In a paper published in the Sept. 2001 issue of IEEE Transactions on Broadcasting (vol. 47, no. 3), the authors state that for HDTV transmission, the average received power could be 12 dB lower than the peak visual power of NTSC signals. This is supposed to provide for service replication to at least the Grade B contour.
If you do the math, a 5 MW peak visual power signal divided by 16 equals 312 kW average power in the UHF band, specifically at 615 MHz. As many DTV signals are radiated in the UHF band with more than this power, they enjoy some signal level margin within their coverage area.
How does this vary with frequency (wavelength)?
The answer begins with an understanding of the Dipole Factor, which relates the power available at the terminals of a dipole antenna resonant at the signal frequency to the incident field strength in dB above 1 microvolt/m (dBuV/m). This is a function of the signal wavelength. Short dipoles will be resonant at higher frequencies than larger dipoles. The Dipole Factor for 615 MHz and for 194 MHz (which was the geometric center of the high VHF band) differ by 20 log 194/615 (10 dB). So, based on this factor alone, this means that for the high-band VHF, the ERP limit should be 1 MW/10, which is 100 kW. But there are other factors that should be considered. Let's start with "sky noise."
A MISSING PARAMETER IN DTV PLANNING
As Dr. Oded Bendov, Yiyan Wu, myself and several others wrote in a paper published in IEEE Transactions on Broadcasting (Sept. 2004 issue; vol. 50, no. 3), sky noise is a significant factor below the UHF band.
So what is sky noise? Let's do a little mental experiment. First we go up on the roof and disconnect the TV downlead from the antenna. Now we terminate this downlead in 75 ohms. Next we go to the other end of the downlead and connect it to a high-quality spectrum analyzer and measure the noise power across a 6 MHz portion of the UHF band, and also in both high and low VHF bands. Now, we go back to the roof, remove the termination and reconnect the TV antenna. Then we return to the spectrum analyzer and repeat measurements of the noise in these same channels.
The antenna is found to be a source of noise whose power is maximum in the low VHF channel; lower, but still significant in the high VHF band; and in the UHF band, it's scarcely higher than that measured with line terminated in 75 ohms. Ours has to be a mental experiment because of the presence of a multitude of noise—actually, DTV signals from afar except, perhaps in the now virtually vacant low VHF band. Sky noise is mainly man-made noise from electrical machinery and high-voltage power transmission lines. As the use of electrical power is increasing about 2 percent per year, this component of sky noise will continue to increase over the years.
|Table I: Sky noise power in the High VHF band |
We conclude that the antenna is picking up energy from the sky: hence the term "sky noise" principally in the VHF TV bands. This experiment has already been conducted and its results are published. The difference in any noise power present between that observed with the antenna connected and that measured when the line is terminated in 75 ohms is excess noise. The median of excess noise power in a residential area in the 174-216 MHz high VHF band is given in Table I.
To overcome this excess noise power, the received DTV signal power must be increased, as the FCC Planning Factors did not consider sky noise.
NOISE GENERATED IN THE DTV RECEIVER FRONT END
In our September 2001 IEEE paper the effects of variations in the impedance of the antenna as seen by the receiver front end were reported. The effect is to change its noise figure with respect to its value under laboratory conditions. As Murphy's Law predicts, these impedance variations seen by the receiver do not reduce its generated noise. It was observed in connection with radar work at MIT during World War II that the generated noise power varies with the source impedance seen by the RF amplifier. What you get when driving the RF amplifier from a 75 ohm resistive source is not what's experienced when the RF amplifier is fed from the antenna via a downlead line many wavelengths long at TV frequencies. Under actual operating conditions, Murphy was right; there is more noise than when the antenna appears to be a 75 ohm resistive source to the receiver. This was not taken into account in the DTV planning process.
In this paper, we also showed that the downlead loss varies with frequency across each TV band. There is excess line loss due to impedance mismatches between the line and the receiver. Table II gives the range of this mismatch loss over each TV band. This mismatch loss adds directly to the matched loss of the downlead, which is what the FCC used as a planning parameter in designing the DTV service.
|Table II: Range of mismatch loss vs. frequency in each TV band |
For planning purposes, the line loss should be the worst case sum of the matched line loss and the mis-match loss. This is given in Table III.
Note the mismatch loss does not increase for longer downlead lines.
DTV RECEIVER NOISE FIGURE
As stated earlier, the noise figure used in planning the DTV service is idealized in that it results from laboratory measurements made with negligible coax cable between the signal generator and the device under test, so standing waves cannot exist in the laboratory bench setup.
Measurements made by the FCC Laboratory reported in FCC/OET Report 07-TR-1003 (March 30, 2007), Figure 4-1 shows that the signal passes through a 25-foot length of low loss coax (not RG-6U) with a 3 dB attenuator at the input to the line and a minimum loss pad (5.7 dB resistive attenuation) at the output end of the coax cable. Thus any reflection is attenuated twice (3 + 5.7 dB per pass) effectively eliminating any reflections from reaching the device under test. This may be good laboratory practice, but it also explains why the desired signal threshold level was so consistent between DTV receivers tested. For example, the minimum signal level at the DUT would be about -84 dBm, but the signal power into the splitter would have to exceed -75 dBm. This is not a realistic test. It forces the DUT to operate under ideal conditions, that is it sees a 75 ohm resistive source, not an antenna whose VSWR on some channels may well exceed 3.
If any one tries to add 8.7 dB of attenuation to the downlead, the minimum usable DTV signal power from the antenna must be increased by 8.7 dB.
|Table III: Total line loss for 50-ft. RG-6 coax at TV frequencies |
A 60-YEAR-OLD DATABASE
The propagation curves, which express distance from the transmitting tower to a receiving antenna 30 feet above ground (simulating a two-story residential structure) date back to the 1950s. At that time, ground clutter 40 miles out from town would have been minimal where huge numbers of residential subdivisions had recently been constructed.
Today, many such sites have mature trees and other foliage that tend to attenuate TV signals. Further, many suburban homes are single story. A rooftop antenna, if there is one, might be 15 feet above ground. The mature foliage surrounding such homes today is sometimes called "ground clutter." It can significantly affect the field strength at the 15-foot height above ground for rooftop antennas in single story residences in suburbia. In the Longley-Rice software a ground clutter value can be inserted. The FCC set its value to zero. This would be valid in a desert, but not suburbia surrounding most major population centers.
With perfect hindsight, we note that fresh data should have been gathered in anticipation that it would be needed for the planning of the DTV service.
I have compiled three DTV link budgets :
- • Range of minimum usable field strengths for a passive rooftop directional antenna for DTV reception in the high VHF band 37-44 dB µV/m.
- • Reception of high-band VHF band DTV signals with a suitable low noise pre-amplifier.
- • Reception of UHF DTV signals with a suitable low noise pre-amplifier.
MORE ABOUT DTV LINKS
It is shown that for a passive rooftop antenna as the FCC assumed in planning the DTV service, that the minimum usable field strength at 30 feet above ground for DTV reception of High VHF Band signals is in the range of 39-44 dB above 1 microvolt/meter (dBµV/m). The lower end of this range would only apply if the effective noise figure of the receiver is 7 dB. The upper end of this range would apply where the receiver's effective noise figure is 12 dB as was suggested in the 2004 IEEE paper by Bendov, et al.
It is quite possible for the effective noise figure to be 7 dB on one High VHF channel and 12 dB on another High VHF channel because of the standing waves on the downlead. Twelve dB effective Noise Figure is a useful figure for planning, but it is not the upper limit on noise figure when the receiver "sees" a VSWR greater than 3, which is possible.
It is also shown that with a low noise amplifier at the antenna end of the downlead that the minimum usable field strength for High VHF channels is 35 dB uV/m 1 dB better than the FCC minimum field strength for a passive antenna.
And a link budget for this same low noise pre-amplifier receiving DTV signals on Channel 38 (615 MHz) in the UHF band shows that the minimum field strength is only 35 dB µV/m 6 dB below that specified by the FCC affording a 6 dB signal level margin at UHF.
These link budgets are available on the online version of this column on www.tvtechnology.com. They will allow you to see exactly how I got to these results. Moreover, you can then speculate with these link budgets to illustrate what other parameter values would yield. For example, by eliminating sky noise, its effect can be observed. By substituting the FCC values (matched line loss) for my total line loss numbers, you will see why mismatch loss should be taken into account.
Please note that many so-called low noise pre-amplifiers are only "low noise" in the context of analog TV signals. You can see the low noise amplifier performance specs I used by downloading these link budgets.
Many viewers using an indoor, passive antenna for reception of analog TV signals are simply out-of-luck. The field strength required (as measured at 30 feet outdoors), needs to be at least 85 dB above 1 µV/m, about 45 dB above the edge of DTV coverage. So indoor antennas cannot be expected to work over a large part of a station's analog coverage.
The FCC had a difficult task to find about 1,800 vacant channels to lend to broadcasters for an orderly transition to DTV. With perfect hindsight, compromises had to be made and they were made and now the transition is over and we must survive the consequences of these compromises. Now it is up to broadcast engineers to optimize their DTV coverage on their assigned permanent channel. In some cases, coverage on the "permanent channel" may be unsatisfactory to both the broadcaster and the public. In such cases, broadcasters will have to find a more suitable channel and prepare to petition the FCC for a change in their "permanent" channel. Today there must be some 1,800 vacant channels to choose from. Ideally, some of those vacant channels are in the same band but able to operate with much higher ERP without causing harmful interference. There are other alternatives too.
Charles Rhodes is a consultant in the field of television broadcast technologies and planning. He can be reached via e-mail at [email protected].
"Idealized DTV Link Budget for a Passive, Rooftop, Directional DTV Antenna" (MS Word)
"DTV Reception Link Budgets for the High-VHF Band" (MS Word)