OTA Antenna Tuning

From MythTV Official Wiki
Revision as of 16:00, 4 October 2007 by Gregturn (Talk | contribs)

Jump to: navigation, search

In the US receiving ATSC (HDTV) signals OTA is a popular option. It's free and with a little bit of time investment very reliable. I've put together this antenna tweaking guide based on my experiences with OTA ATSC.

Equipment

For this guide I'm going to reference my setup. This guide works well for other cards too. The key to making this easy is to have a laptop and somewhere to monitor signal strength from the laptop while you're at the antenna making adjustments. If you can monitor on multiple tuners at once you'll complete the job much faster.

Here is my equipment list

Antenna Attic Placement

A good directional antenna like the CM4228 can be fickle to setup. Moving the antenna just a few feet in any direction can have a large effect on signal quality. When placing the antenna in the attic try to avoid pointing it out the shingles of your roof. Nails and water running down the roof shingles tend to play havoc with signal quality I've found. My best results cam from pointing the antenna out a flat side-wall.

If you're really not sure where the best spot is you can choose a temporary mount point to start with. Mount the antenna on the mast and try securing the mast in a small Christmas tree stand. This may not work depending on the type and size of antenna you have. Or you can do what I did and just screw the antenna mast to the floor with an L-Bracket and clamp. Once you have a location chosen mark it on the floor with a marker. If it's a good spot you want to remember exactly where it was later on.

Once you have the mount point you need to know the direction to point the antenna. Use http://www.antennaweb.org and http://www.tvfool.com to get an idea of the direction from you to the broadcast tower. For me, all the stations I cared about were in the same direction between 132deg and 123deg. If that's not the case for you then ask the mythtv-users list for advice on how to pickup stations using multiple antennas and combining signals.

Getting the Initial Readings

Before tweaking you need some baseline readings. Without these you won't know if the changes you're making are for the better or worse. Start with a channel scan to identify the frequency for each station you care about. Put these into a table along with the initial signal strength and signal to noise numbers:

  1. Run hdhomerun_config FFFFFFFF scan /tmp/ota_initial_scan.txt
  2. From the /tmp/ota_initial_scan.txt file record the frequency, channel, signal strength, and signal noise of each station you care about
FRQ  STATION             Scan
=====================================
707  5.1  WRAL DT CBS    61/56  
719  17.1 NBC17HD NBC    --/--  
683  50.1 WRAZ HD FOX    67/63  
701  11.1 WTVD HD ABC    70/69

Channel scan on the HDHR doesn't always accurately show the signal strength and signal noise. I recommend you tune to each station and watch the status output for about 20 seconds to get an accurate picture of signal strength and signal noise.

  1. Run: hdhomerun_config FFFFFFFF set /tuner0/channel 8vsb:707000000
  2. Run: watch -n 1 hdhomerun_config FFFFFFFF get /tuner0/status
  3. Keep an eye on the signal strength (ss) and signal noise (snq) for about 20 seconds. You want them to be as close to 100 as possible. Add this info to your table Use CTRL-C to stop the watch command.
FRQ  STATION           Scan    132deg
=====================================
707  5.1  WRAL DT CBS  61/56   65/62  
719  17.1 NBC17HD NBC  --/--   43/04  
683  50.1 WRAZ HD FOX  67/63   65/62  
701  11.1 WTVD HD ABC  70/69   77/69  

Notice my initial readings indicate that NBC17 isn't coming in well. On the channel scan the HDHR saw it but didn't get a good SS/SNQ reading. On the manual tune I was able to see some SS but had a very poor SNQ reading. Everythign else isn't to bad however ideally we would see SS/SNQ closer to 75/80 on all channels. Also, watch for wild fluctuations in SS and SNQ. If you see that chances are you are experiencing a multipath signal problem and will need to move the location of the antenna

Making Adjustments

Here is where the laptop comes in handy. You should head into the attic with the laptop, marker, and mapping compass. Connect to your system where the hdhomerun_config program lives (or install a copy on the laptop!) and tune in the weakest channel in the table from your initial scan. Turn the antenna until you see a SS/SNQ of 75/80 or there about. I've had good results with any station showing 70/75 SS/SNQ or better as long as it doesn't fluctuate much!

  1. Start by tuning weakest channel with hdhomerun_config FFFFFFFF set /tuner0/channel 8vsb:719000000
  2. Monitor for 20 seconds with watch -n 1 hdhomerun_config FFFFFFFF get /tuner0/status
  3. Turn the antenna making small adjustments and note the change in real time
  4. Record the new information in your table. Use the mapping compass to get an idea of the direction in case you want to use it again later.
  5. Repeat the first three steps for each channel until all channels are showing a good SS/SNQ reading that is stable
FRQ  STATION           Scan    132deg  128deg
=============================================
707  5.1  WRAL DT CBS  61/56   65/62   78/88
719  17.1 NBC17HD NBC  --/--   43/04   69/85
683  50.1 WRAZ HD FOX  67/63   65/62   71/88
701  11.1 WTVD HD ABC  70/69   77/69   60/69

Improving Signal Strength

If you live in a marginal reception area or would like to get away with a smaller antenna that fits in the attic better you will need an amplifier. There are several different types available but the best one is a in-line amplifier. This style places the amplifier right at the antenna connector and feeds power to it through the coax. No extra wiring needed. Try to avoid the styles that place the amp at the capture card. The main disadvantage is that coax cable has losses and and once the signal is lost the preamp cannot recover it. This is also where an in-line amp shines. If you can find one, a model with adjustable gain can prove useful. Please note that this advise only applies to receiving a signal OTA with an antenna. Cable users wanting more signal (usually because they are splitting to many sets in their house) will want to use a amplified splitter which just makes up for the losses in the splitter. Using a amp designed for antenna use will cause overload and distortion and will degrade the signal rather than improve it.