7/25/08 Common TV Antenna Types Shown above is the radiation diagram for a 9-element Yagi antenna that the authoronce used to receive channel 12. To interpret this diagram, imagine that the antenna is at the origin. The length of a line from the origin to a ny point on the surface is proportional to the gain in that direction. HDTV Antennas An ante nna made fo r analog TV will wo rk fine fo r DTV. There is nothing differen t about an antenna for DTV or HDTV. Unscrupulous people have labeled their antennas “HDTV Antennas” as a marketing ploy. The honest antenna makers have had to re- label their products likewise to avoid losing sales. Some terms Gain – a measure of how much signal the antenna will collect. Beam width – how directional an antenna is.
Shown above is the radiation diagram for a 9-element Yagi antenna that the author once used to receive channel 12. To interpret this diagram, imagine that the antenna isat the origin. The length of a line from the origin to any point on the surface isproportional to the gain in that direction.
HDTV Antennas
An antenna made for analog TV will work fine for DTV. There is nothing different aboutan antenna for DTV or HDTV. Unscrupulous people have labeled their antennas“HDTV Antennas” as a marketing ploy. The honest antenna makers have had to re-label their products likewise to avoid losing sales.
Some terms
Gain – a measure of how much signal the antenna will collect.
Bandwidth – how the gain varies with frequency. A narrowband antenna will receivesome channels well, but other channels poorly.
This is an easy page. But if you want, you can skip or skim the rest of this page withoutcompromising your understanding of the pages that follow.
The Dipole
This is the simplest TV antenna. Variations on the dipole are the bowtie (which haswider bandwidth), the folded-dipole (which can solve an efficiency problem) and the
loop (a variation on the folded dipole). All four have the same gain and the sameradiation field: a torroid (doughnut shape). The gain is generally 2.15 dBi. “dBi” means“dB of improvement over an isotropic radiator ”, which is an antenna that radiatesequally in all directions. This sounds like a discussion of transmitting antennas, and itcould be. An antenna will have the same gain when receiving as when transmitting,and also the same radiation pattern.
The dipole has positive gain because it does not radiate equally in all directions. This isa universal truth. To get more gain, an antenna must radiate in fewer directions.Imagine a spherical balloon. Now press on it from opposite sides with a finger of each
hand. Push in until your fingers meet. The result looks like the torroid above. But moreimportantly, the balloon expanded in the other directions. A-hah! Gain! That’s the wayantennas work.
Keep this balloon analogy in mind. More complicated antennas work by reducingradiation in most directions. They distort the balloon considerably, but the volume of theballoon remains constant.
Another rating system for antennas uses dBd, which means dB of improvement over adipole antenna. To convert dBd to dBi, just add 2.15. Antenna makers specify their gains in dB. They actually mean dBd, but given the way they exaggerate their claims,dBi is usually closer to the truth.
In the US, TV antennas are always horizontal. If you rotate an antenna about theforward axis (a line from the transmitting antenna) the signal strength will vary as thecosine of the angle. In other words, when the antenna elements are vertical, no signalis received because TV signals have horizontal polarization.
Stacked Dipoles
Two heads are better than one, and so it is with dipoles. N dipoles will take in N timesas much RF power as one dipole, provided they are not too close to each other. Thus a4-dipole antenna would have a gain of 8.15 dBi. (That is 2.15 dBi doubled once (plus 3dB) and doubled again (plus another 3 dB).) This assumes their positions and cablelengths are adjusted so that their signals add in-phase. This explanation of gain mayseem at odds with the balloon explanation, but ultimately they are equivalent. (Addingdipoles does not increase the volume of the balloon because phase cancellation occursin some directions.)
Dipoles are commonly stacked horizontally (collinearly), vertically (broadside), and inechelon (end-fire).
When dipoles are stacked horizontally, the horizontal beam width becomes verynarrow. This is because they do not add in-phase for directions not straight ahead.Similarly, when stacked vertically, the vertical beam width becomes narrower.
Lets say you are 20 miles from a city and TV transmitters are scattered all over the city. A medium gain antenna might be too weak, but a high gain antenna would be sodirectional you would need a rotor. Solution: A bunch of dipoles stacked verticallywould give you the gain you need. The vertical narrowness of the resulting beam is of little importance, but the horizontal broadness of the beam means no rotor needed.
Radio waves will reflect off of a large conducting plane as if it was a mirror. A coarsescreen works just as as well. Reflector antennas are very common.
The double bow-tie above has an average gain of 6 dBi. With a bigger screen it wouldhave more. The parabolic reflector focuses the signal onto a single dipole, but itsbandwidth is a little disappointing. The corner reflector has a little less gain but muchgreater bandwidth. The corner reflector has roughly the gain of three dipoles. It is agood medium gain antenna, widely used for UHF. If you need more than 25 dBi thenthe paraboloid dish is the only practical choice, but they are huge.
The LPDA has several dipoles arranged in echelon and criss-cross fed from the front.The name comes from the geometric growth, which is logarithmic.
This is a very wideband antenna with a gain of up to about 7 dBi. For any frequency,only about three of the elements are carrying much current. The other elements areinactive. As frequency increases, the active elements “move” toward the front of the
array. Most VHF TV antennas are LPDAs.
TV LPDAs come in two types: straight and Vee. The Vee type (LPVA) has a veryslightly higher gain for channels 7-13. But this author often favors the straight type
since it has nulls 90° to each side that can be used to cancel out interference.
Yagi Antennas
A Yagi antenna has several elements arranged in echelon. They are connectedtogether by a long element, called the boom. The boom carries no current. If the boomis an insulator, the antenna works the same.
The rear-most element is called the reflector . The next element is called the drivenelement. All the remaining elements are called directors. The directors are about 5%shorter than the driven element. The reflector is about 5% longer than the drivenelement. The driven element is usually a folded dipole or a loop. It is the only elementconnected to the cable. Yet the other elements carry almost as much current.
The Yagi is the most magical of all antennas. The more directors you add, the higher the gain becomes. Gains above 20 dBi are possible. But the Yagi is a narrowbandantenna, often intended for a single frequency. As frequency increases above thedesign frequency, the gain declines abruptly. Below the design frequency, the gain fallsoff more gradually. When a Yagi is to cover a band of frequencies, it must be designedfor the highest frequency of the band.
An antenna has an aperture area, from which it captures all incoming radiation. Theaperture of a Yagi is round and its area is proportional to the gain. As the leadingelements absorb power, diffraction bends the adjacent rays in toward the antenna.
The formula for the aperture area of any TV antenna is A=G 2 /4 where λ is thewavelength and G is the gain factor over an isotropic antenna (not dB).
Nonessential reading:
Question: How does the energy collected by the directors get to the cable?
Answer: It is re-radiated to the driven element as normal radio waves.
Question: Why don’t the re-radiated waves go backward or laterally?
Answer: Because all the directors cancel each other in those directions.
Question: Why don’t these re-radiated waves prevent the diffraction of incident waves inward toward theboom?
Answer: Because the phase of the re-radiated waves has been changed by about 90 degrees, so they
neither subtract nor add to the incident waves.
Question: How did the director currents get changed by 90 degrees?
Short answer: The element lengths control this. The director currents are shifted -90 degrees while thereflector current is shifted +90 degrees.
Long answer: This graph shows how the current induced in a rod is affected by the length of the rod. Thephase changes quickly with a small change in element length.
This boosts the performance on the lower numbered channels without hurting the highchannels. Although the Yagi/Corner-Reflector might not be the best antenna, it is themost common UHF TV antenna, mainly because it can be mounted on the front of aVHF antenna without degrading the VHF antenna.
Comparing a Yagi/Corner-Reflector to an 8-Dipole-Reflector
The graph above shows the gain functions for four TV antennas:
• Plot A is the Channel Master 4228 8-Bay, a stacked dipole reflector antenna.• Plot B is the Channel Master 4248, a Yagi/Corner-Reflector.• Plot C is the 4248 with all of its directors removed, making it a pure corner
• Plot D is the 4248 with its corner reflector removed and replaced by a singlereflector element, making it a standard Yagi. The D2 plot shows the backwardgain where this exceeds the forward gain.
The point of this graph is that a Yagi/Corner-Reflector performs like a Yagi for the highnumbered channels and a corner reflector for the low numbered channels. For themiddle channels it outperforms the sum of the two types.
A UHF Yagi today is designed for channel 69. If you see an old Yagi, it might beintended for channel 82. In the future they will be cut for channel 51. It is not possibleto tell by looking at a Yagi which era it belongs to, so be careful.
As you can see, the 8-Bay is a very directional antenna. If miss-aimed by 5°you can
lose 1 dB of signal. If the skyline is more than 5°above horizontal, you should tilt theantenna up to point at the skline.
The overhead view shows nulls at 30°and 90° to both sides. These can be used toeliminate multi-path (ghosts) or interference. You simply rotate the antenna until theoffending signal is in one of the nulls.
A Yagi also has some forward nulls that can be used as ghost killers. But aYagi/Corner-Reflector acts more like a corner reflector for most channels, and has nonulls. At channel 60 you can finally see the Yagi pattern start to emerge.
