OTA HDTV Reception Q&A

Updates on the DTV transition and how to receive over-the-air digital television for free.

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Location: Richmond, IL, United States

Hello there! I created this blog to share the information about over-the-air HDTV reception you have been wearily searching the Web or calling technical lines for, whether you have decided for less expensive means to get your favorite TV shows, or still adjusting from the over the air broadcast DTV transition that occurred on June 12, 2009. After working for a leading antenna manufacturer for almost 5 years, during which time I've shared my expertise with those who asked on the phone and by email at work, I decided to do the same in the Blogosphere! Confused about getting your local HD channels? Just click through the archives, some of the most useful information is in the early posts from 2005-06. If you want to get in touch with me with antenna related questions, just leave a comment anywhere on this site.

Friday, September 09, 2005

HOW CAN I COMBINE OTA HDTV WITH SATELLITE?

We at Winegard have been supplying DirecTV and formerly VOOM with off-air antennas, so that customers can enjoy their local channels in HDTV in addition to the satellite HDTV. Now DirecTV is already underway providing local HDTV channels via their satellite, but it will take time to deliver to all the US markets. In addition, there is the issue for paying an extra $5 per month for local channels, plus MAYBE the off-air signal could hold better than the satellite in heavy rain.

Customers do not want to run extra cable, so they would prefer a single run with the satellite signals (950-1500 MHz or up to 2400 MHz in a stacked system) with the off-air signals (54-806 MHz). This can be accomplished. However, given the choice, I would prefer doing the extra work running separate cable lines for the off-air system, as I shall explain.

Combining the off-air signal with a satellite dish signal requires a device called a DIPLEXER. It is about the size of a common TV splitter, but is moreso a band separator and combiner. The advantage of SEPARATING bands instead of splitting them is that is, in theory, little loss. However, I say IN THEORY. Here comes the reality. Adding devices along a cable between an antenna and receiver will introduce two forms of off-air signal degradation. One form is INSERTION LOSS. Ideally, separating a signal will have zero insertion loss, but realistically you can see about 10% signal loss of signal through a diplexer. The other form of signal degradation is MISMATCH LOSS. This can be specified by the VSWR (voltage standing wave ratio) of the diplexer. Mismatch is due to variations in the RF impedance through the diplexer (the ideal impedance of a TV system is 75 ohms). If the impedance of the diplexer on all the frequencies, the VSWR would be a perfect 1, which would indicate that the voltages of the signals would be equal going out compared to going in.

Sadly, in a broadbanded device, a VSWR of 1 across all operating frequencies is an RF challenge. Realistically I would consider a diplexer or splitter with a maximum VSWR of 2 to be good, and 1.5 to be outstanding. If the VSWR for a certain frequency is very high, then the line from the output to the diplexer to the tuner would have a standing wave in the cable, and a mismatch to the tuner. As a result, some signal would bounce back on the line, and a secondary signal could return, which results in a form of ghosting on an analog picture. As I mentioned in a previous post, a cause of ghosting in an analog picture, is a cause for picture breakup and loss on the DTV picture, even if the antenna being used provides enough signal.

In addition, there is a double whammy when diplexing. At the tuner, the combined signal needs to be separated AGAIN for the respective signals to connect to the proper inputs. This involves a diplexer in reverse, introducing potentially MORE mismatch and insertion loss. If the combined line has a bad VSWR to begin with, the additional diplexer could multiply the ill effects on the DTV signals over-the-air, and possibly the satellite signals as well. I would figure you would be looking at 30% signal loss MINIMUM for off-air in a diplexing case than for a separate cable run.

Now while my explanation may seem gloomy, what I am saying is not hopeless. As long as diplexers are selected with a good low VSWR, success may be realized. The same, by the way, goes for using multiswitches that add off-air VHF/UHF signals. Paying attention to insertion loss and cable loss would allow an installer to select a proper preamp to overcome the signal losses.

Speaking of preamps, Winegard’s preamps and amplified antennas can be powered through the cable. We normally provide an inline power supply for a separate antenna line, but it is possible to power the amplifier in a satellite system diplexed with an off-air signal. This can be done by using the voltage delivered by the receiver at the satellite port that switches the LNBs and keeps the dish live, allowing program information to be updated by the receiver. It is tricky, though. First, the indoor separating diplexer will need to be DC passive on the SAT side only, not on the off-air side, to keep the desired voltage in the combined line. The diplexer that combines the satellite and off-air signal would then be power passive on BOTH the satellite and antenna side, providing voltage to both the dish and the off-air amplifier. There is a potential pitfall, however, and that is the maximum current rating of the receiver’s SAT port. If the maximum current is enough to power the dish, but that amount of current is close to the rating, then there is little power left to power the off-air amplifier, resulting in possible loss of picture. Also, for the combining diplexer, the power-passive on both side item will work with Winegard Chromstar preamps, SquareShooter SS-2000, and SharpShooter SS-3000, thanks to a voltage regulator in the circuitry. Unfortunately there is no such regulator in the Sensar and Metrostar, so a special combining diplexer is required with a 12VDC voltage regulator at the VHF/UHF port to the amplified antenna.

But as I mentioned at the top, the most effective means on delivering the off-air signal in a satellite system is to use cable runs separate from the satellite, since this setup is very well matched. So if you are planning to get satellite HDTV from an installer, I would challenge your satellite provider by asking if the diplexers what their VSWR ratings are; if they are under 2.0, then the installer is well-prepared to give you a good install. I would also use http://www.checkhd.com/ and the antenna guide to give to the installer, it would be a big help in pointing the off-air antenna the right way, or with practice to realize that your area is not RF friendly for a small antenna, and a larger one may be needed, which they may require you to provide prior to the install. Also, in areas close to the off-air signals (within 20 miles), the installer may provide a non-amplified antenna, but in a diplexed situation, the boost from a preamp may make a difference, so you may have to be ready to add a high-input low-distortion preamp (like the Winegard HDP-269). To put it another way, let the buyer beware, and always prepare.

If you just discovered this page and want more of the HDTV off-air reception tricks you probably have been desperately Googling the Web for, please browse the dated “Archives” sorted by date, there is one column per week. Thanks for viewing!

4 Comments:

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8:49 PM  
Blogger Jimjar said...

Derek, thanks for all of the good information, but I need a little more detail. I have a Winegard Metrostar antenna. You said 'a special combining diplexer is required with a 12VDC voltage regulator at the VHF/UHF port to the amplified antenna'
Can you recommend a combining diplexer like that ? My directv installer used a Winegard DS-8000 powered diplexer a few years ago, but it was damaged and I cannot find a new one anywhere. I would like a recommendation for a new powered or power passing diplexer that will work with my Metrostar.

11:03 AM  

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