Antenna Technology Bootcamp

NTA Show 2017Denver, CO

Review: How a slot antenna worksThe 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.

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.

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Some Useful TermsBeam 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.

How Elevation Patterns Are CreatedLet’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.

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10 BaysNo Beam TiltNo Null FillGain = 12.36 (10.92 dB)Array Electrical Length is3420 degrees

Example 1

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The upper two slots have had their spacing reduced by 20 degrees. Thearray now has 0.25 degrees of electrical beam tilt and the first null has beenraised from 0.0% of peak field to 3.3%. The second, and third nulls havebeen increased slightly from 0.0%. The elevation gain has dropped slightlyto 12.27 and the electrical length of the array has dropped by 40 degrees to3380 degrees.

Example 2

10 Bays0.25 Degree Beam Tilt3.3% First Null FillGain = 12.27 (10.89 dB)Array Electrical Length is3380 degrees

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The upper two slots have had their spacing reduced by 48 degrees. Thearray now has 0.50 degrees of electrical beam tilt and the first null has beenraised from 3.3% of peak field to 6.7%. The second through fifth nulls havebeen increased slightly from 0.0%. The elevation gain has dropped slightlyto 11.85 and the electrical length of the array has dropped by 96 degrees to3324 degrees.

Example 3

10 Bays0.50 Degree Beam Tilt6.7% First Null FillGain = 11.85 (10.74 dB)Array Electrical Length is3324 degrees

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The upper two slots have had their spacing reduced by 76 degrees. Thearray now has 0.70 degrees of electrical beam tilt and the first null has beenraised from 6.7% of peak field to 10.0%. The second through fifth nulls areincreasing nicely The elevation gain has dropped to 11.18 and the electricallength of the array has dropped by an additional 56 degrees to 3268degrees.

Example 4

10 Bays0.70 Degree Beam Tilt10% First Null FillGain = 11.18 (10.48 dB)Array Electrical Length is3268 degrees

Adding a Vertical ComponentWith 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 cross-polarized 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 ?

The E/P or C/P slot antennaPolarizer elements are mountedon either side of the slot. Thepolarizers are about 1/8 λ eachand launch a vertically polarizedelectromagnetic field ¼ of acycle or 90 degrees later thanthe horizontal field inquadrature. When axial the ratiobetween the two fields is equalwe have Circular Polarization(C/P). When the horizontal fieldis stronger than the vertical wehave elliptical polarization. ForATSC 3.0 a 70/30 to 50/50 H toV ratio is ideal.

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.

= slot location

Omnioid CardioidOmni-directional

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

Slot antennas tuning and bandwidthStandard slot antennas are not that broadband, and depending the tuning anddesign can offer good performance over 3 to 4 channels. If you are faced with amove a channel or two away, the best way to see how the antenna isperforming is to sweep it. Antennas are factory tested in a free field. Whenmounted to a tower or support structure, the antenna can detune. The RFcurrents flowing around the antenna pylon can couple to the mountingstructure. That can change the electrical length that currents are flowing.

The best way to see how a given antenna (emphasis on side mounted lowerpower antennas) is to do a sweep of the antenna two channels above and twochannels below the channel of interest (sweep width 30 MHz).

Fast Fact: Omnioid antennas detune more easily as more currents are flowingon 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. isgood at channel 18, but a average of 1.18:1 at channel 19. Will adding a fourplunger fine matcher help ? That is a maybe as the fine matcher may not beable cancel out the reflections. One channel over is maybe a 60% chance, twochannels over is a 25% chance.

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.

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 nowbeen swept and has enough bandwidth for use on channels 28 to 32.

Using our Ellarray program and running a elevation pattern at channel 30, thegain of the array is 13.69. The spacing between the elements or slots is 360degrees. We then calculate the gain at channel 28, with the spacing being setat 355 degrees. The gain has risen to 13.74.

At channel 29, the spacing becomes 358 degrees, which yields a gain of 13.74.

Now we will go up to channel 31 where the slot spacing is now 363 degrees.The gain has dropped to 13.59. Moving up to channel 32, the spacing is now at365 degrees and the gain has dropped a bit more down to 13.50. Going upeven further in spacing above 360 degrees will cause gain to drop even moreas the aperture efficiency of the array lessens.

All broadband antennas have different gains at different frequencies. If beam tiltand null fill have been added, those values will different as well due to the smalldifferences in element short spacing. We will see a plot of this a few slidesahead.

Is an antenna ATSC 3.0 ready ?One of the most overlooked parameters when evaluating complete transmissionsystems, including antennas, is the Group Delay, (sometimes referred to asEnvelope Delay). When a signal from a broadcast transmitter is routed throughcomponents in the system such as transmission lines, filters, switches and finally tothe antenna for transmission, these components in the signal path can alter thecharacteristics of the transmitted signal. If the alteration is severe, (especially fordigital signals, where excessive bit error rates can occur), disruptions in the serviceor degradation to the quality of the service can result. One of these criticalparameters is the rate of change of phase shift as a function of frequency within achannel. 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 aspossible.

Traditionally, Group Delay is evaluated for cavity filters, since filters are capable ofstoring, (and hence delaying), the signal energy for different amounts of time withinthe channel, as a function of a specific frequency evaluated, across the channel.

Mathematically, Group Delay is defined as:τg=-(1/2π)*dϕ/df

Where dϕ/df is the derivative of the transmission phase with respect to frequency,(usually evaluated over the operating frequency band). Within a DTV channelbandwidth, the implications for broadcasters could be substantial, depending on theGroup Delay characteristics of the entire transmission path, including the antenna,since the phase characteristics of the digitally encoded baseband signal over thechannel are crucial.

In many cases, if the radiation moment magnitude of a particular radiating element ofan antenna is small per excitation voltage cycle, the element, and the completeantenna system will exhibit high stored energy per cycle, (also defined as the "Q" ofthe antenna system). If the energy is stored in the electric and magnetic fields in andaround the antenna, the signal is delayed. If the time delay due to this stored energy isdifferent at different frequencies within a channel, then an abnormally high GroupDelay parameter can result.

On the next slide are two plots. The one to the left shows the elevation pattern plottedfor a single channel 16 bay antenna, on the upper edge, middle, and lower edge of a 6MHz 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 slotantenna with elliptical polarization. Over a single channel the group delay isunder 3 nS – excellent for ATSC 3.0 operation .

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16 Bay SFN Low 16 Bay SFN Mid 16 Bay SFN High

16 Bay slot antenna beam sway plotover a single 6 MHz channel

S21 delay plot of a three channel UHF elliptically polarized slot antenna

? You have just taken over the site. There are a number ofYagi’s in storage with no ID stickers. What frequency are they on ?Measure the driven element length. Then use 5540 divided bylength in inches to get the frequency.

Five element YagiThis is a birds eye view of a 5 element Yagi. Depending on element spacingand 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 as20 dB at +/- 90 degrees off the main lobe.

Five element Yagi

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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.

Broader band Yagi

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Element 1 ReflectorElements 2,3 Driven elementElements 4,5, 6 Directors

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

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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 antennais at element 3.

Yagi antennas do not like each other !Yagi antennas are reclusive and want their own territory. When Yagi antennasare 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.

Log Periodic AntennasThe Log Periodic turns 60 next year. It was developed at the University ofIllinois in 1958. The antenna consists of a number of pairs of half wavespaced dipoles. The dipoles are tapered down in size until the last pair isslightly above the higher frequency of interest. Taper factors of 0.92 to 0.95are common.

To the left is a depiction of a 8 elementlog 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 toabout 5 to 10% shorter than a ½wavelength at 108 MHz. The averagegain 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 107.1MHz and we short circuited the booms in the middle of the array, what wouldhappen ?

Log Periodic AntennasTo the left is another variation of the log periodicantenna, a Zig Zag design. The Zig Zag elementsare connected to each other and form thetransmission line. The support booms are nonmetallic. In this design two or four antennas areconnected together. The vertical support brace at therear of the antenna is metallic and connects the ZigZag antennas. The four antenna design tilts the ZigZags into an apex.

The VHF antenna pictured below is a variantof a log periodic antenna. At channel 2 to 6,the elements serve as a standard logperiodic. At channel 7 to 13 the antennafunctions as a 3/2 wavelength or thirdharmonic mode design. The three elementsin the front are directors. The elements areswept forward for slightly more gain. Bothantennas were in the 1969 Allied Radiocatalog.

Panel AntennasPanel antennas are excellent broadband antennas that are commonly usedfor UHF transmission. The panels antennas usually contain 4 pair of elements,mounted on a reflector backplane. Most panel antennas have a single inputthat feeds the center of the array. Depending on the distance from thebackplane (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 backratio of a single panel is in the range of 20 to 25 dB. The front of the antenna iscovered with a radome system. The input power is limited to 1.0 to 1.5 kW perpanel at UHF frequencies

The elements themselves can be shaped anumber of ways. They can be straightelements, vee shaped, bow tie shaped, oreven use large washers. The feed point ofthe panel is at the center of the elements

Panel antenna depiction

Panel AntennasMultiple panel antennas can be combined to form higher gain arrays, directional,and Omni-directional panels. Beam tilt can be added by offsetting the length ofthe top panels feed line system. Since we are controlling the feed line length in 4element increments, the choices become more limited than what a slot antennaoffers.

To get an Omni-directional pattern, 4 individual panel antennas are used perelevation. If possible the panels should be placed so the backplanes of theantennas 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 beammaxima happens, the RF currents flowing around the panels have added inphase at a given azimuth angle. When they are out of phase, a minima or lowerfield value results. These coupling and length of the path that the RF currentsflow change with frequency. As we go lower in frequency, the path lengthdecreases and there is more uniform radiation from the antenna.

For directional patterns, using two or three individual bays at the same elevationcan 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.

Panel AntennasWith the repack, it will be common to see multiple channels that use a singlepanel antenna. With the current operation everything is stable and operatingwell. Will the move to the new channels be as smooth as thought ? The bestthing to do is sweep the transmission line at the output of the combiner. In 75percent or more of applications, the sweep will come back fine. If the sweepcomes 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 canhappen with larger arrays of panel antennas. A quick thing to try is move thetop or bottom bay of panels up or down vertically by an inch or so. If this helpstry moving the second sets of panels (second from top and second frombottom by the same amount. At UHF frequencies it only takes a spacingchange 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. islow, then the antenna will perform well. Since the antenna is branch fed via asystem of power dividers into a low Q system of panels, the differential groupdelay will be very low

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