A spectrum display doesn't always show the extent of a multipath problem unless it is primarily caused by irregular amplitude across the channel.
Bill Naivar had an urban multipath problem that showed on his spectrum analyzer scan, and was able to solve it with what he calls an anti-ghosting antenna:www.prism.gatech.edu/~wn17/
at the bottom of page one there is a link to page two which is:www.prism.gatech.edu/~wn17/Web%20bill%20page%202.htm
On the other hand, Trip in VA
had a reception problem that didn't show on a spectrum display of a channel as multipath, so he thought that it wasn't multipath, but it was.Chattanooga TN: Got Atlanta But Few Localshttp://www.avsforum.com/avs-vb/showthread.php?t=1372416
It wasn't until he was able to borrow an expensive piece of equipment that showed the delay time of the reflections (echos) that he was able to confirm his suspicion of multipath.
My theory is that the reflections didn't show on his original analyzer scans as multipath because they were approx. the same amplitude as the primary signal. The analyzer scan just shows the amplitude across the 6 MHz channel, not the timing of the reflections.
The relative amplitude of the reflections becomes more critical as they increase in time difference from the primary signal.
See the echo table in attachment #1 which comes from page 22 ofATSC Recommended Practice:
Receiver Performance Guidelines
Document A/74:2010, 7 April 2010atsc.org/cms/standards/a_74-2010.pdf
Converter boxes were also required to meet equalizer specifications for multipath to be eligible for the coupon program.
DTV Converter Box Cupon Program
Information Sheet for Manufacturers
The second attachment shows the specs.
What's the best way to infer multipath: signal strength? correction errors in a 24 hour period?
No, not signal strength. Trip has strong signals, but poor signal quality. The equalizer has a limit to how much it is able to correct multipath errors, and some tuners are better able to make corrections than others. So, even if you were able to make measurements, it is still your tuner that makes the final decision as to how much multipath error it can handle.
A tool that I have found useful when aiming an antenna for minimum multipath errors is a converter box or tuner that has two signal bars, one for signal strength and one for signal quality. The ones that I have used are the Apex DT502 and the Centronics ZAT502HD. A photo of the dual signal bars is shown in attachment 3. Credit for the photo goes to douglas-b
who first posted it on the Centronics ZAT 502 HD / RTC DTA1100HD / Digiwave DTV5000HD
but it is no longer there in his post. My photo of the dual signal bars wasn't as good, so I posted his there as an attachment to one of my posts.
I insert an attenuator between the antenna and the tuner to approach the "digital cliff" so that the tuner is more sensitive to errors (quality). Slight changes in antenna aim produce small changes in signal strength, but can produce large changes in signal quality if you can find a signal path with minimum reflections.
When I was testing the DT502 with my CM4221 antenna I got (for 13.1 on RF41):
Signal Quality 60%
Signal Strength 55%
I had aimed the antenna with my SLM (signal level meter), but when I rotated the 4221 slightly to the right I got:
Signal Quality 100%
Signal Strength 56%Note the BIG change in signal quality with only a slight change in signal strength.
It seems that the signal quality indication is a more sensitive aiming tool than signal strength
, because it shows the increase in BER (bit error rate/ratio) from multipath reflections. In my situation the BER is affected by the weak signal, the fixed multipath reflections, and the changing (dynamic) multipath reflections. My antenna is aimed across a well-traveled road, so I get reflections from cars. (This is an example of the need for the new ATSC M/H standard.) When the quality went up to 100%, the car reflections were less of a problem. My stronger signals maintain a good lock inspite of the cars.
Some of the new SLMs can measure BER and MER (modulation error ratio.....similar to SNR). The less expensive ones, like my Sadelco DisplayMax 5000, only measure emulated BER and MER using noise as a reference. The more expensive SLMs contain an ATSC demodulator that measures true BER and MER, but they cost kilobucks.