[mtownsend]

Originally Posted by MikeInEburg

Signal level on 213MHZ at antenna mast as measured on my Sadelco meter is -24dBmV. Much of the time, signal is fine. Then we get tiling and picture breaks up. Digital demod indicates a 20dB SNR - which should be OK. I suspect that combo of marginal signal and multipath are to blame.

Hardware setup is as follows:
-Blonder Tongue BTY-10HB single-frequency antenna (for channel 13) roughly 40' above ground level.
-Blonder Tongue CMA-HB VHF preamplifier on mast
Roughly 250' of RG11 to headend
-Blonder Tongue PS-1536 Preamp power supply
-Toner 3-way splitter:
Leg 1 to Drake DAD860 digital demodulator for WJZ (RF13) (converts to baseband, then we re-modulate on our in-house cable plant)
Leg 2 to Blonder Tongue 8VSB to QAM transcoder (we transmit same signal on QAM on our private cable plant)
Leg 3 to a Drake DAD860 digital demodulator for WBAL (RF 11) that is broadcast from the same tower as WJZ. No problems with this channel!

Your setup seems sound. A 20 dB SNR from the demodulator means that you should have about a 4 dB margin for handling multipath/interference/fading. You might be able to squeeze out another dB or two by switching to an amp with a lower noise figure. The CMA-HB noise figure is 5.0 dB, while something like the Winegard AP-3700 has a noise figure of 2.6 dB (the Winegard also has a built-in selectable FM trap).

Do the pixellation events correlate to certain conditions like rainy weather or windy conditions? You might want to check for antenna wobble and tree blockage.

Multipath does seem to be the most likely cause of the occasional breakups. You might want to try aiming the antenna a little off-axis to see if things get any better/worse. If you have a strong multipath reflection coming off of a hill or building, you might be able to de-tune your sensitivity to it by adjusting the antenna aim. It would help if you had access to a spectrum analyzer and could look at the flatness of the modulated channel while you are adjusting the antenna.

Drake (digital demodulator mfr) says that I should try a newer consumer-grade set-top box to see if newer chipset better deals with multipath. So - any recommendation on a newer set-top box that has tuner that may be better than others? It appears that the set-top box market has fallen off a cliff in the past year...

Yep, the demand for set-top boxes has gone away since all new TVs have built-in tuners, and TV's keep coming down in price. Without the $40 coupon program, it probably makes more sense to upgrade an old TV rather than to spend $60 out of pocket on a set-top box.

Most electronics retailers still carry at least 1 model of converter boxes. Set top boxes are still available, but your selection might be limited to just the 1 model that the store carries. I recall seeing them at Walmart and Radio Shack, and there are probably others.

The LG chipset used in many of these boxes is known to be pretty good (so-called 6th generation chipsets). Broadcom also has a pretty good chipset. These newer chipsets are known to do a better job of dealing with multipath. You might want to check out this list on Wikipedia (http://en.wikipedia.org/wiki/Comparison_of_CECB_units).

BTW, these "low-end" converters downconvert everything to NTSC quality (should be the same as the DAD860), so they will not be able to deliver HD content in their native resolution. Do you have any plans to support HD content in the future?

We are considering a second stacked antenna to help eliminate multi-path, however that's a fairly large nut to crack.

The set-top converter box definitely seems like the easier and cheaper route to try first.