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

Indoor Antenna Adjustment

With the long-range antenna set up, I decided to revisit the indoor antenna that I use for most of the other channels. Due to a strange polarization effect, the Silver Sensor antenna that I was using as the main antenna was set up at a crazy angle. The angle was an attempt to balance the vertical position with the best reception for channel 2-1 and the horizontal position with the best reception for all the other channels.

Now that I was no longer using the DB-2 as my long-range antenna, I decided to try it as the main antenna. It has significantly higher gain than the Silver Sensor, and can be mounted in a location where it picks up all channels well. With the Silver Sensor, I had a weekly ritual of fiddling with it to make sure that I had it in the right position. The DB-2 was easier to set up, and is a bit more tolerant of changes in position. Figure 4 shows the position of the DB-2, as well as its major drawback. It does not come with a stand, so you need to make one. My "stand" is composed of two stacks of books to get an inclination angle towards the transmitter. (I live in a valley below Sutro Tower.) To hold the correct incline, I use the antenna's mast clamp as a support, with a bit of foam to keep it from slipping across the surface of the book.

Although the antenna is somewhat directional and pointed away from the new transmitter for channel 11 on Mt. San Bruno, it still comes in loud and clear. I have direct line of sight to the new transmitter. The signal is so strong that my HD receiver can lock on from just a 6-foot length of coaxial cable without an antenna. I was quite pleased with this because I was not looking forward to joining a third antenna into the setup to receive channel 11.

Figure 4: AntennasDirect DB-2 and "stand"

Wiring It All Together

I had initially hoped when using the antenna joiner that I could receive both channels 50 (54-1) and 52 (36-1). Channel Master JoinTennas are channel-specific, so I tried using JoinTennas for channels 49, 50, and 52. Although the JoinTenna uses a band-pass filter to let the "alternate" channel through, the filter is not perfect and will allow some of the signal from adjacent stations through. In the course of experimentation, I found that the signal from Monument Peak was so weak that I needed to use a JoinTenna for channel 52. As much as I tried to receive channel 54-1, I could not get a strong enough signal to do so.

In the testing phase, I was using a single HD receiver, the Hughes HTL-HD. When I went to connect the new antenna system to the MythTV system, I needed to split the signal three ways. Two outputs fed the MythTV tuners, and one the existing HTL-HD. My first attempt at connecting everything was to buy a single 4-way splitter. Splitters work by dividing the signal. A perfect splitter will cut the signal in half, and send half each way. In practice, there is a slight amount of additional loss, and most two-way splitters are rated for a 3.5 dB loss. Passive 4-way splitters impose a 7.5 dB loss. When I connected the two antennas to a 4-way splitter, none of the receivers could lock on to any channel.

To split the signal while ensuring that each component could be used for recording, I replaced the passive splitter with a Channel Master 3044 distribution amplifier. Unlike a preamplifier, distribution amplifiers can introduce a bit more noise because they work with signals that are already strong. Once I replaced the splitter with a distribution amplifier, I could recover high-definition signals on all three outputs.

Even with the distribution amplifier, there is a noticeable difference in quality between the Hughes receiver and the pcHDTV tuner cards. The Hughes receiver is able to lock up on weaker signals, and shows a visual glitch about once every month, while the pcHDTV cards will have glitches every week or two. The Hughes receiver has a much better capability to deal with the weak signal on channel 36-1.

I connected the Hughes receiver only to the TiVo because it was much more reliable when was hooked into the TiVo after hitting the distribution amplifier, and the signal is too weak to be reliably recorded by the pcHDTV boards in the MythTV system. Figure 5 shows the end wiring diagram, with both antennas, the antenna joiner, and related amplifiers. It also notes the other change I recently had to make because a TiVo software update disabled serial control and forced me to use an IR emitter instead.

Figure 5: How it all fits together

Final Thoughts

Although there is still some fiddling left to do, the antenna system is finally complete. There are occasional picture glitches on channel 36, but they are small and short-lived. Given the distance I live from the tower, an occasional hiccup is worth the perfect digital picture the rest of the time. Although I could almost certainly get a better picture by doing a roof mount, I am perfectly happy to forego the climbing on the roof and put up with the occasional glitch.

Matthew Gast works in the Office of the CTO at Trapeze Networks, where he works on product architecture and industry standards. He is a voting member of the IEEE 802.11 working group, and serves as chair of 802.11 Task Group M. At the Wi-Fi Alliance, he chairs the Wireless Network Management marketing task group and the Security technical task group. In 2007, Matthew was a founder of the OpenSEA Alliance, a group which supports the development of open-source network security solutions. He currently serves on the engineering steering committee and on the organization's board of directors.

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