Design
and Installation of Outdoor TV Antenna
Tom Schmidt
http://www.tschmidt.com/
10/12/2008
Summary
February 17 2009 digital television (DTV) transition motivated us to review existing outdoor antenna system and undertake upgrade. This paper documents steps we took to evaluate requirements and install new outdoor TV and FM antenna system.
Over the air (OTA) reception has fallen out of favor over the years with rise of Cable and Satellite providers. About 10% of American households currently receive free OTA TV. Transition to DTV may impact OTA use. On one hand OTA quality is often better then Cable or Satellite because program is not recompressed (transcoded) to fit available channel capacity. Being free, except for cost of antenna, OTA may be preferred to paying monthly fee for cost conscious customers. On downside due to DTV transmission cliff effect coverage area for many TV stations will be somewhat smaller then analog.
Digital TV uses same channel assignments as analog TV. Due to nature of digital transmission picture is either perfect, or non-existent. Whereas with analog transmission picture quality slowly degrades becoming snowier and snowier until it becomes unwatchable. In many cases antenna used for existing analog TV will work fine for digital. In others optimum viewing experience requires better antenna.
We live in southern NH. This gives us access to both NH and Boston channels. Being 42 miles from Boston in hilly and wooded New England require a good outdoor antenna. Current system is 20 years old. Though it was a good time to see if anything could be done to improve reception.
There is no such thing as a digital TV antenna. Antenna does not care if it is receiving an analog or digital channel. That being said digital reception is more demanding then analog. There is widespread concern coverage area of digital TV will be less then analog. Analog reception slowly degrades as noise increases showing up as snow. Digital remains perfect until signal falls below a critical value. When that occurs receiver is no longer able to recover video and audio. This phenomenon is called digital cliff effect. FCC and stations use Longley-Rice propagation modeling to estimate coverage area. Transmit power and antenna design are engineered so digital coverage is same as analog. However cliff effect makes determination of identical coverage area problematic.
Interested readers are encouraged to visit Ken Nist’s HDTV primer site, particularly the antenna section. Much of the information in this paper was derived from Ken’s site.
Historically TV has been transmitted as an analog signal. Color and stereo audio enhancements were done in the analog domain. For last several years US TV stations have been authorized by FCC to simulcast programs in both digital and analog format. Digital transmission offers a number of advantages but with any change of this magnitude there are bound to be teething problems along the way.
Digital TV (DTV) refers to way program is transmitted. Just because a program is digital does not mean it is HDTV. Many station are using DTV to transmit one or more SDTV programs. Digital transmission allows station option to transmit higher resolution programs called High Definition TV (HDTV). HDTV picture quality is much better then Standard Definition TV (SDTV). HDTV is more like watching a movie then TV. Digital is more efficient then analog making it possible for station to deliver not only better image quality but also multiple programs, called sub channels, within existing 6 MHz RF TV channel.
From an over-the-air (OTA) viewer’s perspective most important DTV change is cliff effect. Where analog gets progressively worse as signal decreases digital programs are displayed perfectly until signal level drops below a critical threshold. When that happens picture/audio either breaks up or disappears completely.
On Feb 17 2009 full power US TV stations stop broadcasting in analog. In some cases they move to a different channel and/or change transmit power. This makes antenna design challenging as need to take into account current and post-transition channel allocation.
First step is to find local stations and determine how situation changes Feb 17 2009. The most popular RF modeling site is AntennaWeb run by the Consumer Electronics Association. Another is TV Fool run by Andy Lee. Antenna Web is very conservative so it misses some stations receivable with good antenna. TV Fool is more technical and reports distant stations and estimated signal strength.
Pre Transition DTV Channel
Assignment
|
||||
|
RF Chan |
Virtual Chan |
Call Sign |
TVFool Rx(dBm) |
Old Antenna |
|
2-13 |
VHF |
|||
|
14-17 |
|
|
|
|
|
18 |
62 |
WMFP |
-84.4 |
Mid |
|
19 |
2 |
WGBH |
-80.9 |
Mid |
|
20 |
5 |
WCVB |
-81.1 |
Mid |
|
21 |
|
|
|
|
|
22 |
|
|
|
|
|
23 |
66 |
WUTF |
-87.4 |
No |
|
24 |
41 |
WVTA |
-102.5 |
No |
|
25 |
31 |
WNNE |
-107.2 |
No |
|
26-28 |
|
|
|
|
|
29
|
27 |
WUNI |
-88.4 |
No |
|
30 |
4 |
WBZ |
-82.1 |
Mid |
|
31 |
25 |
WFXT |
-98.6 |
No |
|
32 |
68 |
WBPX |
-99.0 |
No |
|
33 |
21 |
WPXG |
-63.4 |
Mid |
|
34 |
60 |
WNEU |
-55.3 |
Mid |
|
35 |
50 |
WZMY |
-80.5 |
No |
|
36 |
|
|
|
|
|
37 |
Reserved non-TV |
|||
|
38 |
|
|
|
|
|
39 |
38 |
WSBK |
-91.8 |
No |
|
40 |
|
|
|
|
|
41 |
56 |
WLVI |
-90.7 |
No |
|
42 |
7 |
WHDH |
-88.8 |
Mid |
|
43 |
44 |
WGBX |
-86.9 |
No |
|
44-46 |
|
|
|
|
|
47 |
48 |
WYDN |
-100.6 |
No |
|
48-56 |
|
|
|
|
|
57 |
11 |
WENH |
-70.9 |
Mid |
|
58 |
|
|
|
|
|
59 |
9 |
WMUR |
-29.0 |
Strong |
|
60-69 |
|
|
|
|
|
Converter Box: Insignia NS-DXA1-APT (Yellow) Stations of interest (Green)
Viewable stations |
||||
Prior to Feb 17 2009 stations are simualcasting in analog and digital format. FCC authorized second RF channel for DTV. After transition some stations will move to different channel assignment. This is important for planning purposes. In our area three stations, two in NH and one in Boston move from UHF to VHF. VHF frequencies are split into two blocks channels 2-6 and 7-13. VHF low is not ideal for DTV due to man-made noise. In most markets there will be no digital VHF low stations. This provides an opportunity to use a VHF high antenna. VHF high antenna is optimized for channels 7-13. For a given gain eliminating VHF lo results in a smaller antenna. Even though not optimized for FM (located between channels 6-7) and VHF low if signal is strong enough VHF high antenna may still provide adequate signal. This is convenient if need to receive some analog VHF lo stations prior to transition.
The other important consideration is station azimuth. Fringe viewers like us typically have access to multiple stations serving different markets. Because we are distant need a directional antenna to pick up station with adequate signal level. Down side of directional antenna is pointing accuracy. If antenna is not pointed directly at transmitter signal level is significantly reduced.
Antenna rotator changes antenna direction as needed to pick up wildly dispersed stations. Rotors are the bane of multi TV households and digital video recorders. Changing antenna direction determines which stations are viewable. Rotors cause problems for DVR because DVR is not able to change antenna direction.
In designing antenna system need to carefully consider which stations are of interest to minimize need to constantly change rotor position. Another option is to use multiple antennas permanently orientated and combine them. In our case decided to offset VHF high antenna by –105 degrees relative to UHF antenna. When UHF antenna is pointing at Boston VHF antenna is aligned with Channel 11 WENH. NH. It is slightly misaligned for Channel 9 WMUR NH but transmitter is so close antenna still delivers adequate signal.
With this arrangement able to receive all stations of interest except Channel 7 WHDH in Boston and Channel 50 WZMY in Derry NH. That means unless someone wants to watch either of these channels there is no need to move antenna.
DTV Channel Assignment as of
Feb 17 2009
|
||||||
|
RF Chan |
Virt Chan |
Callsign |
Network |
TVFool Rx(dBm) |
Dist |
Az True |
|
2-6 |
|
|
|
|
|
|
|
7 |
7 |
WHDH |
NBC (MA) |
-83.3 |
41.6 |
147 |
|
8 |
|
|
|
|
|
|
|
9 |
9 |
WMUR |
ABC (NH) |
-36.1 |
12.5 |
19 |
|
10 |
|
|
|
|
|
|
|
11 |
11 |
WENH |
PBS (NH) |
-68.5 |
34.3 |
43 |
|
12 |
|
|
|
|
|
|
|
13 |
13 |
WYCN |
LP
– Analog Religious |
-87.7 |
12.2 |
108 |
|
Ý VHF (offset -105 degrees) UHF ß |
||||||
|
14-17 |
|
|
|
|
|
|
|
18 |
62 |
WMFP |
Ethnic
(MA) |
-84.7 |
41.6 |
136 |
|
19 |
2 |
WGBH |
PBS (MA) |
-80.7 |
41.1 |
148 |
|
20 |
5 |
WCVB |
ABC (MA) |
-80.9 |
41.1 |
148 |
|
21-23 |
|
|
|
|
|
|