ATTIC ANTENNAS VS. ROOFTOP ANTENNAS: A PERFORMANCE COMPARISON
CHANNEL MASTER TECH BLOG #2 - NOVEMBER 2020
Yes, you can install a TV antenna in your attic. But it won’t work as well as one on your roof.
Back in the day, it wasn’t unusual to see TV antenna mounted in attics. In particular, large houses like Victorians and Tudors had walk-up attics with lots of room and high ceilings. These houses were also characterized by very steep roofs, some of which were made of slate - tricky to walk on and very slippery in wet weather.
For those homeowners, installing a TV antenna in the attic was the simpler and safer path to take. And if those houses were located in urban areas, TV signals were usually strong enough to make it work. However, ghost images created by signal multipath and reflections from nearby buildings were often seen on-screen.
Older houses rarely had insulating material in attics, especially in outside walls. And while slate roofs (copper flashing) and tin roofs played havoc with TV signals, conventional wooden roofs passed those signals reasonably well. We just learned to live with interference, ghosts, and all those other drawbacks to analog TV.
Today, “ghosts” are a thing of the past, thanks to digital TV. The signal is either perfect, or it isn’t there at all. The adaptive equalizers in ATSC receivers do a very good job of correcting for the echoes caused by multipath as long as the carrier-to-noise (C/N) ratio is high enough. In a perfect environment, that level is just 15.3 dB above the noise floor; a level at which analog reception would be impossible. In a real-world (Rayleigh) environment, the minimum C/N ratio has to be higher; more like 25 dB C/N.
Okay - what does any of this have to do with attic antenna installations? Plenty. For reliable TV reception, signals need to hit the C/N target we just saw, and multipath echoes should be minimized as much as possible. All things being equal, TV signal levels from an attic antenna will be lower than those from an equivalent outdoor antenna, and are more likely to experience multipath.
Today’s homes have foil-faced insulation in walls (even attic walls) and ceilings to conserve energy, better regulate hot and cool air, and prevent moisture from getting in. That foil will cause signal reflections and echoes with attic antennas and indoor TV antennas, which will also be affected by metal air conditioning and heating ducts.
To more clearly see the difference between attic and rooftop TV antenna reception, let’s fire up our spectrum analyzer. This instrument will let us see the actual waveforms on each active TV channel we receive, and show the strength of each station relative to the noise floor along with any distortion of the signals from multipath.
Lowband VHF TV channels 2 and 6 as received on an indoor Generic panel antenna.
Channels 2 and 6 as received on a Digital Advantage 45 mounted in an attic.
Figure 1 shows VHF TV channels 2 and 6 received with a generic amplified, flexible indoor panel antenna. This antenna is mounted high on a sheetrock wall in a 2nd-floor bedroom, on the side of the house closest to the TV tower sites. The walls are lined with foil-faced (vapor barrier) insulation, so there is mild signal distortion. However, both channels are plenty strong enough for reliable reception, with at least 35 dB C/N strength.
Now, Figure 2 shows the same channels as received on a Channel Master Digital Advantage 45 VHF/UHF directional yagi antenna, mounted in the attic on a mast attached to joists. It sits about four feet below the roofline and about five feet above the panel antenna, and is connected to a low-noise, Titan-series VHF/UHF preamp with the output signal split two ways.
While channel 6 now appears to be stronger by about 10 dB with this antenna, the signal level on channel 2 is about the same except there is now a pronounced notch in the signal, likely caused by multipath from nearby insulation and HVAC ducting. (An increase of 3 dB represents a doubling of signal levels, while a 10 dB increase means the signal is ten times as strong.) Again; both signals are sufficiently high enough that the TV receiver equalizers can easily handle the distortion. Neither the indoor antenna or the attic antenna has a substantial advantage with these channels.
Highband VHF TV channels 9 and 13 as received on an indoor Generic panel antenna.
Channels 9 and 13 as received on a Digital Advantage 45 attic antenna.
Let’s move up to highband VHF. Figure 3 shows VHF channels 9 and 13 as received on the panel antenna. Geographically, the channel 9 transmitter is located to the side of the antenna, so we see substantial multipath distortion on its signal. Meanwhile, channel 13 is in a more favorable location, orthogonal to the front of the antenna, so its signal is cleaner.
However, neither channel is as strong as they appear using the amplified Digital Advantage 45 in the attic, as Figure 4 shows. While channel 9 is also off to the side of this antenna, its signal has fewer echoes, and both channels have high C/N ratios. In this case, the attic antenna is out-performing the panel antenna.
Channels 17 - 33 as received on a Digital Advantage 45 attic antenna.
Now, let’s check out UHF reception on both antennas. Figure 5 shows the UHF TV spectrum from channel 17 to channel 33, as received on the panel antenna. Channel 17 is moderately strong with multipath; channel 28 is stronger and cleaner, channels 30 and 31 strong but also exhibiting quite a bit of notching, and channel 33 somewhat weaker with strong multipath.
In contrast, Figure 6 shows channels 17 and 33 with lower signal levels and severe multipath as received on the attic antenna, while channels 30 and 31 are also distorted by echoes. But that doesn’t make sense: The panel antenna is an omnidirectional antenna, while the Digital Advantage 45 is definitely directional and should have much higher gain. Yet, something is creating multipath echoes and reducing UHF signal levels with the attic antenna. Here, the panel antenna is outperforming the attic antenna.
Now, let’s move up to the peak of the roof and look at some signal plots from another Channel Master Digital Advantage 45, this time mounted at the base of a mast, next to a chimney at the opposite end of the house. It sits about four feet higher than the attic antenna and uses the exact same Titan-series low-noise preamplifier, making for a true A/B comparison.
Channels 2 and 6 as received on a rooftop Digital Advantage 45 antenna.
Starting again with lowband VHF TV stations, Figure 7 shows VHF channels 2 and 6 as received by the roof antenna. Both channels are considerably stronger and cleaner (so much so that a 3 dB signal attenuator was installed ahead of the analyzer for this test) and we’ve also picked up a station on channel 4, likely a low-power station. Channel 2 is now 12 dB stronger while channel 6 has also picked up a few dB, and both signals are showing minimal multipath distortion.
Channels 9 and 13 as received on a rooftop Digital Advantage 45 antenna.
Channels 17 - 33 as received on a rooftop Digital Advantage 45 antenna.
Moving up to highband VHF, channel 9’s signal is also improved with the roof antenna by 5 dB as seen in Figure 8. Oddly enough, the signal level from channel 13 is lower on this antenna, but still plenty strong enough and very clean. A big improvement is also seen across the UHF band using the roof antenna, as shown in Figure 9. Not only have all UHF channels picked up 3 to 4 dB in signal strength, they’re also cleaner with reduced multipath distortion.
In conclusion, this A/B comparison clearly shows that a roof-mounted TV antenna will outperform the same model mounted in an attic, as well as an indoor panel antenna. On some channels, the improvement can be substantial. And there’s another advantage to using a roof antenna - impulse noise and other types of interference from computers, personal electronics, appliances, motors, heat pumps (and even attic fans) is greatly reduced, further improving TV reception.
NOTE: It should be noted that in townhouse and HOA developments, there are frequently restrictions on exterior antennas. However, the FCC Over the Air Reception Device (OTARD) rule (47 C.F.R. Section 1.4000) in Section 207 of the Telecommunications Act of 1996 prohibits restrictions that impair the installation, maintenance, or use of antennas used to receive video programming.
Specifically, it prohibits most restrictions that: (1) unreasonably delay or prevent installation, maintenance or use; (2) unreasonably increase the cost of installation, maintenance or use; or (3) preclude reception of an acceptable quality signal.
While the FCC has ruled that you can install exterior antennas to receive TV stations, you may want to avoid any hassles by trying an attic installation first, if the attic space is easily accessible. If you do, be sure to securely mount the antenna as high as possible using a mast and support brackets, and keep it as far away from all metallic surfaces as possible and aimed in the direction of the TV transmitters.
More information on OTARD can be found at https://www.fcc.gov/media/over-air-reception-devices-rule.