Originally Posted by Konrad2
Awhile back, Bob Chase posted a pdf with actual spectrum analyser graphs from a
field test done 2005-09-18 in Pearland Texas. If I'm reading it right, on channels
5 & 9, the CM4228 is significantly more ragged than the Scala, HD7210, and CM5646.
Nearly 10 dB difference within channel 5, about 4 dB within channel 9.
Compare the slope of the various antennas on channel 9.
Compare the slope of channel 35 with channel 36.
FYI: Here are links to Bob Chase's UHF-only outdoor vs attic antenna comparison and an attic location test:http://www.avsforum.com/avs-vb/showt...ic#post5399471http://www.avsforum.com/avs-vb/showt...la#post5410432
and here's Bob's VHF/UHF outdoor antenna test you cited above:http://www.avsforum.com/avs-vb/showt...60#post6246260
When you see two strong adjacent channels, the transmitter and antenna have frequently been designed
as a single integrated structure. For example, NTSC CH8 and ATSC CH9 are different versions of KUHT.
However, antennaweb.org shows that ATSC CH31 and CH32 are on different towers in slightly different locations,
as are also ATSC CH35 and CH36.
Since ALL of the antennas experienced the SAME frequency response irregularities for CH35/36,
I would suspect either the transmitters or perhaps multipath conditions that affected all antennas equally.
The CM4228 (cyan) has a variation of about +/- 4 dB on Low-VHF CH5,
but ya still have to be amazed that it has ANY gain for low-VHF channels and has more gain that any
of the other UHF antennas, including the SS-1000 Square-Shooter that still claims to be a "VHF/UHF" antenna.
Also bear in mind that most locations don't need much of an antenna for low-band VHF.
I've seen people strip off 12-18 inches of coax, leaving just the center conductor.
And Kerry Cozad's "coat hanger" antenna (see slides 11/12) also might be a viable candidate,
although I would take the time to unwind and separate the ends:https://secure.connect.pbs.org/confe...ns/TC05_43.htm
It would be interesting to see how the gain changes as the CM4228 is rotated,
since the max VHF gain isn't always aligned with the UHF beam direction....
And it is also possible that the spectrum analyzer was responding to multipath variations in
that particular antenna location vice gain irregularities as it slowly scanned across the channel.
Long experience has taught me that it is difficult to conduct on-air tests in "controlled" test conditions.....
Requiring numerous additional steps (usually not reported) to exclude other confusion factors....
on High-VHF CH9 the CM4228 variation across the channel is only about +/- 2 dB.
Compare this to the best VHF/UHF Combo antennas which all experience a slight roll-off with frequency,
resulting in a variation of 2-3 dB. So on CH9, CM4228 has 4-10 dB less gain than purpose built
VHF/UHF Combos and is actually within +/- 2 dB, just like the Combos...
Note that the CM4228 is maintaining good energy across CH9, rather than rolling off like the VHF/UHF Combos.
An adaptive equalizer can overcome freq response irregularities much easier than trying to recover energy that is no longer there.
In NTSC, the audio carrier should be about 8-15 dB below the visual carrier:http://www.sencore.com/newsletter/No...files/HDTV.htm
However, each station can chose a specific setting and what you see on a spectrum analyzer depends on
the bandwidth and integration time settings.
Hence overall, it appears that most NTSC channels are displayed with the audio carrier about 10 dB below the visual.
On NTSC CH11 and CH13, let's look at the relative strengths for the triple peaks (visual, chroma, audio).
On CH11, the three best VHF/UHF Combo antennas have about the same gain as the CM4228 for the visual carrier,
however the Combos also experience perhaps 6-8 dB EXCESS of gain for the audio carrier, the worst being the CM3671.
The CM4228 seems to be experiencing a gradual frequency rolloff, increasing to perhaps 6-8 dB for the audio carrier.
On CH13 the CM4228 APPEARS to be matching the gain of the best VHF/UHF Combos on the visual and audio carriers.
However, if you download and blow up the chart, you'll see that the CM4228 is providing 3-4 dB less gain than the combos.
This overall gain loss of 3-4 dB is also seen across the mid-channel frequencies, illustrating a generally flat response for the CM4228 on CH13.
So on CH11 and 13, the best VHF/UHF Combo antennas only provide a few dB more gain than the CM4228
and on CH11 experienced frequency flatness issues that are on the same order as the CM4228.
BTW: STATIC, SMOOTH frequency response irregularities on the order of a few dB can be readily accommodated in an ATSC receiver's adaptive equalizer.
DYNAMIC and/or NOTCHED frequency response irregularities displayed on a spectrum analyzer are evidence of underlying multipath problems.
YMMV: Anytime you measure antenna gain in a real-world location, the results will also reflect current multipath signal conditions. The nature of multipath can cause signal nulls and peaks that vary with location, meaning that one antenna may
experience a frequency response problem differently than the others.
Also, the Ground-Bounce reflection can cause as much as a 3 dB increase in measured gain at certain frequencies,
including those reported by Kerry Cozad using an outdoor antenna range. Reflections from nearby buildings can also inflate the measured gain.....