Last month I described how Andrew Corporation tests its UHF slot antennas. This month I'll delve a little deeper into how slot antennas work. I'll also have a brief progress report on Acrodyne's 60 KW single Diacrode Au-60 UHF transmitter.
In my December and January columns I discussed the role of slot array antennas in UHF transmission. To help visualize the technical details I'll be covering this month, go back and take a look at the photo in last month's column. It shows one of the slots on an Andrew TRASER antenna. My reaction the first time I saw the "workings" of a UHF slot antenna was "The polarization's wrong - the slot is vertical and I need horizontal polarization!". After figuring that one out, I wondered how RF was coupled to the slot.
When I see a new antenna design and want to figure out how it works, I try to see the "dipole" in it. Most antennas in use today are some mutation of a dipole or loop --only the shape and method of feeding them varies. Knowing that dipoles are usually half a wavelength long, it is easy to see the dipole in the slot antenna if you focus on what isn't there (the hole) rather than the metal around it. While intuitively it might seem the slot merely lets some RF out of the cylinder, in reality it actually does the radiating. How this works is explained by "Babinet's Principle", which, although commonly applied to optics, also works at the longer RF wavelengths. Quoting from Constantine Balanis' "Antenna Theory - Analysis and Design" (Harper and Row 1982), Babinet's principle "...states that when the field behind a screen with an opening is added to the field of a complementary structure, the sum is equal to the field when there is no screen." H.G. Booker extended this principle to include conducting screens and added an analysis of polarization.
The rigorous analysis of this is very math intensive and I won't get into it here. The easy way to visualize it is to see the slot as a complement to a dipole made from the piece of metal removed from the cylinder to make the slot. If the combination of the two elements (the slot and its dipole) must combine so that the "the sum is equal to the field when there is no screen", the electric and magnetic fields of the two complementary structures have to be equal and opposite. Because the electric and magnetic fields are 90 degrees apart, the slot must have the opposite polarity of its complementary solid dipole. This makes fabricating slot array antennas for TV transmission, which is predominantly horizontally polarized, much simpler. Imagine how hard it would be to make a self supporting slot cylinder antenna with horizontal slots!
This simple analysis of the slot as a dipole works reasonably well when calculating the elevation pattern from an array of slots. Unfortunately, the analysis falls apart when we try to calculate the azimuth pattern. You already know the familiar fat "8" pattern of a horizontal dipole (maximum signal perpendicular to the antenna with the minimum off the ends). Because of the metal around a slot, the pattern from it is quite different. Let's look at the pattern of a single, axial slot or collinear array of slots in a cylinder. If the diameter of the cylinder is less than a wavelength at the operating frequency the pattern is almost omni-directional. The popular Scala SL-8 1 KW UHF slot antenna is approximately 3 inches (8 cm.) in diameter and has a scull shaped pattern with nulls less than 3 dB below the peak field. As the diameter of the cylinder approaches and exceeds one wavelength, the pattern takes on a more cardioid pattern with reduced radiation behind the slot.
Skull or cardioid patterns aren't the only ones possible with slot antennas. Other patterns can be obtained by adding additional slots around the cylinder. External director elements can be added to the outside of the cylinder to shape the pattern. The test probe in last month's photo is sitting on one of the directors used on KVEA's Andrew TRASAR antenna. External grid reflectors can be used in cases where very narrow patterns are needed.
It's easy to identify the feed point of a conventional dipole antenna. It isn't as obvious looking at a slot antenna. After all, nothing's there! A probe on the edge of the slot couples energy from a coaxial inner conductor into the slot. While you'd expect the feed to be in the center of the slot (lengthwise), it's often offset to better match the high impedance of the slot. Because slots are seldom used alone, the phase and amplitude of the RF coupled off of the inner conductor for each slot is critical to the performance of the antenna. It is possible to use a single inner conductor to feed all the slots in a slot array antenna, although manufacturers often center feed the antenna. This is done either by bringing the transmission line to the center of the antenna instead of the bottom, which works only on side mount antennas, or by placing another coaxial line inside the first one so that while the transmission line enters the bottom of the antenna, the actual feed point is near the middle. To be effective a high gain UHF slot array must have beam tilt and null fill, which means the power fed to the slots won't be equal.
Although most slot antennas made today have a coaxial inner conductor to feed the slots, waveguide is sometimes used. I'm familiar with waveguide slot antennas made by Harris and Andrew. While waveguide slot antennas are simpler in construction (no inner conductor), adjusting the phase and amplitude of their individual slots after manufacture is more difficult.
Most UHF stations have found that adding a vertically polarized component to their signal improves reception on indoor antennas. Often full circular polarization isn't practical (limited transmitter power) or desirable (increased ghosting from multiple reflecting surfaces). One way to add a controlled amount of vertical polarization to a slot antenna is to couple a small amount of energy from the slot and use it to excite a vertical dipole. Such a dipole is visible in last month's photo on top of the slot.
As we've seen, slot antennas consist of cylinders (or waveguide) with holes (slots) cut in them. Obviously something has to be done to keep the weather out of the antenna. Rex Niekamp of Harris Corporation wrote an excellent paper on this topic, which he presented at the World Media Expo / Society of Broadcast Engineers convention in New Orleans last year. It is available in the "1995 SBE Engineering Conference Proceedings" available from the Society of Broadcast Engineers, 8445 Keystone Crossing, Suite 140, Indianapolis, IN 46240, telephone (317) 253-1640. I didn't have room to cover Rex's paper in my WME/SBE report in November's TV Technology so I'll hit some of the main points here.
Mr. Niekamp noted that the environment has both short term and long term effects on slotted antennas. The short term effects are detuning and associated high VSWR due to heavy rain and ice build-up. He also noted that lightning can cause failures. I've observed that a lightning strike has the effect of shorting out the slot. Although it shouldn't make much difference, I've found transmitters using waveguide transmission line usually survive these strikes better than those using coax line. Long term effects include corrosion inside the antenna around contact areas caused by pollution, salt spray and such. Rex Niekamp compared the protection offered by three commonly used methods for protecting slot antennas - slot covers, slot covers with a deicer and a full radome. The full radome was rated best or equal to best in all areas except wind area, lightning protection and initial cost. If you are considering purchase of a new slot antenna and have questions about the best way to protect it, I suggest you obtain a copy of Mr. Niekamp's paper from Harris or the SBE. If you already own a UHF slot antenna, Rex offered some tips on maintaining it. Annual or semi annual inspections are essential. Look for "loose or missing hardware, lightning damage, RF heating and evidence of water accumulating in the antenna." Repair any damage found. Failure to take these steps may result in major problems down the line.
As you probably know, I felt the two most significant products for TV transmission at NAB this year were Acrodyne's single tube 60 KW transmitter based on Thomson's Diacrode and Comark's PS-Squared HV supply for IOT transmitters. As I write this, I'm not aware of any PS-Squared shipments. I'll have more details on PS-Squared as it becomes available.
I can report this month that the first Diacrode transmitter went on the air the end of November at Channel 50 on Sandia Crest above Albuquerque, New Mexico. Acrodyne's second Au-60 Diacrode transmitter is scheduled to go on the air at Channel 53 in Tulsa, Oklahoma the end of January. As of the end of December it was operating successfully into a dummy load. Au-60 number three is scheduled to ship to WQRF in Rockford, Illinois in early January and should be on the air by early February. Acrodyne is rapidly gaining field experience with the 60 KW single tube Diacrode transmitter. The current design has been upgraded since first unit I saw last year. Improvements have been made in the HV power supply and the tube protection circuitry. Improvements made for the 60 KW units are also finding their way into their 30 KW transmitters. Acrodyne's Mitch Montgomery said he expects to build a 120 KW dual Diacrode transmitter this year.
Many broadcasters are watching to see if the Thomson Diacrode will be a reliable alternative to the klystron and IOT / Klystrode for high power UHF amplifiers. At the current rate Acrodyne is shipping Diacrodes, by the end of next year we should have a good indication how the technology is performing. If the technology proves out, it's my opinion Thomson will have to license a second-source for the Diacrode in order to make significant headway against the IOT / Klystrode, which is now available from several sources, including Thomson.
In my report on the SBE convention in New Orleans I summarized the paper Gordon Allison from Larcan-TTC presented on IOT protection devices. As I mentioned in the column, Gordon's paper was very balanced and didn't overtly promote one technology over another. In fact, it was so balanced I drew the wrong conclusion as to which device Mr. Allison actually did prefer! He left me a message explaining that while the spark gap was the least expensive protection method and did adequately protect the IOT when combined with Larcan-TTC's SCR controlled high voltage supply, Larcan-TTC decided the extra protection offered by the vacuum gap was worth the extra cost.
That's it for this month. Next month I'll have a brief introduction to digital signal processing and offer my observations on how it will affect TV transmitters. I'll also cover some of the new options available for transmitting digital data over TV today.
Back issues of my RF columns, computer programs and more are available on my RF Page on the Internet. If all goes well, you will find the RF Page at a new address: http://www.transmitter.com. The new server will give me room to offer new features over the next few months. If you have any problems, I'll be keeping the old site at http://www.gate.net/~dlung/rf.html on-line for another month or so, although it won't be updated as often. You can contact me at firstname.lastname@example.org, email@example.com or firstname.lastname@example.org. You may also fax me at 305-884-9661 or phone me after 6 PM eastern time (when things quiet down a bit) at 305-884-9664. Both numbers are at the Miami Telemundo office, so expect a delay in a response if I'm traveling. My mail service address is 2265 Westwood Blvd., Suite 553, Los Angeles, CA 90064. Because I'm often traveling, if time is critical (response needed in less then ten weeks) please contact me for a local address before sending items by mail. Your comments are always welcome!
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