Building a MythTV Antenna System
Pages: 1, 2, 3

Antenna System "Glue"

With a plan in hand, I also need to get some spare parts for the whole system to tie it all together. Depending on your situation, some or all of these may be important.

Cable. Without cable, you don't move signal around. There are several grades of cable. RG-59 is the standard quality, but has quite high loss, especially over long runs. Putting a large high-gain antenna at the end of a long cable run and then pushing that signal through RG-59 is pointless. Use RG-6 or better. The cable is thicker and slightly harder to work with, but the performance is much better. It may also help to buy high-quality cable. In the course of one weekend's experiments, I picked up some RG-6 cable at a local RadioShack. That turned out to be a regrettable decision because the connectors are hard to tighten or unscrew, which makes modification of the system more time-consuming and frustrating than it needs to be, especially if you are reaching around to the back of any equipment.

Preamplifier. Antennas at the fringes of reception areas are going to be picking up weak signals. In such cases, one of the best investments you can make is in a high-quality preamplifier. Rather than send a weak signal from the antenna down a long cable and try to amplify a very weak signal at the other end, a preamplifier makes the weak signal stronger at the antenna, and sends the strengthened signal down the cable. One of the reasons this is effective is because very weak signals are lost in the noise, so amplifying the weak signal and the noise just results in a very noisy signal. With a preamplifier, the signal is strong enough at the antenna that it can be amplified again relatively cleanly.

High-quality preamplifiers mount on the antenna mast. Power is supplied over the antenna cable from an AC transformer at the other end. Putting a preamplifier in place will require a number of short cables to "splice in" both the preamplifier and its power supply.

One of the key specifications in a preamplifier is its noise figure. Preamplifiers that introduce noise are not effective because they do not improve the signal relative the background noise. Low-noise mast-mounted preamplifiers are a common antenna accessory, and the best models are made by the same companies.

• The Channel Master 7775 is a UHF-only preamplifier with a 2.0 dB noise figure.
• The Channel Master 7777 is a VHF/UHF combination that has the same 2.0 dB noise figure.
• You can also use Winegard preamplifiers, such as the AP-4800, but they have a 2.9 dB noise figure and roughly the same gain.

Of the widely available preamplifiers, I chose the Channel Master 7775 because I only need amplification for UHF, and it has the lowest noise figure.

Joiners. Joining two antennas is easy, but doing it correctly is hard. A two-way splitter can also be used as a two-input combiner. However, the energy on each of the inputs will be split between the other outputs. Channel Master makes a passive device called a JoinTenna that has two inputs: one for a specific channel, and one for the rest of the channels. To add a single channel from a second antenna, hook it up to the specific channel input, while using the main antenna input for everything else. I am fortunate that the JoinTenna works in a way that is ideal for my situation. Most of the stations come from Sutro Tower, and I only need to add a single channel from farther away. JoinTennas can be hard to find, so you may have to be persistent. (I could not find them online, but eventually found Schad Electronics in San Jose, which carries a full line of Channel Master equipment.)

Choosing Antennas

With knowledge of the distance and direction of each signal, you can pick out the appropriate antenna. Most digital TV stations are broadcasting on UHF, which simplifies the antenna design. UHF-only antennas are smaller, and are often the "panel" type that can be much more compact. Although a panel is taller, it requires less space than a long antenna boom. It is also easier to rotate a panel than a boom in limited space.

All of the stations I want to receive are in the UHF bands, with the exception of KNTV, the Bay Area NBC affiliate. I am, however, quite lucky in that I have a nearly unobstructed line of sight from my front door to the transmitters at Mt. San Bruno. The signal is so strong that no antenna is necessary. A 75-ohm shielded coaxial cable plugged into my HD receiver can get a perfect signal from the tower. Even using a UHF antenna, I am able to receive the signal easily. (VHF HDTV broadcasting is relatively uncommon, but broadcasters value it because the much lower frequency require less electrical power to run the transmitters.)

I started by considering the four main options for long-range UHF reception:

• The Channel Master 4228, which is rated for 60-mile reception by Channel Master
• The Channel Master 4221, which is a half-size panel rated for 45-mile reception (the 4228 is two 4221s coupled together)
• The Winegard PR8800, which competes with the 4228
• The Winegard PR4400, a half-size version of the PR8800

With such a long-range reception, I wanted to get the biggest antenna possible. Ken Nist's HDTV Primer has a chart of gain plots calculated from mathematical models of the antennas. The Winegard 8800 and Channel Master 4228 are similar antennas, but the Winegard is better at the low end of the UHF spectrum, and the Channel Master's strength is at the high end of the spectrum. My target channel is 52, so I decided to try the Channel Master first.

Installing and Experimenting With Antennas

It was my initial intent to install the new antenna for channel 36 indoors. Indoor installations are safer because there is no risk of severe electric shock from overhead power lines, and much less risk of severe falls. Antenna grounding is also not necessary in indoor installations. Mounting an antenna indoors is not without challenges. Severe multipath interference can create "hot spots" for signals throughout the building, and the hot spots for the signal may not always match up with aesthetically pleasing mounting locations.

I started off by using the 4228 on the floor of the living room, wired directly to my HD receiver. Surprisingly, it worked quite well and was able to deliver a stable picture without any special effort. However, the cable linking the antenna and the receiver was only 15 feet long. When I tried joining the 4228 to the existing set-top antenna, the signal was just shy of being able to lock on. Adding a preamplifier to the antenna compensated for the loss through the joiner, but when I moved the assembly to the attic, the antenna was just too tall to be able to rotate it towards the signal.

Through experimentation, I discovered that a Silver Sensor pointing in exactly the right direction would bring in the signal, so I took it up to the attic and connected it to the preamplifier. I could get a signal lock, but it was just shy of delivering a quality picture. After consulting the HDTV Primer antenna graph, I decided to try the AntennasDirect DB-2, which has several dB higher gain on channel 52 than the Silver Sensor. Even with the preamplifier, the receiver was unable to lock on.

At this point, I had exhausted all of the common antennas, but I was tantalizingly close. All I needed was a few more dB of antenna gain. Most mainstream antennas constructed with a central boom would likely offer the gain, but would be too large to fit in my confined space. Many all-band antennas, or even UHF boom antennas, are several feet long.

In desperation, I turned to a "last resort" antenna. Most mainstream antennas are designed to work over a relatively wide range of frequencies, but a few specialized antennas are designed for a single channel or a relatively tight channel range. Most of those are designed for use with the lower VHF frequencies.

The only narrow-band UHF antenna I know of is the Blonder-Tongue BTY-10-U, shown in Figure 2. There are six models, each of which is specially designed for a sub-band of the UHF range and has slightly different dimensions. Even though I was turning to the BTY-10-U out of desperation, it had an additional advantage in that the narrow band design allows it to be smaller. Even the largest of them is over four feet long, with a foot-long element at the widest point. The Blonder-Tongue antenna generally sells for $150 new, but I was lucky enough to find one on eBay.

Figure 2: Blonder-Tongue BTY-10-U

When the BTY-10-U arrived, I tried using it to lock on to the signal from channel 52. To start, I tried locking in channel 52 with a preamp from the living room floor. Once I was able to lock on to a signal, I moved it to the attic. Rather than set up a mast mount straightaway, I used empty cardboard boxes to prop it up. The signal received was somewhat sensitive to antenna position. To get a slight upward angle, I propped up the front of the boom. Interestingly, the antenna is not aimed at the station. The strongest signal must be an internal reflection from the material inside the roof.

Figure 3: BTY-10-U installed