8/14/08

It doesn’t work! Now what?

Your antenna doesn’t meet your expectations. What is going wrong? The place to start is with the symptoms.

Symptom: No evidence of station. (No digital lock).

Assuming the station is on the air, the likely causes are:

· Weak signal (You might need a better antenna system.)

· Interference (See Interference below.)

· Overload (See Overload below.)

· Antenna system malfunction

The following experiment will verify that the mast-mounted amplifier is working: Tune the TV to an analog station, then observe the image before and after unplugging the amplifier’s AC power cord. If there was no change then either the amplifier is dead or there is a bad connection between the amplifier and the power injector. The bad connection could be a short between the coax center conductor and the shield, or it could be an open in either conductor. If AC power makes the image worse then you have overload.

Symptom: Persistent dropouts.

On DTV channels you will never see snow, ghosts, or interference, but you will see dropouts. When the signal is corrupted or becomes too weak, you will see “block errors” (parts of the screen that are shifted or obviously wrong), sound dropouts lasting a few seconds, or image freezes lasting a few seconds. All of these errors are crude, unsubtle errors. If these are not present, your image is perfect.

The causes of dropouts are:

· Weak signal (You might need a better antenna system.)

· Interference (See Interference below.)

· Fading (See Fading below.)

Symptom: Day-to-day variation or time-of-day variation.

If you are more than 40 miles from a UHF station the weather will significantly affect the signal strength. At only 20 miles a hard rain can diminish a UHF signal. Your antenna must have enough gain to make up these losses.

But if the change in signal strength is sudden and occurs at about the same time each day then you have interference from another station. (See Interference below.)

I followed the directions carefully. So why is my antenna too small?

The one problem that usually cannot be predicted is skyline multi-path. This will cause a 3-dimensional pattern of weak and strong spots. See “Short delay multi-path” below.

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Fading

Depending on the terrain at your location, there are often two paths for the signal to follow to your antenna.

When the sun warms up the land, a warm air layer near the ground can add a third path. The warm air causes a bending called refraction, which is identical to the “mirage effect”.

Which of these paths will be the strongest is hard to predict. The ground-reflecting path is usually weak if the reflection point is forested. The bent path can be enhanced by focusing, making it stronger than the direct path.

These paths will add together at your antenna, and considering phase, subtraction is a possibility. Whether subtraction occurs depends on the length of the bent path. Since the warm air layer is always either growing (sun up) or shrinking (sun down) this path length is always changing. For UHF the path length need change by only ten inches to turn addition into subtraction. If both path signals are about the same strength, your DTV channel will dropout momentarily. If you see two dropouts that were N minutes apart then you will probably continue to see dropouts every N minutes. This is fading. There is no cure for it, but a stronger antenna will make it less likely.

The above is but one scenario for fading. There are many variations on this depending on the terrain.

Overload

If you add a good amplifier to your antenna system and your results get worse instead of better then you have overload, and you need to reconsider more carefully what you are doing.

Signal amplifiers are supposed to be linear. That is, the output is a magnified but otherwise unaltered version of the input. But too much signal can make an amplifier non-linear, usually clipping off the tops and bottoms of the sine waves. When this happens, the signals of all channels are distorted, not just the one that is too strong. In fact, the too strong signal is usually not a TV station. A close FM station or police station is more likely the cause.

An attenuator is a resistor network that can be used to reduce the gain of an amplifier. An attenuator is often the simplest solution to overload. 3 dB and 6 dB attenuators are commonly available.

If you are close to an FM station, there might be a narrow range between too much and too little gain. You can make that range larger by using an amplifier with an FM trap or by using a more directional antenna. See “Nulls in radiation pattern”.

VHF preamplifiers usually include FM traps that can optionally be disabled. Freestanding FM traps are also available. FM traps can either cover the entire FM band or can be single frequency traps that you tune to the offending station. The latter are more effective. If the FM station is close enough you might need more than one.

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Interference

Common sources of interference include:

· Adjacent channel interference (a very strong station one channel up or down).

· Co-channel interference (two weak stations on the same channel).

· Multi-path interference (usually caused by the direct path being blocked).

· A very close transmitter (a neighborhood FM station, police station, taxi company, etc.).

· An industrial noise source (a factory, a clinic, a malfunctioning power transformer).

· Household appliances, including motors, light dimmers, and fluorescent lights.

You might need an antenna that can reject the interfering signal. See “Nulls in radiation pattern” below.

Adjacent channel interference

When the station you want is not receivable because of a much more powerful station in the next channel above or below, you have adjacent channel interference.

Before 1998, the FCC would never allow stations in the same city to occupy adjacent channels. (There were exceptions for channels 4-5, 6-7, and 13-14 because gaps exist between those channel-pairs.) But due to improved receiver technology, the FCC now allows any channel assignment.

When a signal is 10 to 15 times more powerful than that of an adjacent channel station, most receivers become unable to receive the weaker station. To receive a far away station, you might need to use a directional antenna to reduce the strength of a nearby adjacent channel station. See “Nulls in radiation pattern”. But if both stations are in the exact same direction you might be out of luck. There are some frequency selective filters that can “trap” out a channel, but they are seldom able to reduce an adjacent channel by more than half. (e.g.: Winegard UT-2700 Dual Trap)

When adjacent channel stations broadcast from the same tower or adjacent towers they must have an agreement that neither will exceed 10 times the other’s power.

Co-channel interference

When the station you want is not receivable because of another station on the same channel, you have co-channel interference. The interfering station can be very far away and very weak, yet it can contribute enough “noise” to make the close station hard to receive. The remedy is a new antenna that is both stronger in the forward direction and weaker in the interfering direction. See “Nulls in radiation pattern”.

Multi-path interference

This is a problem that, if severe, can prevent DTV reception even if the signal is strong. The signal is reaching the antenna by more than one path due to diffraction around hills and trees and sometimes reflections off of structures.

There are two distinct categories of multi-path interference. The first is “short delay” multi-path, delays of less than about 20 nanoseconds. On analog channels there will be no visible ghosts.

Short delay multi-path - This is always caused by something directly in front of the antenna. One common cause is a tree in front of the antenna. There will be chaotically overlapped signals behind a tree. This will mainly affect UHF reception. The solution is to relocate the antenna (or cut down the tree). If the antenna stays behind the tree, you will likely see dropouts on UHF channels when the wind blows. And that’s for strong-signal areas. In weak-signal areas you will likely get no UHF reception at all behind a tree.

The other common cause is an irregular skyline (structures and trees in the distance). These will cause overlapping fields, which will result in a regular pattern of strong and weak spots. For UHF, moving the antenna right or left three feet or so can make a huge difference. Moving the antenna is usually the solution. Unfortunately a strong spot for one channel can be a weak spot for a different channel. The same phenomenon happens for VHF, but since wavelengths are ten times as big, the strong and weak spots are ten time further apart, so moving the antenna to a strong spot is often too far to be practical. For VHF the solution is usually a bigger antenna. (The author faces a severe case of skyline multi-path. His UHF strong spots are always about 12 feet apart, but they can move with the weather. His solution: He erected two UHF antennas 6 feet apart, selected by a switch. Whenever one antenna is in a weak spot, the other is guaranteed to be in a strong spot, so at least one antenna always works.)

Long delay multi-path - This is caused by a large obstruction like a hill or a large building. If you tune in an analog channel close in frequency and from the same direction, you will see ghost images. The offending signals are approaching the antenna mostly from the front and sides, but also in rare cases from the rear. Actually all analog images have these ghosts, but without the direct path blocked they are too dim to see.

One solution is to move the antenna to a new spot where signals from the offending directions are less strong. A move of 20 feet or more will likely be necessary.

The other solution is to select an antenna with little or no reception in the offending directions. There are two workable strategies here. If the analog channels show one really strong ghost (and maybe a number of weaker ones) then selecting an antenna with a null will work. See “Nulls in radiation pattern”. Otherwise a highly directional antenna is the way to go. The higher the antenna’s gain, the less reception it will have to the side. (More advice on multi-path can be found at http://www.hdtvprimer.com/ANTENNAS/silver.html . Although that article is written for the Silver Sensor antenna, the same principles apply to larger antennas.)

Appliance noise

Household motors and fluorescent lights often produce noise of the “120 sparks per second” variety. If you tune to an analog station (especially channels 2-6) you may see intense sparkles that are somewhat confined to a broad horizontal band. If so, you must find the appliance and fix it or replace it. Identifying the appliance is sometimes difficult. You might have to shut off the house breakers one at a time, watching to see when the sparkles go away. If every breaker but the TV is off and every appliance on that breaker but the TV is off and the sparkles remain then the noise source is either in a neighbor’s home or is a bad transformer on a utility company pole. (If you can walk around with a portable AM radio tuned to an unexplained buzzy hum, you might be able to further isolate the offending device.) If the source is in your neighbor’s home, brush up on your diplomatic skills. If it is the utility’s transformer, call them. They are obligated to fix it.

If the noisy appliance is in your house, you might need one or more of these fixes:

· Try fixing or replacing the device.

· Try replacing the device with a device containing some RF filtering.

· Try putting an RF noise filter on the power cord of the bad appliance.

· Try putting an RF noise filter on the power cord of your TV.

· If the TV and the appliance are on the same house circuit breaker, move one to a different breaker.

Nulls in radiation pattern

Nulls in the radiation pattern can be useful. If you rotate the antenna so that a null points toward an interfering signal, that signal is eliminated. Some interference situations that might benefit from this trick include:

· Adjacent channel interference (a very strong station one channel up or down)

· Co-channel interference (two weak stations on the same channel)

· Multi-path interference (usually caused by the direct path being blocked)

· A very close transmitter (a neighborhood FM station, police station, taxi company, etc.)

· An industrial noise source (a factory, a clinic, a malfunctioning power transformer)

For example, the Channel Master 4228 has nulls on both sides at 30 and 90 degrees:

Yagi/Corner-reflector antennas have no nulls. LPDA antennas have nulls at 90 degrees, but LPVA antennas have no nulls. To make rabbit ears have nulls (at 90 degrees) lower them into a straight dipole.

In a multi-path situation, a null will work if there is only one strong ghost. (Find an analog channel close in frequency and from the same direction. Examine it for ghosts.) If there are multiple very strong ghosts then a better approach is a very high-gain (very directional) antenna with little reception to the side or rear.

See also The Two-Antenna Trick (indoor version) and The Two-Antenna Trick (outdoor version) . These explain how to create a null in a controlled direction.

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