RF Column 28 - February 1994 Copyright (c) 1994,1995 H. Douglas Lung ALL RIGHTS RESERVED TOPICS: Some notes on mountain top transmitter site Those darn plastic packing pellets! Lighting protection for remote control phone lines Alternatives to phone lines for remote controls Calculating the proper feed horn position for parabolic dishes Correct address for obtaining the NADCON coordinate program ----------------------------------------------------------------------- Last month I promised I'd cover some short items I didn't have room to include in earlier columns. One of the problems of traveling a lot is that you tend to leave things in various locations. The parts for the prototype of my WWV receiver / frequency calibrator project I spoke about a few months ago disappeared somewhere between Los Angeles and Miami. They'll probably re-appear as soon as I order the replacements! As I sit writing this in Los Angeles, I found material for part of the column was in Miami. That gives me the room to air a pet peeve I've been meaning to write about for a while now. If you don't do much work on mountain tops, after reading this you'll probably think I've gone crazy. If you do, you might still think I'm crazy, but I think you'll agree with me. While transmitter days tend to be long, the views and nature help compensate for middle of the night schedule. The last thing I want to do is mess up the surrounding countryside. Unfortunately, most transmitter manufacturers do not consider this when shipping parts. What am I talking about? Plastic pellets! Open up a box anywhere at the site and the wind that always seems to be blowing at those elevations (or the transmitter blower) will send the parts all over the landscape. I know most of the U.S. transmitter manufacturers read this column. Here's what you can do to help us out. If you must use plastic pellets, put them in small plastic bags first, so they won't blow away. N.E.C. used this approach for some of their studio equipment. That expanding foam works nice too, but isn't too practical for small shipments. Big pieces of closed cell foam rubber should work well for things like filters. Microwave Filter Company used cardboard and bubble wrap to pack a 1 kW bandpass filter I bought recently. No mess! At the same mountain site, I unpacked a notch filter from another manufacturer in a wooden box, with pieces of foam rubber and wood securing the filter. That would have been fine by itself, but they had to fill the entire long box with small, non-biodegradable plastic peanuts! We chased them all over the mountain top. A well packed box shouldn't need peanuts! I'm thinking about putting a line on purchase orders for transmitter gear saying "Plastic peanut or pellets are unacceptable packing material and will be returned, freight collect". What do you think? I'll mention any manufacturer here that will swear off peanuts for transmitter items. Besides wind and plastic peanuts, another thing mountain top sites seem to attract is lightning. Actually, lighting can be a problem even on flat ground if a tall tower is present. I could use an entire column talking about grounding and power line protection (and probably will sometime), but now I'll limit it to phone line protection. Most phone lines will have some sort of spark gap or gas tube at the service entrance. When new, they work quite well. After a few hits, performance deteriorates. If they short, they may be removed as a "quick fix" if no replacements are available. In any case, you shouldn't depend on the phone company to protect your remote control or modem line. Gentner realized this and provides a phone line surge protector with each of their VRC-2000 remote controls. I've tried a number of surge protectors at our Midland Texas LPTV site. The phone line is near the end of the line with all wire above ground and a couple towers in the area. I've had good luck with the TrippLite 4 outlet ISOTEL (TM) in the office and at home, but it didn't cut it in Midland. I fried two ISOTEL's before replacing them with a phone line protector made by PolyPhaser. It comes in an impressive metal box and uses screw terminals instead of RJ-11 jacks. So far, its survived. The ISOTEL is a good unit, and to TrippLite's credit the modem was never damaged. On the other hand, two ISOTEL's cost as much as the modem! PolyPhaser distribution isn't too friendly to small users - you'll have to call them to find a regional distributor, establish credit or send a company check and wait for the unit. PolyPhaser's number is (800) 325-7170 (outside the U.S. dial +1 702-782-2511) voice, (702) 782-4476 fax. I've found two way radio tech's like PolyPhaser equipment. They also make AC power line surge protectors, data line protectors and even RF coaxial lightning protection equipment. When I'm on the road, I hate to bother with the paperwork to get a company check out to order a relatively inexpensive piece of gear. It's a lot easier to charge it and get reimbursed later. The last two sites I built I used the much easier to obtain TrippLite TSB phone line protector. It costs around fifty dollars and uses three stage filter with gas discharge tubes. The RJ-11 jacks make it easy to install. So far, no problems. This unit is probably available from your local parts store. If not, most major electronic distributors, including Digi-Key, carry it. Here's a tip if your modem or remote control does get zapped by a surge on the phone line. If it hasn't burned, it may be easy to fix. Many of these units have a limited built in surge protector - often a pair of low value (47 ohm or less) resistors in series with the phone line and some capacitors. You can track them down from the RJ-11 jack on the modem/remote with an ohmmeter. Look for open resistors or a shorted capacitor. If these are good, check the modem's ring detector circuitry. Government regulations (FCC Part 68 in the U.S.) require equipment connected to the phone line to meet certain minimum standards for isolating you from the phone line. The transformers used to do this also do a pretty good job protecting the modem electronics. The ring detector, however, is located before the transformer and thus isn't isolated. All the ring detectors I've seen use a resistor in series with the LED in an opto-isolator. The opto-isolator provides the isolation. If either of these components fails, the modem won't answer. If you're lucky, the opto-isolator will have a standard part number you can cross reference. If not, look in one of the replacement semiconductor directories for a suitable replacement. There's nothing critical about the opto-isolator except that it provide the isolation required. Be aware that substituting components may void the manufacturer's government type acceptance for the modem and make it illegal to use it on the public telephone system. If all this talk of lightning and failed phone lines makes you nervous, there are a number of RF based alternatives for transmitter remote control. Audio tones on a subcarrier on a TV microwave link are the most common. The first remote controls were analog devices where the frequency of the audio tone was related to the voltage applied to the remote at the transmitter site. These units were difficult to calibrate and were replaced with systems that converted the remote readings to digital data. Since the audio subcarriers were already there, most units contained their own simple AFSK (Audio Frequency Shift Keying) modems. Now, most remote controls are based on microcomputers (IBM Compatible PC's are common) and require external modems to convert the computer's RS-232 data to audio tones. Usually this modem will be based on the old Bell 212 standard. The data rate is only 1200 baud, however, it does work on four wire systems (separate receive and transmit circuits) like microwave subcarrier links. I've tried to use regular computer modems using two wire connections on microwave links without much success. If the hybrid used to convert the two wires out of the computer modem to separate transmit and receive circuits isn't precisely adjusted feedback results. Bell 212 modems are becoming harder to find. The newer four wire modems replacing them are pricey. There is an alternative - RF modems. Wegener (404-623-0996 in Georgia) is one of the biggest suppliers. Many cable and broadcast satellite operators use their modems. The Wegener modems use FSK (Frequency Shift Keying) directly on an RF subcarrier. This is more efficient and more robust than systems which convert the data to an audio tone then modulate the subcarrier with the audio. Wegener's 2061 modulator series and 2020D demodulator can operate up to 14.4 kilobaud, twenty times faster than the old 212 modems. To connect the Wegener modem to the microwave, use the microwaves "baseband" outputs and inputs. Both units have loop through connectors, so they can be inserted in series with the microwave units subcarrier modulators or demodulators. Look for a lot of these demodulators to show up on the used market. As satellite operators convert to digital compressed video for transmission, the data subcarriers will no longer be required. Two other RF data transmission methods deserve mentioning. In some areas broadcasters are using packet equipment originally design for ham radio use. The packet nature of the data permits several stations to share one 450 MHz. "P-channel" frequency for remote control. Packetizing the data slows it down, but since most older systems didn't even push 1200 baud, the speed is acceptable for small remote control systems. The second method is to encode the digital data in the vertical interval of the main video signal, like Teletext. Note that I'm talking about using this system on microwave links (duplex), not on the TV transmitter itself. Although several companies sell Teletext equipment, I'm don't know anyone marketing the equipment for this purpose. While we were installing some satellite dishes at our NOC in Miami some questions came up about the proper location for the feed horn. The dish manufacturer hadn't included this information with the dishes. I found a simple formula for calculating the focal distance of parabolic dishes. The results from the formula matched what the manufacturer's engineer told us. Here's the formula: D x D Focal distance = -------------------- 16 x C D = the diameter of the reflecting surface of the dish (don't include supporting rings or structures outside the parabola) C = The depth of the dish. Run a string from side to side across the center of the dish and carefully measure the distance to the center. Be sure to stay consistent with the units. If the "D" is in meters, everything else must be measured in meters. Once you've calculated the proper focal length of the dish, another problem arises. The feed assembly must be mounted so that the phase center of the feed is at the focal point. Where is the phase center of the feed? Technically, it is the point where the energy appears to come from. In practice, because of the size of the opening of the feed horn, there isn't a neat point. Some manufacturers say it is at the center of the circle formed by the front edge of the scalar rings. Looking through some text books on microwave feeds, I'd say it is usually a little bit further back in the feed. If the beam width of the feed is 90 degrees, the signals coming in from the 45 degrees either side of center will intersect inside the feed, not at the front edge of the feed. Draw it out on paper to see what I mean. The trigonometry is left as an exercise for the reader. Don't get carried away with the precision though. Constantine Balanis, writing in his book "Antenna Theory - Analysis and Design" ((c) 1982, Harper and Row) concluded a discussion on calculating the phase center by saying "The analytical formulations for locating the phase center of an antenna are usually very laborious and exist only for a limited number of configurations. Experiment techniques are available to locate the phase center of an antenna." Some tweaking will be required! One final item - Robert Pritchard, Chief Engineer at WHSP in Vineland, New Jersey informed me the address I gave for obtaining the NADCON program was wrong. Refer back to my SBE column if you missed the discussion on NADCON. Here is the correct address to order NADCOM: NOAA, NGS, N/CG174 SSMC3, Station 9202 1315 East-West Highway Silver Spring, Maryland 20910 Telephone 301-713-3242 Attn.: National Geodetic Survey Information Branch The cost is $30 and they will take credit cards. Robert told me he had to take a telephone tour of the Washington bureaucracy to obtain this information. Thanks! If you have information, tips or techniques that would be of interest to other RF engineers, please share! You can reach me on CompuServe through E-MAIL at 70255,460 or on the Internet at 70255,460@compuserve.com. On occasion, I'm in my office at 305-884-9664. Call after 6 PM eastern local time if possible. My fax number is 305-884-9661. You can write me at 2265 Westwood Blvd., Suite 553, Los Angeles, CA 90064. If you mailed me anything between Thanksgiving and Christmas in 1993, it probably went up in smoke when the building was burned the day before Christmas. If you haven't gotten a response to letters or requests sent in December or late November, please contact me. Copyright (c) 1994,1995 H. Douglas Lung ALL RIGHTS RESERVED