10/24/08

Choosing a mounting site

 

Diffraction

Diffraction is the ability of a wave to bend around into the shadow formed by an obstruction.  It doesn’t matter whether it is an absorbing or reflecting obstruction.  Most OTA viewers depend on diffraction for their reception.  The only exceptions are:

·         Where the transmitting tower can be seen.

·         Sometimes in cities with tall buildings reflection is more effective than diffraction.

 

VHF Diffraction

http://www.hdtvprimer.com/ANTENNAS/VHFdiffr.jpg

 

 

The direction the signal is moving is always perpendicular to the wave fronts.  Thus if an antenna is mounted in the shadow of a building, the antenna should point at the top of the building, because that is where the wave is coming from.

 

Low frequencies diffract efficiently, but VHF diffracts poorly.  UHF is another ten times worse.

 

UHF Diffraction

http://www.hdtvprimer.com/ANTENNAS/UHFdiffr.jpg

 

These diagrams use linear shading and thus are perhaps overly pessimistic.  Reception might be possible where these diagrams show no signal.  (Logarithmic shading would convey more optimism.)

 

TV Transmitting power allowed by the FCC:

To make up for the inability of UHF to reach into valleys, the FCC allows UHF stations to have higher power.

 

                                    Analog:                      DTV:

Channels 2-6 :               100 kilowatts             50 kilowatts

Channels 7-13 :             316 kilowatts             160 kilowatts

Channels 14-69 :           5 megawatts              1 megawatt

 

The above numbers are approximate.  The actual power rules are more complicated than this table, and stations can argue for and get a higher limit.  But the goal in most cases is a 60-mile reception radius.

 

The above power numbers are ERP numbers (effective radiated power).  ERP is defined as the transmitter’s RF power output times the gain of the transmitting antenna.  UHF transmitting antennas usually have higher gain, so the disparity in transmitter electric bills is not as great as this table suggests.

 

Ground reflections

Often the signal waves are angled downward slightly, usually the result of diffraction over an obstacle in the distance.  If there is mostly-flat ground in front of the antenna, the ground reflection can be efficient.

Instantaneous voltage diagram:

http://www.hdtvprimer.com/ANTENNAS/GroundReflectInst.jpg

These two waves pass through each other without affecting each other.  But the antenna responds to the instantaneous sum of the two overlapping waves.  Where the two waves subtract, there will be places where reception is very weak.

Average power diagram:

http://www.hdtvprimer.com/ANTENNAS/GroundReflectAve.jpg

The result is a striped region of alternating strong and weak layers parallel to the ground.  Thus there are cases where lowering the antenna might put it in a stronger signal.  So, while the following siting technique is unorthodox, the result is very credible:

http://www.hdtvprimer.com/ANTENNAS/comic.gif

ă King Features Syndicate.  Reproduced here with permission.

Unfortunately a strong spot for one channel can be a weak spot for a different channel, so compromise might be necessary.

 

The ground doesn’t have to be as flat as you might guess for these layers to form.  Weeds, shrubs, and trees are mostly transparent to VHF.  A surface wet from rain will usually be 100% reflective.

 

Average power diagram for UHF:

http://www.hdtvprimer.com/ANTENNAS/GroundReflectUHF.jpgThis layering problem is greatest for UHF.  The distance from a very strong spot to a very weak spot can be as little as three feet.  But weeds might save you.  Stand where the antenna will go and look toward the ground in the direction of the transmitter.  If you see weeds or shrubs or trees, you are OK.  If you see lawn or dirt or pavement then you likely have some layering.

 

http://www.hdtvprimer.com/ANTENNAS/GroundReflectZero.jpgAn antenna has an aperture, over which all incoming signal is collected.  In this diagram the aperture is positioned to collect signal from two layers.  But adjacent layers always have opposite polarity, and subtract.  Thus this antenna is picking up no signal at all (assuming a 100% efficient ground reflection):

 

When layering is present, if a larger antenna is necessary, choose an antenna whose aperture is wider, not taller.  Otherwise you may find the new antenna works no better than the old one.

 

http://www.hdtvprimer.com/ANTENNAS/GroundReflectPower.jpgThe ground reflection can be very helpful.  Assume the power in the incident wave is P.  If the reflection is 100% efficient, you might expect the power in the overlapped area to be 2P.  But instead it will vary from 0P to 4P.  (Power is the square of voltage.  Where the voltage doubles, the available power goes up by 4.)

 

If you can put the antenna in the most intense spot, it will collect 4 times as much signal as with no ground reflection (assuming a 100% efficient ground reflection).

 

Trees and UHF

If a tree loses its leaves in the fall, reception behind it will improve dramatically.  Many people get a TV for Christmas, and erect an antenna for it in January, and then wonder why it quit working in May.  It’s the trees.

 

In the following simulation, a tree is modeled as a perfect sphere blocking 90% of the signal.

 

http://www.hdtvprimer.com/ANTENNAS/Tree35I.jpg

http://www.hdtvprimer.com/ANTENNAS/Tree35.jpg

(The simulation was in 3 dimensions.  The diagrams show the field strength in a plane through the tree center.  Due to symmetry the diagrams look the same when viewed from above.  The obstruction was coded as a disk, not a sphere, but the difference is minuscule in most places.)

 

If the antenna is behind a tree, it is in overlapping fields: a weak field that passes through the tree plus a weak field that is diffracted around the tree.  Overlapping fields are complicated, with strong spots and weak spots.  This will be true even if the tree is not a perfect sphere.  If you get a UHF antenna to work behind a tree, you will likely see dropouts when the wind blows because the strong and weak spots will move around as the tree deforms.  Even in a good-signal neighborhood it is inadvisable to put a UHF antenna behind a tree.

 

The farther away a tree is, the less of a problem it is.  For far away trees, assume no signal penetrates the tree, and reception will be by diffraction around the tree.  Trees block 100% of satellite signals.

 

 

Trees and VHF-high  (The tree blocks 60% of the signal.) 

http://www.hdtvprimer.com/ANTENNAS/Tree10.jpg

In this case the wake tendrils are very broad.  The tree is not likely to deform enough to cause a dropout.  Reception might be slightly sensitive to wind.

 

Trees and VHF-low  (The tree blocks 30% of the signal.  )

http://www.hdtvprimer.com/ANTENNAS/Tree4.jpg

An antenna in its wake will work fine for channels 2-6.

 

Skyline Multi-path

The following four diagrams are identical except for the view rotation.

http://www.hdtvprimer.com/ANTENNAS/overlap.gif

Rays from the transmission tower come to earth after passing over a skyline ridge.  This ridge could be a tree line 50 yards away or a mountain ridge 5 miles away.  The rays diffract at the ridge, staying in a plane perpendicular to the ridgeline.  The result is often overlapping rays.

 

Overlapping fields will result in weak signal spots and strong spots arranged in a regular pattern.

http://www.hdtvprimer.com/ANTENNAS/overlap.jpg

For UHF the strong and weak spots are often 5 to 20 feet apart.  If you are in a neighborhood with overlapping fields, moving your antenna a few feet can make a huge difference in signal strength.  The chimney might seem like the perfect site, but if the chimney is in a weak spot then the chimney is a mistake.

 

To make matters worse, the pattern of strong and weak spots will be different for different frequencies.  You will want to find a spot that is strong for all the channels you want.  But such a spot might not exist above your roof.  In this case you must search for a spot that is the best compromise for your must-have channels.  In the worst case you might need two antennas and a switch.

 

To make matters even worse, you will not likely discover that you are in such a neighborhood until after you have purchased and installed the antenna.  To prove that you have strong and weak spots, you move the antenna (leftward and rightward, higher and lower) while keeping it perfectly pointed at the signal and watching the DTV signal strength indicator.  (What?  Your TV is not on the roof?  Well maybe with some cordless phones you can get your wife to help you.  Tape the phone to your head.)  It is hard to keep a large antenna pointed correctly while devoting half of your attention to not falling off the roof, but a smaller antenna might not achieve a digital lock.

 

At this point a professional installer starts to look like the smart choice.  But will he stick with it, or will he too quickly declare further improvements impossible and walk away?  He will hesitate to raise his estimate, but he will not work at a loss.

 

These problems are UHF problems.  VHF does the same thing, but with strong and weak spots 50 to 200 feet apart they are not very evident and there is usually not much you can do about them.

 

Non-uniform fields

Overlapping fields result in non-uniform fields: layered and continuously varying.  An antenna in a non-uniform field doesn’t perform quite like one would guess.  Normally an antenna captures all the radiation within its aperture.  But in a non-uniform field some signal gets rejected.

 

http://www.hdtvprimer.com/ANTENNAS/nonuniform.jpgMany people in this situation conclude they need a bigger antenna.  But a bigger aperture gathers signal from a larger area, and this larger area is usually even more non-uniform, causing greater signal rejection.  Many people have switched to a larger antenna and found no improvement.

 

There is more than one way to explain this counter-intuitive result.  (These explanations sound totally different, but they are equivalent.)  Probably the simplest explanation is that the antenna’s beam width gets narrower as the aperture gets bigger.  The bigger antenna is now so directional that it cannot be pointed directly at both sources that produce the overlapped field.

 

A solution to this dilemma is an asymmetric aperture.  Choose an antenna whose aperture is large in the direction of the layer, but small in the direction across the layer.  Stacked antennas will have such an asymmetrical aperture.

 

Is a higher antenna always better?

No, for the first house below:

 

http://www.hdtvprimer.com/ANTENNAS/skim.jpg

 

But after the signal has skimmed over several equal-height obstacles it will be necessary to go up about 5 wavelengths to find a full-strength signal, even if the transmitting tower can be seen from a lower spot.

 

For channel 2, five wavelengths would be 86 feet.  A mast that long is impractical.  But below 86 feet the signal strength is roughly proportional to the square of the height.  Thus the rule of thumb: “Higher is always better” for VHF.

 

For the following text, “skyline” will mean the highest obstruction your antenna can see.  The skyline could be the top of a house or distant hill.  The top of a tree could be the skyline for UHF, but not VHF-low (for which trees are transparent).

 

The rules for UHF are a little more complicated than for VHF.  UHF is more affected by obstructions and less affected by height.  For UHF, 5 wavelengths is only about 10 feet.  But a UHF antenna should be higher than this in these cases:

1.    If at all possible, get the antenna above any obstructions.

2.    If your skyline is less than 200 yards away then raising the antenna makes a significant difference.  You would be a candidate for a tower.  (When the skyline is farther than 200 yards, the benefit is usually too small to justify the effort a high mast requires.)

 

You will probably want to attach a VHF antenna to your chimney.  That is also likely the best place for a UHF antenna.  But if your chimney mount is still obstructed (by trees, etc.) then an unobstructed site closer to the ground will work better for UHF.  The essential goal is to find a spot where your UHF antenna can see a distant skyline in the direction of the station.

 

Note that on the front of a hill, the antenna height often makes little or no difference (VHF and UHF):

http://www.hdtvprimer.com/ANTENNAS/hillside.jpg

 

Power lines

Power lines will reflect the signal.  But that is just one reason to keep antennas away from power lines.  If there is RF noise in the power lines, the lines will transmit the noise to a close antenna.

 

Many people have been killed when their antenna fell into power lines.  Channel Master recommends that the antenna be kept away from power lines by a distance of twice the mast length plus twice the antenna length.

 

Attic antennas

If an indoor antenna is not as reliable as you want, an attic antenna is the next step up.  If you are in a neighborhood with moderately strong signals, an attic antenna might work.  But you are wasting your time installing an attic antenna in a poor-signal neighborhood.  Most successful attic antennas are within 20 miles of the transmitter. (30 miles often works if you are on a hillcrest.)  The problems with attic antennas are:

  1. The antenna might not be high enough above obstacles outside the house such as trees.
  2. It is hard to estimate the signal loss caused by the wood and other construction materials.
  3. Metal objects in the attic can block the signal.

 

Estimating the signal loss in ordinary construction materials requires knowledge of their water content.  Exceptions are aluminum siding, stucco (which has an embedded metal screen), and foil-backed insulation, all of which totally block all signals.  Concrete and most bricks have moderate water content, but their thickness is enough to block all signals.  In a desert, plywood becomes so dry that it causes no signal loss at all, even for UHF.  In any other place, there will be some moisture.  Exterior wood is generally always wet inside, especially in north facing surfaces.  (Paint does not prevent this.)  The amount of water varies with the weather.  Dry asphalt shingles are mostly transparent to TV signals, but the way they overlap encourages water to persist between them.  The vapor barrier is often wet on one side or the other.  The bottom line is that there is no way to predict the signal loss in these materials.  It is usually a mistake to point an antenna through a surface that gets totally wet in rain.

 

Metals reflect signals.  A metal object 8 inches long is big enough to reflect UHF.  Smaller objects, such as nails, are of no concern.  Wires and metal pipes effectively reflect VHF, as do plastic pipes containing water.  If these reflecting objects are positioned to the side, to the rear, above, or below the antenna, they will have little effect on it, provided they are not too close.  These objects should be further away than 2 feet for UHF, 4 feet for VHF-high, or 6 feet for VHF-low, and an even larger separation will help a little.  (Some might wonder why these numbers are not proportional to the wavelength.  It is because the lower frequency antennas are lower in gain.  An antenna’s aperture depends on the gain as well as the wavelength.)

 

There should be no horizontal or diagonal wires or pipes in front of the antenna.  A perfectly vertical metal vent pipe is invisible to TV signals, but its flashing at the roofline might not be.

 

Inner-city antennas

In good-signal areas, small low-gain antennas may work fine.  Indoor antennas nearly always work up to 10 miles from the transmitter.  They often work up to 20 miles for people who live on hillcrests, and sometimes 30 miles if the transmitting tower is visible.  But if http://www.antennaweb.org says you are not a candidate for an indoor antenna then don’t waste your time and money on this.

 

If you are in a city and the transmitters are all around you, you might get tired of getting up to re-aim the antenna when you change channels.  In that case, an omni-directional antenna, like a disk antenna, mounted in your attic might serve you well.  But an omni-directional antenna will not work for DTV in a multi-path situation.  Multi-path in cities occurs mostly when there is a big building blocking the direct path to a TV tower.  If you see ghosting on some of your analog channels then you are probably not a candidate for an omni-directional antenna.  See Omni-directional antennas.

 

Common misconceptions  (optional reading)

 

What about reflections?

Many websites say ghosts are caused by reflections off nearby objects.  But this is true maybe 1% of the time.  Most such reflections are spherical waves that die off quickly due to spreading, too quickly to be delayed enough to produce a ghost.  Only a plane surface can produce a reflection that will persist long enough, and most natural flat surfaces are at an angle that will reflect upwards away from any antenna.  Building surfaces can produce ghosts.  But like for a mirror, the reflection off a building surface is very directional.

 

What about front-to-back ratio?

Many websites say a high front/back ratio will block ghosts that arrive from the rear.  But ghosts normally come from the front and are caused by the most direct path being blocked.

 

Conceivably an obstruction could be just high enough to block the antenna but not high enough to block the signal reflecting off a building to the rear.  But the blockage will necessitate a high gain antenna, and all such antennas have a high front/back ratio.  So again the front/back ratio is something that never has to be considered.  See Front/back ratio.

 

Related topics

Fading  (Why a signal drops out for no apparent reason)

Field strength meters  (for finding the strongest spots)

Fringe area reception

Multi-path interference  (There are two kinds)

Painting an antenna  (Is this OK?)

 

 

 

 

 

This page is part of “An HDTV Primer”, which starts at    www.hdtvprimer.com