ANTENNA TRICKS2/15/2012 2:44 AM Eastern
Attention, all commercial and residential A/V designers and installers: The FCC just made your life more interesting and possibly much more aggravating. In mid-August 2002, the commissioners voted to make ATSC-compatible digital TV tuners mandatory for all TV sets sold after July 2004, beginning with large (36-inch) screens first and gradually working down to the smallest TVs by the end of the analog-to-DTV transition on January 1, 2007. VCRs and PVRs with NTSC tuners would also be similarly affected.
On November 1, 2002, the six-month extension that many broadcasters received from the FCC — in response to petitions from broadcasters who said they weren't ready to go on the air with digital signals — passed. The passing of that deadline brought more stations to the airwaves. In the Philadelphia market, there are 11 DTV stations to choose from, many of which are not available on digital cable.
With the major networks now offering just about all of their prime-time programs as well as many live sports and entertainment events in HDTV, your customers are slowly becoming aware of this premium broadcast service and want in on it. Residential customers in particular are opting for a combination of big-screen plasma monitors and HDTV, but the trick remains to pick up the signals reliably where they are available.
In previous columns, I have detailed my tests of indoor and outdoor reception of DTV signals as well as comparative tests of different antenna designs for both reception modes. This month I'm going to discuss ways to get more signals from your antenna to your digital TV or set-top receiver. RF signals usually behave in predictable ways. They have mathematical relationships to time and space that are rarely violated, and that makes it a bit easier to determine how these signals will behave with antennas.
I am often asked if more signal can be had from a given DTV station by adding a second antenna, a practice those in the amateur radio world (and also staging and rental people) call stacking. Just as stacking two projectors yields about twice the light output on the screen, stacking two antennas doubles the gain, increasing it by 3 dB. But there is a science to stacking.
To test out some of these theories (and dispel some others), I climbed up into my attic with a bunch of test equipment, some work lights, and my digital camera. Previously, I had installed a modified Channel Master 4308 suburban UHF yagi for indoor reception of Philadelphia DTV stations. (A yagi antenna, also known as a yagi-uda array or, simply, a yagi, is a unidirectional antenna commonly used in communications when a frequency is above 10 MHz.) Having left enough mast for other hardware, I brought up a second modified 4308 to try some antenna tricks (see Fig. 1).
|DTV Channel||Single Yagi with Preamp (Signal Strength)||Two Yagis, Splitter, and Preamp (Signal Strength)||Two Yagis and Preamps, One Splitter (Signal Strength)|
|KYW-26||24.2 dBmV||24.2 dBmV||23.3 dBmV|
|WPSG-32||7.1 dBmV||4.5 dBmV||4.2 dBmV|
|WTXF-42||17.5 dBmV||16.5 dBmV||15.1 dBmV|
|WPHL-54||13.4 dBmV||8.5 dBmV||14.0 dBmV|
|WHYY-55||-5.4 dBmV||-12.1 dBmV||-9.8 dBmV|
|WPVI-64||13.6 dBmV||11.9 dBmV||13.0 dBmV|
|WCAU-67||-1.7 dBmV||-1.7 dBmV||-5.1 dBmV|
The first step was to look at DTV signal strength using the existing antenna and a Channel Master Titan 2 UHF preamp. It wasn't easy, but I managed to shoot some screen captures off the B&K Spectrum Analyzer of clusters of DTV channels. I also logged signal strength readings with a Sadelco MiniMax 800 RF signal level meter (see the table, “Signal Strength Readings Under Different Antenna Scenarios”).
In all but one case, the signal levels were high — way above the ATSC Gaussian threshold of 15.3 dB carrier-to-noise and also above the real-world limit I have observed of about 20 dB C/N. The one exception? WHYY-DT, whose antenna sits on the south side of its tower and thereby puts my location 25 miles NNE in a deep null.
With the setup I have, all seven DTV stations tested were reliably received on a Samsung SIRT-151 ATSC set-top receiver, including WHYY-DT. But I wanted to see if I could squeeze a few more decibels out of the signal and give myself some headroom during times of fluctuating signal levels, such as in heavy rains or high winds.
I used the plots of WPHL-54 and WHYY-55 as my references each time I made a change to the setup. Fig. 2 shows the two waveforms with the single 4308 and UHF preamp. There's a pretty good slope on WHYY's signal, but it is otherwise clean and about 20 dB above the noise floor.
The Sadelco signal strength readings in the table don't appear to have much in common with the displayed waveforms. Because of multipath and signal fading, signal levels were up and down by as much as 3 dB during a measurement. That is why knowing the signal strength isn't enough — you need to see the waveform, as well, to determine if your efforts are working for or against you. Compare the readings with the waveforms shown, and you will see wide variances between the two.
This was an obvious first choice. Assuming I could get a good match to the two antennas, I might be able to get more signal out of a combined array — except for a catch.
To combine antennas, you need more than a simple resistive signal splitter. What you really want is a power divider that uses ¼-wavelength matching lines to make the coupling between antennas. You can also use two ¼-wavelength pieces of 50ž coax and a conventional T connector to do the job.
In my case, I grabbed a two-way signal splitter and hooked both antennas into it, using coaxial lines that were the same length. In theory, I would lose 3 dB of signal through each port of the splitter, which would offset the 3 dB theoretical gain of the second antenna. What I would gain, however, is a narrower aperture for the antenna and possibly better side-lobe rejection and front-to-back ratio.
When stacking antennas, you need to place the second antenna a certain distance below or above the first antenna. Usually, that is a multiple of wavelengths and is determined by the size of the antenna. Given that the UHF TV channels run from 14 to 69, or 470 MHz to 800 MHz, I opted for about one wavelength spacing at 635 MHz, or right around channel 41, the middle of the band.
Fig. 3 shows the channel 54 and 55 waveforms with this new setup. It doesn't look like there's much of an improvement. If anything, WPHL-54's signal looks more distorted; in fact, it is lower in level by about 6 dB. That could be attributable to the indoor setup, because one antenna is seeing echoes that are not in phase with the other antenna. Foil siding in the house would be a likely culprit.
Another possibility is phasing problems between the two antenna feed lines, even though they were cut to be as close as possible to the same length. One port inside the splitter may also have a higher standing wave ratio than the other. Whatever the cause, WHYY is also weaker by about 5 to 6 dB and not receivable now.
I left the two antennas as they were and instead added a pair of UHF preamps ahead of the two-way splitter. The Titan 2 power supply has enough current capacity for this trick, though I would be more comfortable with a nice 3 dB “pure” 75ž pad at the output of each preamp to make sure they are driving a clean 75ž load.
Fig. 4 shows the results of this antenna trick. For some odd reason, the channel 54 waveform is now level and slightly higher than with one yagi and preamp. Could that be the result of improved matching by using the preamps ahead of the splitter? That doesn't seem likely, but it could be that the inputs to each Titan 2 preamp present a better 75ž load, hence lower SWR to each antenna.
When doing something like this, you have to be careful not to overload the receiver. It's best to use a high-quality power combiner that looks flat at the desired frequencies. Otherwise, you'll amplify all kinds of standing wave problems and create more problems than you solve.
I have done this only once before with a pair of yagi antennas for 432 MHz and two preamps running about 10 to 12 dB gain, along with 3 dB resistive pads at the output of each preamp before feeding a two-way strip-line power combiner. That was to eliminate any possibility of oscillation owing to impedance mismatches.
Can you get away without a rotor if you wish to receive DTV signals from two or more directions? Possibly. The trick is to minimize out-of-phase echoes that each antenna will pick up. For example, my main antenna will do a good job with Philadelphia's DTV stations, but other stations such as WLVT-62 in Allentown, Pennsylvania, or WNJN-43 in Trenton, New Jersey, are off the back or sides of the antenna.
These signals present too much multipath to be demodulated by the ATSC receiver. By aiming a second yagi at the desired station, I can clean up the pattern enough to enable reception of that station as well as the original Philadelphian stations, provided I don't introduce too many out-of-phase signals through the second antenna.
Fig. 5 shows the waveforms of WTXF-42 from Philadelphia and WNJN-43 from Trenton. Although channel 42 is nice and clean, 43 looks pretty sad. By adding a second antenna aimed toward Trenton, I wound up with the waveforms in Fig. 6. WNJN-43 looks a lot healthier, and WTXF-42 doesn't seem much the worse for it.
However, I reduced all signals by 3 dB by using the splitter, and that may hurt me with WHYY-55's miniscule waveform. Sure enough, Fig. 7 shows I reduced WHYY's carrier by about 3 dB and introduced more multipath to it. I might have to sacrifice one PBS digital TV station for another to pull off this trick.
The dynamic equalizers in ATSC receivers are designed to correct for a certain amount of signal multipath, intentional or otherwise. With a strong 8 VSB signal such as that observed from channel 54 or 42, I could add a small, low-gain yagi to the array and aim it toward another digital station I want to pick up. I wouldn't have to worry too much about any multipath or phasing problems it might cause with my primary DTV stations; only the low-power stations would suffer as a result.
You can get DTV reception in many ways, but there aren't many hard and fast rules for anything other than theoretical environments. You may be able to get clever with antennas and solve your indoor and outdoor problems with a bit of experimentation. To do so, you need a spectrum analyzer to view the results of your work — simply looking for more gain in your system won't cut it.
Peter H. Putman owns PHP Communications, in Doylestown, Pennsylvania. The author of The Toastmasters Guide to Audio/Visual Presentations, Putman is also a regular columnist in S&VC.