Antenna Technology Bootcamp. NTA Show 2017 Denver, CO

Transcription

1 Antenna Technology Bootcamp NTA Show 2017 Denver, CO

2 Review: How a slot antenna works The slot antenna is a TEM-Mode coaxial structure. Coupling structures inside the pylon will distort and couple to the fields in this coaxial antenna, causing a voltage to be applied directly across each of the slots in the antenna. This voltage alternates from plus to minus and back again at the channel frequency of operation. The length of the slots are adjusted so that the oscillating electric fields that develop across the gap that the slot creates will launch a radiating system of fields, propagating away from the antenna. 1λ ~ 0.8 λ If the coaxial pylon antenna is oriented vertically, with the slots cut in the outer conductor oriented vertically as well, the electric fields across these slots will be oriented horizontally.

3 Some Useful Terms Beam Tilt The amount of tilt in degrees that the main lobe is tilted downward by electrically short spacing the top elements of the array, or by mechanically tilting the antenna downward. Electrical and Mechanical beam tilt can be used at the same time to increase the over all depression of the main lobe Null Fill The amount of field that is added between the main lobe and the first and/or second secondary lobes. Null fill keeps the field values from going to zero close to the main beam. Values of 5 to 20% are common and are added by short spacing the top elements of the array. Beam Sway The difference in relative field over the bandwidth of the antenna. At a given depression angle of let s say -10 degrees, the field value of the low end the operating band is 0.16 of full field and at the top end of the band it s 0.19 of full field thus giving you a beam sway of 0.03 of peak field (or signal level) across the band.

4 How Elevation Patterns Are Created Let s look at how null fill and beam tilt are formed on a 10 bay slot antenna. We will use a specially spaced low RFR design for this demonstration Example Bays No Beam Tilt No Null Fill Gain = (10.92 db) Array Electrical Length is 3420 degrees

5 Example Bays 0.25 Degree Beam Tilt 3.3% First Null Fill Gain = (10.89 db) Array Electrical Length is 3380 degrees The upper two slots have had their spacing reduced by 20 degrees. The array now has 0.25 degrees of electrical beam tilt and the first null has been raised from 0.0% of peak field to 3.3%. The second, and third nulls have been increased slightly from 0.0%. The elevation gain has dropped slightly to and the electrical length of the array has dropped by 40 degrees to 3380 degrees.

6 1.00 Example Bays 0.50 Degree Beam Tilt 6.7% First Null Fill Gain = (10.74 db) Array Electrical Length is 3324 degrees The upper two slots have had their spacing reduced by 48 degrees. The array now has 0.50 degrees of electrical beam tilt and the first null has been raised from 3.3% of peak field to 6.7%. The second through fifth nulls have been increased slightly from 0.0%. The elevation gain has dropped slightly to and the electrical length of the array has dropped by 96 degrees to 3324 degrees.

7 Example Bays 0.70 Degree Beam Tilt 10% First Null Fill Gain = (10.48 db) Array Electrical Length is 3268 degrees The upper two slots have had their spacing reduced by 76 degrees. The array now has 0.70 degrees of electrical beam tilt and the first null has been raised from 6.7% of peak field to 10.0%. The second through fifth nulls are increasing nicely The elevation gain has dropped to and the electrical length of the array has dropped by an additional 56 degrees to 3268 degrees.

8 Adding a Vertical Component With ATSC 3.0 on the near horizon, your viewers are on the move in dynamic reception environments. The antenna in most cases is not in a horizontal position. The main reason for the desirability of circularly- or elliptically-polarized transmit antennas is because, with a linearly-polarized transmit antenna, as the television signals propagate from the transmitting to the receiving site, the polarization can be rotated due to the influence of external magnetic fields from sources such as the earth itself or large metallic structures like buildings that may have a magnetic moment. This is referred to as Faraday Rotation. If the signals arrive crosspolarized from the transmitting to the receive antenna, the attenuation can be up to 20 db, severe enough to cause the loss of signal at the television receiver. Adding a vertical component to your signal can greatly enhance reception of your station. How do we add the vertical component?

9 The E/P or C/P slot antenna Polarizer elements are mounted on either side of the slot. The polarizers are about 1/8 λ each and launch a vertically polarized electromagnetic field ¼ of a cycle or 90 degrees later than the horizontal field in quadrature. When axial the ratio between the two fields is equal we have Circular Polarization (C/P). When the horizontal field is stronger than the vertical we have elliptical polarization. For ATSC 3.0 a 70/30 to 50/50 H to V ratio is ideal.

10 Azimuth Pattern Differences In many cases there are distinct differences in a slot antenna s horizontal and vertical azimuth pattern. Vertical and horizontal polarized currents flow at different values around the pylon and directional parastitics hence different patterns form. Omnioid Cardioid Omni-directional = slot location

11 V Pol Azimuth H Pol Azimuth A wide cardioid UHF antenna. Pylon diameter to parastitic length were optimized to keep the H and V Pol azimuths close. H Pol azimuth gain is 2.01, V Pol azimuth gain is 1.87.

12 Slot antennas tuning and bandwidth Standard slot antennas are not that broadband, and depending the tuning and design can offer good performance over 3 to 4 channels. If you are faced with a move a channel or two away, the best way to see how the antenna is performing is to sweep it. Antennas are factory tested in a free field. When mounted to a tower or support structure, the antenna can detune. The RF currents flowing around the antenna pylon can couple to the mounting structure. That can change the electrical length that currents are flowing. The best way to see how a given antenna (emphasis on side mounted lower power antennas) is to do a sweep of the antenna two channels above and two channels below the channel of interest (sweep width 30 MHz). Fast Fact: Omnioid antennas detune more easily as more currents are flowing on the back side of the pylon! So let s say you are on channel 18, and want to move to 19. The V.S.W.R. is good at channel 18, but a average of 1.18:1 at channel 19. Will adding a four plunger fine matcher help? That is a maybe as the fine matcher may not be able cancel out the reflections. One channel over is maybe a 60% chance, two channels over is a 25% chance.

13 The plot above is for a channel 50 slot antenna. The plot indicates the antenna has very poor bandwidth. An antenna with good bandwidth would plot within the red circle. If you sweep your antenna out and the plot forms within the red circle, all is well.

14 How about the gain of the slot antenna as it is operated on a different channel? We will use a 12 bay antenna that was tuned for use on channel 30. It has now been swept and has enough bandwidth for use on channels 28 to 32. Using our Ellarray program and running a elevation pattern at channel 30, the gain of the array is The spacing between the elements or slots is 360 degrees. We then calculate the gain at channel 28, with the spacing being set at 355 degrees. The gain has risen to At channel 29, the spacing becomes 358 degrees, which yields a gain of Now we will go up to channel 31 where the slot spacing is now 363 degrees. The gain has dropped to Moving up to channel 32, the spacing is now at 365 degrees and the gain has dropped a bit more down to Going up even further in spacing above 360 degrees will cause gain to drop even more as the aperture efficiency of the array lessens. All broadband antennas have different gains at different frequencies. If beam tilt and null fill have been added, those values will different as well due to the small differences in element short spacing. We will see a plot of this a few slides ahead.

15 Is an antenna ATSC 3.0 ready? One of the most overlooked parameters when evaluating complete transmission systems, including antennas, is the Group Delay, (sometimes referred to as Envelope Delay). When a signal from a broadcast transmitter is routed through components in the system such as transmission lines, filters, switches and finally to the antenna for transmission, these components in the signal path can alter the characteristics of the transmitted signal. If the alteration is severe, (especially for digital signals, where excessive bit error rates can occur), disruptions in the service or degradation to the quality of the service can result. One of these critical parameters is the rate of change of phase shift as a function of frequency within a channel. This is the specific definition of Group Delay. In the analog days with until there was ghosting or bands caused by reflections, group delay, it was mostly ignored. Now we want the signal path to be as linear as possible. Traditionally, Group Delay is evaluated for cavity filters, since filters are capable of storing, (and hence delaying), the signal energy for different amounts of time within the channel, as a function of a specific frequency evaluated, across the channel. Mathematically, Group Delay is defined as: τg=-(1/2π)*dϕ/df

16 Where dϕ/df is the derivative of the transmission phase with respect to frequency, (usually evaluated over the operating frequency band). Within a DTV channel bandwidth, the implications for broadcasters could be substantial, dependingonthe Group Delay characteristics of the entire transmission path, including the antenna, since the phase characteristics of the digitally encoded baseband signal over the channel are crucial. In many cases, if the radiation moment magnitude of a particular radiating element of an antenna is small per excitation voltage cycle, the element, and the complete antenna system will exhibit high stored energy per cycle, (also defined as the "Q" of the antenna system). If the energy is stored in the electric and magnetic fields in and around the antenna, the signal is delayed. If the time delay due to this stored energy is different at different frequencies within a channel, then an abnormally high Group Delay parameter can result. On the next slide are two plots. The one to the left shows the elevation pattern plotted for a single channel 16 bay antenna, on the upper edge, middle, and lower edge ofa6 MHz channel. The slight difference in elevation patterns is called Beam Sway. On the right side is a plot of Differential Group Delay for a three channel UHF slot antenna with elliptical polarization. Over a single channel the group delay is under 3 ns excellent for ATSC 3.0 operation.

17 16 Bay SFN Low 16 Bay SFN Mid 16 Bay SFN High S 21 delay plot of a three channel UHF elliptically polarized slot antenna Bay slot antenna beam sway plot over a single 6 MHz channel

18 Five element Yagi This is a birds eye view of a 5 element Yagi. Depending on element spacing and the taper of the directors, the forward gain will be about 7 db (gain of 5), with a front to back ratio of 12 to 18 db. Side lobe rejection can be as much as 20 db at +/- 90 degrees off the main lobe.? You have just taken over the site. There are a number of Yagi s in storage with no ID stickers. What frequency are they on? Measure the driven element length. Then use 5540 divided by length in inches to get the frequency.

19 2 1 Five element Yagi Depending on the taper and length of the directors this could be a single or dual channel antenna at high band VHF. For good dual channel operation the taper or shortening of the directors needs to increase. The RED director elements are a depiction of a two channel design Element 1 Reflector Element 2 Driven element Elements 3,4,5 directors

20 Broader band Yagi To make the Yagi more broad band we can add a second driven element that is driven out of phase with the other driven element. Element 2 is cut near the low end of the band. Element 3 is cut near the high end of the band. This design would work well for a channel 7 to 13 antenna Element 1 Reflector Elements 2,3 Driven element Elements 4,5, 6 Directors 5 3 Gain will be slightly higher at the upper end of the band. If designed right the V.S.W.R. would be less than 1.25:1 over the band. The feed point for this antenna is at element 3.

21 Yagi antennas do not like each other! Yagi antennas are reclusive and want their own territory. When Yagi antennas are placed to close together several things happen. First they detune quickly, increasing V.S.W.R.. The azimuth pattern distorts, and off main beam lobes can quickly form. In the good old analog days you could see that distortion either in ringing in the picture or ghosting. With digital the MER rate gets clobbered until the blue screen of death comes on. The best way to mount a Yagi is by having a rear mounted antenna. There is much less pattern distortion because the reflector element stops more of the received or transmitted field from coupling to the mounting structure behind. If there are several Yagis on a given tower, separate them by at least 1 wavelength vertically 1-1/2 wavelength is even better. Horizontal separation of Yagis is more, with 2 wavelengths minimum recommended.

22 Log Periodic Antennas The Log Periodic turns 60 next year. It was developed at the University of Illinois in The antenna consists of a number of pairs of half wave spaced dipoles. The dipoles are tapered down in size until the last pair is slightly above the higher frequency of interest. Taper factors of 0.92 to 0.95 are common. To the left is a depiction of a 8 element log periodic antenna for the FM band. The longest element is cut about 5 % longer than a ½ wavelength at 88 MHz. The element pairs taper in size down to about 5 to 10% shorter than a ½ wavelength at 108 MHz. The average gain is 7 db or a dipole gain of 5. Gain is fairly flat across the antenna. Another way of thinking of this antenna is, that is a group of three element Yagis. The two booms form the transmission line. The antennas impedance is a function of the spacing of the two booms. So here is a question If this antenna was used to transmit a signal at MHz and we short circuited the booms in the middle of the array, what would happen?

23 Log Periodic Antennas To the left is another variation of the log periodic antenna, a Zig Zag design. The Zig Zag elements are connected to each other and form the transmission line. The support booms are non metallic. In this design two or four antennas are connected together. The vertical support brace at the rear of the antenna is metallic and connects the Zig Zag antennas. The four antenna design tilts the Zig Zags into an apex. The VHF antenna pictured below is a variant of a log periodic antenna. At channel 2 to 6, the elements serve as a standard log periodic. At channel 7 to 13 the antenna functions as a 3/2 wavelength or third harmonic mode design. The three elements in the front are directors. The elements are swept forward for slightly more gain. Both antennas were in the 1969 Allied Radio catalog.

24 Panel Antennas Panel antennas are excellent broadband antennas that are commonly used for UHF transmission. The panels antennas usually contain 4 pair of elements, mounted on a reflector backplane. Most panel antennas have a single input that feeds the center of the array. Depending on the distance from the backplane (reflector), spacing between the elements and operating frequency, a single panel will have about 11 db of gain, with a narrow cardioid pattern. - 3dB beam width will be in the range of 50 to 60 degrees. The front to back ratio of a single panel is in the range of 20 to 25 db. The front of the antenna is covered with a radome system. The input power is limited to 1.0 to 1.5 kw per panel at UHF frequencies The elements themselves can be shaped a number of ways. They can be straight elements, vee shaped, bow tie shaped, or even use large washers. The feed point of the panel is at the center of the elements Panel antenna depiction

25 Panel Antennas Multiple panel antennas can be combined to form higher gain arrays, directional, and Omni-directional panels. Beam tilt can be added by offsetting the length of the top panels feed line system. Since we are controlling the feed line length in 4 element increments, the choices become more limited than what a slot antenna offers. To get an Omni-directional pattern, 4 individual panel antennas are used per elevation. If possible the panels should be placed so the backplanes of the antennas nearly touch. This will help to reduce scalloping of the azimuth pattern. Scalloping goes up as the frequency of interest goes up. Where an inter beam maxima happens, the RF currents flowing around the panels have added in phase at a given azimuth angle. When they are out of phase, a minima or lower field value results. These coupling and length of the path that the RF currents flow change with frequency. As we go lower in frequency, the path length decreases and there is more uniform radiation from the antenna. For directional patterns, using two or three individual bays at the same elevation can form a number of useful cardioid pattern. Since panel antenna is branch fed, uneven power division can of columns of panels can create additional patterns.

26 Panel Antennas With the repack, it will be common to see multiple channels that use a single panel antenna. With the current operation everything is stable and operating well. Will the move to the new channels be as smooth as thought? The best thing to do is sweep the transmission line at the output of the combiner. In 75 percent or more of applications, the sweep will come back fine. If the sweep comes back not that great on one of the new channels you are operating on, there is something that is causing a reflection at a given frequency. This can happen with larger arrays of panel antennas. A quick thing to try is move the top or bottom bay of panels up or down vertically by an inch or so. If this helps try moving the second sets of panels (second from top and second from bottom by the same amount. At UHF frequencies it only takes a spacing change of ¼ to ½ inch to know out reflections that are adding up. So what about panels and being ATSC 3.0 ready? If the swept V.S.W.R. is low, then the antenna will perform well. Since the antenna is branch fed via a system of power dividers into a low Q system of panels, the differential group delay will be very low

27 Our Repack Guide 87 pages of antenna applications and engineering information Available on line at:

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29 Questions? Contact Information William Ammons Gendron Drive Lewiston ME U.S.A