Z Technology 's RF Newsletter - DTV Edition
Broadcast News, Edition 4
This is the fourth issue of Z Technology, Inc.'s DTV Newsletter. We have settled towards a stable subscriber list of about 1500 station engineers. Let me know if you want to be added or removed. Thanks, and keep your suggestions coming.
U.S. DTV Scorecard
As of January 28th, 2004, the FCC (http://www.fcc.gov/mb/video/files/dtvsum.doc) reported 1629 stations, 97%, had been granted a DTV license or construction permit. 628 Television stations are now on the air with licensed DTV facilities, plus another 749 stations are operating under special or temporary authority, for a total of 1377 on-air. http://www.fcc.gov/mb/video/files/dtvonairsum.html. The new FCC view is set manufacturers will have until June 2007 to include DTV capabilities in new television receivers 13" to 35.999" diagonal (Does this rule out the great 2006 spectrum sale and phase out of NTSC? Congress may not think so.). Sets 40" or bigger will have to be DTV capable by July 1, 2004.
Z Technology, Inc. to co-sponsors DTV Seminar in Several Cities.
Z technology has participated in co-sponsoring several Zenith/ATSC Digital VSB Transmission Seminar's over this last year. And we are still learning lots of good stuff. These are seminars not-to-miss when they come within a few states of your location. See http://www.zenith.com/index.asp?url=./digitalbroadcast/tradeshows.html for latest news on these great seminars.
We had a great time in Sacramento August 27th. SBE Chapter 43 was a great host, and I met lots of nice folks. Our talk was on DriveTest Coverage Testing of 8VSB and NTSC signals.
Measuring RF coverage of multiple signals
Everybody knows how to measure strength. Just hook an antenna to your field strength meter and take the measurement, right? Well, almost. There are several things to consider, so you will know how to interpret your measurement, and several things we can do to make measurements easier and more reliable.
First we must know how each item of equipment contributes to the measurement. Second, we must take that all into account and write down the correct answer. In dBuV/m. But antennas perform differently at different frequencies. And Field Strength Meters measure power, not field intensity. And cables have losses that increase as the frequency goes higher.
Happily, your calibrated measurement antenna comes with manufacturer's instructions on how to set it up (adjust the element length), and a table of antenna correction factors for the range of frequencies it was designed to cover. These correction factors are generated for your specific antenna, so don't use them for any other antenna, even if it looks the same. The measurement feed line will also be calibrated, so keep the serial numbered calibration table with its serial numbered cable.
You can take measurements manually, and sometimes that is appropriate, but that will severely limit the number of measurement points in your survey. The trend, today, is to automate the measurement process, and let the hardware record and plot actual field intensity as you drive throughout your coverage area. Since the system must be completely automated, requiring no attention at all from the measurement vehicle's driver, it might as well measure your NTSC signal, your DTV signal, your FM station's signal, and even a few of your competition's signals as well.
Several such automated measurement systems are available from Z Technology, Inc., and we can talk privately if you want one (or visit www.ztechnology.com). What we want to talk about in this edition of the DTV Newsletter is the equipment setup for automatically taking and recording field intensity measurements for multiple stations.
Measurement cables and antennas are factory calibrated and shipped with a correction factor table. These factors are used by the controlling laptop PC to calculate Field Intensity in dBuV/m. But how do you measure two different frequencies with a single antenna length adjustment? And how do you account for the filters and combiners required to isolate and match two antennas to the Field Strength Meter input? Really not that hard, since you have all the tools, traceable to the National Institute of Standards and Technology, in your measurement system.
A setup for multiple frequency signal measurements can be described as looking something like this:
A range calibrated UHF dipole antenna would be used for UHF channels, and a VHF dipole for FM and VHF TV frequencies. The high and low pass filters prevent power from the unwanted antenna from reaching the meter and provides isolation between the antennas. The power combiner provides a reliable 50-ohm match from the antenna receiving power, through to the field strength meter.
When there is only a single frequency being measured with each dipole antenna, that antenna will be adjusted to the correct length and the Antenna Correction Factor can be taken directly from the calibration table. When a single antenna must measure two different frequencies, its length should be adjusted for one of the two frequencies. A revised antenna factor can be found for the frequency with the incorrect dipole length by adding the difference in the calibrated Field Intensity Meter reading between the correct length and the actual length (for the other frequency) at which automatic measurements will be taken. These correction factors for each frequency are entered into the Windows™ PC DriveTest application (from the Antenna Correction Factor table supplied by the manufacturer for the correct length frequency, a revised antenna factor for the frequency with the incorrect antenna length).
The antenna factors, cable losses, filter losses, and combiner losses all go into the antenna correction factors in the Job File when setting up the PC application. The Windows™ DriveTest application can then calculate, display, record, and map field strength directly in dBuV/m.
By carefully considering all of the factors involved in NIST traceable field intensity measurements, you can be sure your measurement setup is measuring the signal that is really there. Modern Windows™ based DriveTest applications can make the gathering and processing of thousands of data points, from multiple signal sources, both simple and reliable.
Question of the day…… Have you found a TV set sales clerk that can tell a customer how to display the over-the-air digital signal from your station on his NTSC television set?