HEAVY METAL, PART 2
Comparisons of indoor antennas for VHF-UHF DTV reception
Last month, I covered my extensive side-by-side-by-side tests of VHF and UHF antennas for signal reception and gain. That marathon session took two days to pull off, but once it concluded, I thought to myself: Why not run a similar set of tests for indoor reception?
Why not, indeed. Armed with a bunch of aluminum and some test equipment, I spent a full day setting up and testing a variety of indoor antennas - some amplified, and some not - just to see how well they pulled in analog and digital TV signals.
The location was the center room of my studio, which is where I normally test plasma monitors, projectors, and other display devices. This office location is almost equidistant from a pair of DTV stations in Allentown, PA (about 23 miles due north) and five DTV stations in Philadelphia (about 23 miles south). The view north is mostly unobstructed; however, a 50' hill and the four-story county courthouse block me to the south. In other words, it's a pretty typical indoor DTV reception location.
Photo A shows the test equipment
setup. Photo B shows a mounted antenna on a 5' mast section, while Photo
C shows the tabletop antenna location. Photo D shows an antenna being
tested with an external preamp, and Photo E shows an actual indoor waveform
(with multipath) from KYW-26.
Photo A
Photo B
Photo C
Photo D
Photo E
Many of the antennas used in my first test were unsuitable for indoor
reception. Mostly, they were just too darn large, unless you have a big
attic space. So I opted only for those models which could reasonably be
used free-standing indoors, or mounted to a wall bracket (or even an attic
stud, if need be).
Unlike the outdoor tests, I checked some of the antennas with and without an external UHF preamplifier. Signal levels are crucial for indoor DTV reception, and I don't think it unreasonable to add such a preamp if it can make the system work better. A Channel Master model 7775 was the stock-box preamp tested with the Antiference Silver Sensor and two of my own compact yagi designs.
Once again, the receiver was a Samsung SIR-T150, with a small spectrum analyzer fed from its antenna loop output. The Sadelco Mini-Max 800 was used to take signal strength readings in all modes (as far down as -20 dBmV in analog mode and -13.2 dBmV in digital mode). Reception for the carrier-to-noise tests was verified for at least three minutes using my reference Princeton AF3.0HD monitor.
Here's how to read the tables. All measurements in Tables 1 thru 3 are logged in as dBmV levels. Signals below 0 dBmV are in red, and signals above that level are in blue. Signal levels that fell below the Mini Max 800 threshold sensitivity are colored gray. It doesn't mean they wouldn't work with the tuner, just that they were pretty weak (less than -67 dBm).
The fourth table is the approximate carrier-to-noise measurement data. While not as accurate as precision lab equipment, it will give you a good idea of how strong each DTV signal was. Once again, the DTV correction factor for each measurement has been applied.
Numbers in blue mean that these signals locked up the Samsung SIR-T150 tuner successfully. Numbers in brown indicate moderate to heavy multipath, but that sync and EQ lock were still successful on the Samsung. Numbers in red indicate there were dropouts of video, audio, or both during the three-minute test window, and a gray "DNR" designation means exactly that - the signal was not received.
There is a direct correlation between an antenna's physical size and its
gain at a specific frequency. You can make antennas much smaller, but
the active (driven) elements must resonate close to or at the wavelength
of the desired frequency. Tricks such as capacitors, inductors, and even
winding antennas into coils and helixes have been used to reduce physical
size while maintaining reasonable performance.
In this category, the best performer hands down was the Radio Shack 15-1862 combo antenna. This design uses a pair of rabbit ears and a rotatable UHF double loop against a mesh reflector. It has a variable-gain VHF/UHF preamp that isn't too noisy, and the resulting gain figures are almost 6dB better than the runner-up, Terk's TV55 with the internal amplifier switched on.
The performance of this $60 antenna was quite a bit better than another, more expensive "designer" Radio Shack indoor antenna, the 15-1890 dipole/helix antenna. The box for the 15-1890 is actually marked "High Definition Antenna" on the box (you know that would have caught my eye immediately!), but it doesn't have a lot of gain on VHF channels in any mode. Without the internal amplifier, it's pretty deaf. With it, it's marginal.
The Antiference Silver Sensor went up against two of my own designs with and without a preamp, but since these antennas are designed to have little or no gain at UHF frequencies, they basically bombed out on this part of the test. (At least they aren't susceptible to overload from a strong VHF station.)
I expected this part of the test to be a more closely-fought battle, but
once again the RS 15-1862 blew away the field on every analog channel.
What's more, its analog UHF signals looked pretty clean with a moderate
amount of multipath that could be cleaned up by repositioning the double
loop antenna. The 15-1890 was once again a bit of a disappointment, as
the Channel Master 3000A and Terk TV55 both outperformed it substantially
on channels 17 and 29. Only on channel 57 did the RS antenna show a bit
more gain.
The UHF-only antennas were pretty evenly matched across the band, with the unamplified Silver Sensor doing best on channels 17 and 29, and either of the two ROAM antennas pulling out a little more signal on channel 57. Adding the external preamp didn't change things much - the Silver Sensor still had the most gain on channels 17 and 29, and the ROAM antennas were a bit more sensitive on 57.
You'd pretty much expect the digital signal measurements to follow the
analog signals. But here's a caution - a signal strength meter cannot
distinguish multipath from the 'true' signal, which would affect all of
the readings in tables 1 and 2. Just because a signal is strong doesn't
necessarily mean it is clean enough to be locked up by a DTV receiver.
You could be measuring a strong multipath (echo) reflection, which is
not usable.
Among the amplified antennas, the RS 15-1862 once again provided the most signal level across the band. None of the other antenna configurations provided much signal at all except with channel 26 and channel 64 - not coincidentally, the two strongest signals in the office, and the two running the most transmit power (787 kW and 500 kW ERP, respectively).
The 15-1890 didn't yield much signal in any mode, except with KYW-26 and WPVI-64 when using the amplifier with either the internal dipole or helix antenna. Things were a bit tighter with the UHF-only antennas with and without the preamp. There was no clear advantage to any antenna except on channel 42, a signal that usually has lots of multipath. Since the ROAM yagis were designed for best SWR match and moderate multipath, they did a bit better here.
Here's where the rubber met the road, so to speak. Each antenna was tested
for actual reception of each of the four DTV signals present. Successful
reception required a steady 3-minute observation with no audio or video
dropouts or hangups. Again, the actual measurements are not terribly precise,
but close and take into account the correction factor for measuring DTV
waveforms.
In the amplified group, the Radio Shack 15-1862 swept the board, locking up each of the four stations (one with moderate multipath) and receiving all just fine. The only other antenna to come close was the RS 15-1890 (surprise!) using the internal helix antenna and switching the amplifier on. While this antenna didn't show much gain in the previous four tables, it did overcome moderate multipath to pick up a very weak signal from PBS affiliate WHYY-55.
The remainder of the UHF antennas jockeyed back and forth through each channel with no clear victor to be named. Since all of these antennas were directional yagi designs, their placement was much more critical, but once they were aimed correctly, plenty of signal was seen on the spectrum analyzer. The Silver Sensor had an edge on channels 64, while my own yagis did a bit better on 26, 42, and 62.
CONCLUSIONS
Most of the antennas tested will do the trick for DTV reception, provided signal levels are strong enough. In most cases, you'll need some kind of amplifier to get signal levels high enough for the DTV tuner's equalizer to work with. The quality of internal and external amplifiers can vary quite a bit - the cartridge UHF preamp sold by Radio Shack for $39.95 is not terribly stable and a bit noisy, so spend a few more dollars and use a premium-grade version from Channel Master, Blonder-Tongue, or Winegard.
The "trick" antennas that save space will yield mixed results for DTV reception. Terk's TV55 did okay on the lower UHF DTV channels (26 and 42), but was out-performed by the much smaller Radio Shack 15-1890 compact antenna from channel 42 on up. Even the rather pedestrian-looking Radio Shack 15-1862 cranked out some serious DTV signals on all channels, and it doesn't take up much room either.
If you are working with UHF signals only, then don't bother with a VHF antenna - maximize your gain where you need it the most. Similarly, choose a good design with lots of gain for VHF if that is where the signals are, and don't worry too much about UHF performance. It's a good idea to check into a compact spectrum analyzer such as those sold by Avcom, B&K, and Blonder-Tongue - it's the only way you'll know for sure what the received waveforms look like.
Copyright ©2002 Primedia Business Media / Peter H.
Putman.
This article appears in the July 2002 issue of Sound
and Video Contractor.