A Discussion On HDTV OTA Reception

Introduction
There have been many questions about the Over The Air (OTA) reception and with the TV industry moving to DTV there will be more and more. There is no better method of getting HDTV material to your HDTV than the signal from an OTA receiver. Edit: Since I wrote this they have released the HD DVD and Blu-ray players and they probably should have the honor of delivering the best HD to your set, but for broadcast TV, OTA is still the best method.

When it comes to pulling in OTA signals it can pretty much be a hit and miss situation. In reverse order of importance:

The worse thing would be to have no antenna at all.
The next best thing would be to have an indoor antenna.
The best thing would be to have an outside antenna.

There is no getting around it, size and placement matters when it comes to an OTA antenna. The difference between an indoor antenna and an out door antenna is not double or triple, it is multifold. Unless there is a situation that prevent you from installing an outdoor antenna, that is what you should do.

Purpose
The purpose of this essay is to provide information that will allow one to be more informed when setting up an OTA antenna system rather than to recommend particular equipment.

Before we get into the subject from a technical standpoint of receiving a signal, there is a matter that needs to be discussed.

HOA Regulations
Before we get into the subject from a technical standpoint of receiving a signal, there is a matter that needs to be discussed.

Many people think that their HOA covenants and restrictions prohibit the installation of outdoor antennas. And many HOA documents that were created before 1996 do have such prohibition provisions in them, but the Telecommunications Act of 1996 has made these provisions invalid. For more information see: Complete F C C Fact Sheet…

Being on a HOA board myself, I know from first hand experience that many people just read their deed restrictions (or covenants and restrictions) and think everything in them is enforceable. Well they are not in the case of satellite dishes for TV or internet reception or antennas for OTA TV reception. The Telecommunications Act of 1996 made antenna provision in HOA documents obsolete.

There are circumstances that will not allow for an outdoors antenna, such as condos and apartments where the outside exclusive use areas do not face the proper direction. Then there are other people who use the HOA as an excuse to try other indoor antennas, because they don’t want an outdoor antenna on their roof or are nervous about having to deal with their HOA. Some use the HOA excuse as a ploy to try to get someone to come up with some magical antenna. That is going to be fruitless. I would just say that none of that would change the type of antenna you will need. If one is stuck on having only indoor antennas, unless you are very near the transmitter, you will have to go through a trial and error method to find an antenna that will work and it is possible that a single indoor antenna will not work for all stations and you may not be able to receive all stations.

Physics
Receiving a RF signal is basically a matter of physics. RF signals have a wavelength that is a factor of the frequency the signal is broadcast on and the optimum length of an antenna element is one that is equal to this wavelength or a multiple of this wavelength. Since channel 2 starts at 54 Mhz with a wavelength of 6 meters, the next best element length of ½ wavelength is normally considered the optimum for antenna design. The ½ wavelength element is called a dipole antenna and as you can see it will still represent a pretty large antenna. For channel 2 the dipole length is 3 meters or 118.11 inches. Any other element size will reduce the signal level received. All antenna designs are a series of compromises to be able to cover the entire TV broadcast range and the Channelmaster Crossfire a antennas all are 110” wide with differing number of elements. The more elements the better chance there is to match more channels ½ wavelength, thus the stronger overall. Other techniques include making the elements form a V shape to reduce the overall width and resulting turning radius.

There is a direct correlation to antenna size and the signal received. Double the antenna size and double the signal. There are some techniques can slightly extend antenna area beyond its physical size and poor designed large antennas can effectively reduce its size, the fact remains that there is no substitute for size and a large antenna is going to give you a better signal than a small antenna.

Miles Rating
First of all I’m one of those people that think the miles rating given by the manufacturers is only useful for relative comparison between antennas offered by a given manufacturer. There is not any standard as to how these figures are derived and each manufacturer has their own method of determining their ratings. Bottom line trying to determine which antenna would be better between two different manufacturers is not possible, in my estimation.

Secondary Signals
Secondary signals, also called multipath, are those that have been reflected off of a large object, such as a large building, and arrive a short time later than the primary signal. If these reflections are strong enough, they can totally confuse the DTV receiver and you will get no signal as far as the receiver is concerned. Directional antennas have a front and a rear. The front is pointed toward the transmitter. Directional antennas have elements at the rear of the antenna to shield the active elements from these signals that come in from the rear of the antenna.

Outdoor Antennas
These are the larger antennas usually consisting of a boom with perpendicular elements of varying lengths to cover the frequency range required. For VHF this is from channel 2 at 54 Mhz to channel 13 at 210 Mhz with FM radio between channel 6 and channel 7. FM covers 88 to 108 Mhz. UHF channels start with channel 14 at 470 Mhz with each channel being an adder of 6 Mhz up to channel 69 at 794 Mhz.

Smaller antennas are available like the Channel Master Stealthtenna for urban areas that work very well, but if you are very far from the transmitters these loose their effectiveness very quickly. Since most DTV is broadcast in the UHF or upper VHF bands the Channel Master 4228 is my antenna of choice.

Fringe Area Reception
There is not going to be any substitute for a large outdoor antenna if you live 50 miles plus from the transmitters. Next to the size of the antenna, the height is as important. The higher the better as there is more signal strength the higher you go. The reason is pretty simple. TV signals for the most part are line of sight signals and anything that is between the transmitter and your antenna will reduce the signal. This can be trees, buildings and even the ground itself. The higher the antenna is the more chance it has not had to go through some of the obstructions.

Amplifiers
There are two basic types of amplifiers, indoor and outdoor. Beyond that there are four specifications that are important. Gain in db, noise, output power and distortion. Gain is the amount of signal increase where 10 db is x10, 20 db is x100, 30db is x1000 and so forth. The noise figure should be a low number, the lower the better. The output power will be rated in dbmv and distortion is how faithful the output signal is to the input signal. Too high of an input signal to an amp can cause distortion and that is why if you are very close to the transmitters an amp is probably not called for. Some of the more expensive amps will have both gain control and a tilt compensator that allows the boost of the higher frequencies only to compensate for the fact that higher frequencies will suffer more loss in the coax than the lower frequencies.

Indoor amplifiers are used to boost the signal due to the use of splitters needed for inside the building. Typically these will be lower gain something on the order of 6 db or so. There are distribution amplifiers that have multiple outputs, building in the distribution splitters.

Outdoor amplifiers are meant to be located as near the antenna as is possible, sometimes built right into the antenna. These units will be furnished with a power injector that is mounted inside the building to feed the power for the amplifier into the downlead to be fed up to the antenna. Typical gain for this type of antenna is 15 db and up. Too much gain can be as bad as no gain if it causes the connected device to overload.

Indoor Antennas
If an indoor antenna is a must, the best bet is to try different antennas until you find the best possible. Since you are dealing with a smaller antenna that is not big enough to cover the ½ wavelength of the HDTV signals, none are going to be optimum and they will be a compromise. Get the antenna from a store that will allow you to return it. I would also get a long RG6 cable so I could move the antenna about in the house.

Try different locations in the attic if you have an attic. If you must put antenna in the attic, you should bear in mind that objects near (within ½ wavelength) the antenna effectively become part of the antenna and will reduce its effectiveness. Many times when the signal has to penetrate radiant barriers, it will be close to non-existent.

If you don’t have an attic, try different places in the house or apartment. Also try different windows so the signal doesn’t have to go through foil on insulation or brick. Once you find the optimum location for the indoor antenna, then address how to get the wire to it. The chances that the antenna placement on top of your TV will be the best place is not likely. Partly due to the interference that will come from your TV and digital receivers themselves as well the probability that the signal has to pass through more walls. Computers and other digital processors will interfere with HDTV reception in many cases.

Bottom Line
If you seriously want good OTA reception there is no substitute for a big well made antenna mounted as high as is possible. Normally a single antenna will do the trick with a motorized rotator if stations are located in different directions. The alternative could be poor reception or cable TV service.

Copyright 2005 - Richard Binckley

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