TV Technology - January 7, 2009 - (Page 23)

FEATURES in the above photo works well. I’ve had good luck with the Antennas Direct C1 and C2 antennas for UHF measurements. They are compact, fairly rugged and the broad azimuth pattern makes aiming easy. The antenna is connected to a splitter feeding a USB tuner and laptop to check reception and to a spectrum analyzer capable of channel power measurement. Since the goal is to identify problems that may require more detailed coverage studies, I focus on the difference in signal strength between multiple stations after correcting for the difference in antenna pattern. I don’t recommend this technique if nearby “reference stations” are not available. To minimize the impact of reflections, buildings and foliage, only use sites with a clear view to the tower where a SPLAT coverage study or terrain profile shows no blockage. Try to find sites equal distance from the antenna in the directions of interest. At each site, orient the antenna for best reception. If the signals seem too weak, one station is much weaker than expected or the optimum orientation is not in the direction of the transmitter tower, move the antenna or find another site nearby. For example, in a recent test I saw signal levels much lower than expected. Looking at the antenna orientation, we saw the antenna was pointing at a large pine tree! Moving the antenna a few feet to clear the tree provided more realistic results. Once the antenna is aimed, take a wide-span spectrum analyzer shot of channels in the market. After doing this, take channel power measurements of the station being measured and as many reference stations as possible. Stations using omnidirectional patterns make comparisons easy. Back in the office, calculate the difference between the predicted signal level and the measured signal level for each station. Ideally, the difference should remain constant. If it doesn’t, as may be the case at a site with a lot of foliage or other unavoidable obstruction, the difference should be close for all the stations, after taking into account differences in antenna gain and greater foliage loss at higher frequencies. I’ve used this technique at three different locations on the East Coast and results were as expected. The impact of the tower and transmission lines on sidemounted antennas was obvious. I’ve also taken readings along one radial to attempt to verify the null in the elevation pattern but haven’t had a chance to analyze the data to see how it worked. CHECK THE TRANSMITTER it is worth some time off air to test the transmitter, filter and antenna in the exact configuration it will be switched to on Feb. 18. Coordinate with other broadcasters if necessary to do some on-air testing, even if it has to be overnight and be sure to have people monitoring the signal in enough locations to know it is working. Compare the signal with other stations and, if possible, your current DTV signal. The FCC requires proof of performance for DTV transmitters. Most of the proof involves checking out-of-channel emissions (see the IEEE P1631/D3 draft recommended practice for how to do this with a spectrum analyzer and notch filter, www.ieee.org/organizations/society/bt/D TVEmissionsD3.pdf), but the output power calibration and signal-to-noise ratio measurements are also important. Test the transmitter into a dummy load for a few days to reduce the chance of component failure in new or converted transmitters. Good luck! If you uncover some interesting problems or unique solutions, let me know by e-mail. Even though time constraints may make it difficult to respond to all e-mail, I read all comments. Your question could provide the seed for a future RF Technology column. E-mail me at Dlung@transmitter.com. As a final check, if any changes to the DTV transmission plant are required on Feb. 18, as noted earlier, www.tvtechnology.com • TV Technology • January 7, 2009 23 http://www.ieee.org/organizations/society/bt/DTVEmissionsD3.pdf http://www.ieee.org/organizations/society/bt/DTVEmissionsD3.pdf http://www.newtek.com http://www.newtek.com/goplaces http://www.tvtechnology.com

Table of Contents Feed for the Digital Edition of TV Technology - January 7, 2009

TV Technology - January 7, 2009 NFL Enters a New Dimension Mobile DTV Looms Large in 2009 Contents A ‘Flexible Framework’ Doing More With Less ‘X’tra-Streamlined Finish Line in Sight for BAS Transition Ahead of the Relocation Curve DTV Transition Survival Guide ENG and the Lines of Communication DTV Transition Survival Guide Who Do We Really Work For? Obama to Expand Internet Access The Wizard Takes a Holiday The Solid-State Disk Revival To Light the Ear... or Not Hulu Gets It Right—the First Time User Reports—Video Servers & Recording/Controllers Reference Guide Product Showcase Classifieds Marketplace TV Tech Business

TV Technology - January 7, 2009

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.