain – and to a lesser extent, bandwidth – were the key specifications. In 2002, antenna VSWR and pattern are equally important – if not the most important – specifications.
FCC OET-69 interference-free coverage is based on the use of a directional receive antenna. DTV viewers are assumed to have an antenna with a gain of 4 dB at Channels 2 to 6, 6 dB at Channels 7 to 13, and 10 dB at Channels 14 to 69. To squeeze in more channels, front-to-back ratios for receive antennas were also specified. For analog TV, a front-to-back ratio of 6 dB is used for Channels 2 to 69. For DTV, much higher ratios are required – 10 dB for Channels 2 to 6, 12 dB for Channels 7 to 13, and 14 dB for Channels 14 to 69.
In determining how well an antenna will work with DTV signals, VSWR has to be considered. The Association of Federal Communications Consulting Engineers (AFCCE), in its Consolidated Opposition to Petitions for Reconsideration to the FCC's Sixth Report and Order on Advanced Television Systems, noted "The effective noise figure, the noise figure subject to a typical mismatch between the receive antenna and the receiver's input, is higher by at least 3 dB for a VSWR of 2:1. VSWR as high as 5:1 can be expected in practical installations."
The AFCCE concluded that the DTV receiver's noise figure would have to be better than 4 dB when a 7 dB noise figure is used as the basis for determining coverage.
In addition to the signal loss associated with a high antenna VSWR, group delay will also be affected. The effect on the signal-to-noise ratio or error vector magnitude is the same on the receive side as on the transmitter side.
Fortunately, if a low-noise amplifier (LNA) is mounted at the antenna, the impact of antenna VSWR is greatly reduced. The output of the LNA should provide a good match for the coax lead-in and a low VSWR at the tuner.
COMPARISON SHOPPING
There are several ways to cover the span from 54 to 216 MHz. Log-period antennas perform well over frequency ranges larger than this while maintaining a low VSWR. However, gain is low relative to boom length required, and the feed system is complex.
Did you ever look at an outdoor TV antenna and wonder why it was built that way? VHF TV antenna design is a challenge, since antennas have to operate from 54 to 88 MHz (a 1.6:1 range) and 174 to 216 MHz (a 1.2:1 range) – a total range of 4:1!
A half wavelength dipole at Channel 2 (54 MHz) will be 9.1 feet long. At Channel 13 (210 MHz), the dipole is only 2.3 feet long. Although it isn't practical to use one dipole to cover the entire 54 to 216 MHz range, dipoles also work well at odd harmonics of their half wavelength resonant frequency. A dipole antenna that covers 54 to 88 MHz should also work from 164 to 264 MHz.
Some consumer antennas take advantage of the 3-to-1 relationship between the low VHF and high VHF bands. Antennas that depend on this harmonic relationship are swept forward. The reason is that above resonance, the dipole pattern starts to change from a figure 8 – with lobes perpendicular to the dipole – to one that looks more like a four-leaf clover.
Bending the dipole elements forward reduces the beam splitting in the direction the elements are swept. Other tricks to improve the pattern include adding "whiskers" to each element about a quarter wavelength from the feed point.
The picture shows a common low-gain urban VHF/UHF antenna. Each of the dipole elements is driven, giving the antenna a better front-to-side ratio than would be possible if parasitic elements were used and – if the dipoles are tapered – improves the bandwidth.
"Thicker" dipole elements (or an array of elements) would also provide a wider bandwidth. Many antennas use interlaced high-band and low-band VHF elements. The driven element can be a series of swept dipoles described above with interleaved parasitic elements for each band.
How do these VHF antennas compare with planning factors used to determine DTV coverage? At least for textbook designs, it isn't too difficult to obtain the correct gain and front-to-back ratio. VWSR, however, is another issue.
As noted by the AFCCE, VSWR of up to 5:1 can be expected. UHF-receive antenna designs tend to be much simpler, because they only need to cover 470 to 806 MHz, a 1.7:1 range. When UHF TV moves to the core channels, the range will drop to 470 to 698 MHz (1.5:1).
A popular design of the past used four stacked dipole-driven elements in front of a screen. As at VHF, well-crafted log-periodic designs provide excellent performance across the UHF band. The gain versus boom length is not as much of an issue at UHF.
Another antenna well-suited to the UHF TV frequency range is the V or rhombic antenna. These are easy to construct, but their size makes them impractical for most consumer uses. For this discussion, it helps to know that a half wavelength at 470 MHz (Channel 14) is 12.6 inches and at 806 MHz (Channel 69), 7.3 inches.
A typical single UHF bow-tie (triangular) dipole antenna in front of a reflector will have a gain above a dipole ranging from just under 6 dB at Channel 14, to 10 dB at Channel 69. A stacked four-bay bow-tie dipole antenna in front of a screen provides gain ranging from around 11 dB at Channel 14, to approximately 15 dB at Channel 69.
A typical UHF bow-tie using a corner reflector has about 2 dB more gain than one using a flat screen: 8 dB to 12 dB from Channel 14 to 69.
As with VHF antennas, differences between manufacturers make it hard to specify a typical VSWR for a particular type of antenna. However, the simpler feed system of the single dipole in front of a corner reflector should give it an advantage over the more complicated multiple-bay bow-tie design. I've found the performance of the simple corner reflector bow-tie antenna to be similar to that of the four-bay bow-tie and often better than some corner reflector/ dipole/parasitic director combinations.
How does the performance of these UHF antennas compare with the DTV planning factors? The gain numbers seem reasonable – at least at midband and higher – for the four-bay bow-tie and corner reflector antennas. At the low end of the band, they may come up short. But what happens to the front-to-back ratio?
For a corner reflector or a dipole in front of a screen, the size of the corner reflector or screen will determine the ratio at the low end of the band and the size of the opening (or spacing between horizontal members of the reflector) will determine the performance at higher channels. The corner reflector dipole/Yagi in the photo (Consumer VHF/UHF TV Antenna) would probably not meet the required 14 dB front-to-back ratio at all UHF frequencies.
BUILD YOUR OWN!
Although the bow-tie dipoles have a reasonable response over the UHF TV band, there are better options. Log-periodic antennas are the usual choice, but a "do-it-yourself" option is a V or a rhombic. The V antenna consists of two elements forming a "V." Ideally each of the elements should be four wavelengths or more in length at the lowest frequency, although gain is reasonable even at three wavelengths.
To build a V antenna, it can be as easy as arranging two six-foot rods 50 degrees apart. Gain will be over 5 dB at the low end of the UHF band, where each side of the V is just under three wavelengths – and increase smoothly toward the high end of the band, to over 8 dB.
Two V antennas are easily stacked about 12 inches apart to provide gains over 12 dB at Channel 69. The impedance of the V antenna will be higher than the 300 ohms available from the ubiquitous 300-ohm TV balun, but it is consistent and close enough that the VSWR should be better than 2:1 across the UHF band.
One problem with the V antenna is that it is bidirectional. The front-to-back ratio is 0 dB. If you need a high front-to-back ratio and are only interested in one or two channels, one of the two stacked V antennas can be moved a quarter of a wavelength behind the other and fed 90 degrees out of phase. However, there is an easier approach – the rhombic.
Fig. 1 shows a drawing of a rhombic antenna with four elements. Each element of the antenna ("L" in Fig. 1) should be equal and ideally 60 inches or longer. The angle between the elements is still 50 degrees (130 degrees on each side). At the end opposite the feed point, terminate the antenna with a 470-ohm resistor. (The value of this resistor can be adjusted to maximize the front-to-back ratio to 20 dB or more.)
Gain is over 12 dB with elements five wavelengths long. If this seems low, remember that half the power is lost in the 470-ohm terminating resistor.
Rhombic antennas can be stacked to improve the pattern and increase gain. Gains up to 17 dB are practical. Although the rhombic takes some space and the design of stacked rhombus antennas can be complex, construction is simple because the rhombic doesn't have parasitic radiators or reflectors.
There are many other UHF-receive antenna designs I didn't have room to discuss. What's your favorite TV receive antenna? Drop me an e-mail at dlung@transmitter.com!
