Understanding Antenna Specifications and HDTV

Television antennas are designed for specific applications.  The most familiar antenna, the VHF/UHF/FM antenna is designed to receive television channels 2 through 69.  Manufacturers make many models of this type of antenna to address signal reception requirements for a variety of locations.  Other antenna designs, though less numerous, include a VHF antenna that receives channels 2 though 13 and a UHF antenna for reception of channels 14 through 69.

Manufacturers of high quality television antennas describe the features of their designs though specifications.  Antenna specifications provide valuable information about performance that can be expected from a particular antenna model.  Antenna specifications also help the technician select an antenna model to resolve a specific reception problem. 

When you view an antenna specification sheet or download it as a file, you immediately see a lot of information.  If you are not familiar with the terminology, the following will help explain the details:

Gain
Antenna gain is the value by which a signal is increased as referenced to a dipole antenna.  Antenna gain is not an absolute value but rather a relative comparison.  The concept of an antenna dipole as a reference is rather abstract.  Suffice to say that antenna gain is a good quality and an important part of antenna design.  Higher gain is better and it is related to the general size of the antenna.  Generally, large antennas have higher gain than smaller antennas.

Antenna Beamwidth
Antenna beamwidth is a measure of how wide an angle the antenna will accept signals when the front of the antenna is orientated in a desired direction.  It is desirable for television antennas to be directive so the narrower the beamwidth, the more desirable.  Signal reflections that appear as “ghosts” for example, when viewing an analog signal may be the result of an antenna that has a very wide beamwidth or an antenna that “sees” unwanted signals from the sides.  Generally, larger antennas have narrower beamwidths and smaller antennas have wider beamwidths.

Front-to-Back Ratio
This is an important specification as it describes how much a signal received at the rear is reduced as compared to a signal received at the front of the antenna.  As noted, we desire a very directive antenna for television reception or as directive an antenna as we can select for our application.  If you are in the city, strong signal reflections from nearby buildings can arrive at the back of the antenna and compete with signals arriving at the front of the antenna.  The combined signal energy would confuse the HDTV tuner and thereby impede reception.  If you are in the country, the signal of your desired channel is weak and it may be impaired by a signal arriving at the rear of the antenna from an adjacent city.  A higher front-to-back reduces the effect of the undesired signal. 

OK, so how do you use antenna specifications to select the right antenna for your location?  Well, consider what the Federal Communications Commission’s (FCC) engineers used to allocate the HDTV channels throughout the United States.  The FCC engineers employed a planning model that assumed a minimal antenna design that would be used by the viewing public for the reception of HDTV. 

Gain +3 VHF (Ch 2 – Ch 13)
Front-to-Back Ratio:  10 dB VHF (Ch 2 – Ch 13)
Gain: +9 dBUHF (Ch 14 – CH 69)
Front-to-Back Ratio: 12 dB UHF (Ch 14 – Ch 69).

Given the FCC planning information, you can see that specifications of moderate to large size antennas meet or exceed the planning model requirements.  Whether you select an all-band (VHF/UHF/FM) antenna or separate VHF and UHF antennas, comparing antenna specifications with the FCC planning model assures that a selected antenna will provide reliable HDTV reception.

Are the antenna specifications a little too much engineering information for your comfort?  No problem, use any one of the on-line antenna selection web sites and get all the antenna specifications and engineering considerations done for you.  The on-line antenna selection programs use the same criteria and compare the specifications of many antennas before providing you with a selection.  In most cases, you simply provide your location and the program returns with the antenna manufacturer and model number. 

If your business includes antenna installations for HDTV, you most certainly want to know how antenna specifications affect the selection of antenna for your customer.  Web site antenna selection models are effective but nothing is more informative than one’s experience with antenna installations in a specific area – especially antennas installed for HDTV reception.  Record antenna models and specifications that are successful in your installations. 

Perhaps at this point you are asking why not simply select the largest antenna and use it in every installation?  This would be OK except that in most situations, you are selecting an antenna that is acceptable to your customer with some trade-off in size and performance.  Therefore it is important to be able to quantify the difference of gain, beamwidth, and front-to-back ratio between antenna models.  Also, the largest antennas require substantial support hardware – a heavy duty tripod or antenna tower for mechanical stability.  Larger antennas require higher elevation to realize the performance stated in the specifications.  Without the elevated position, large antennas perform only somewhat better than smaller antenna models.  Often, a smaller antenna will perform better if simply elevated above the roofline.  A general rule of thumb is that the antenna should be positioned at least ½ wavelength above the roofline for optimal performance.  If the HDTV channels include a station on Ch 2 or Ch 3, this relates to about 10 ft. above the roofline for optimal performance.  Of course, this may not be an option when considering the impact upon the appearance of the home. 

Research the HDTV channel assignments for your area.  You may discover that a UHF antenna is all that is required to receive local HDTV broadcasts. Separate UHF antennas provide equivalent or greater performance as compared to the VHF/UHF antennas.  On the other hand, if you have one or more HDTV channels assigned to the VHF band (Ch 2 through Ch 13), an all-in-one antenna (VHF/UHF/FM) offers the most compact solution with high performance. 

Commercial systems contractors should not overlook the use of separate VHF and UHF antennas for a multiple dwelling unit (MDU) HDTV antenna system.  The separate VHF and UHF antennas provide higher performance and generally in MDU situations there is ample space for the installation of separate antennas. 

Investigate the performance of your next television antenna purchase by reviewing its specifications.  Make comparisons with the next larger and next smaller antenna models.  If you have questions, consult the antenna manufacturer’s customer support or contact your electronics dealer.

Credits:
Ruler:  Fermilab, US Department of Energy, Batavia, Illinois
Map: American Map, Langenscheidt Publishing Group, American Map Corporation, Maspeth, New York 11378

By:
Steve Zahn, Doc. #120405
© 2005 / Used By Permission
szahnengineering@msn.com