TV upgrade – MER optimisation

At TV upgrade I reported a change in TV antenna pointing to a different and distant transmitter, and gave a Spectrum Analyser plot at the main TV receiver.

At that time, I adjusted the antenna accurately (within 1°) based on compass heading, but antennas are not perfect and two significant path obstructions may have bearing on best signal.

I could have run up and down the ladder making small adjustments and observing amplitude or better, RF S/N on the Spectrum Analyser but that is tedious and suboptimal so I purchased a DVB signal analyser.

Importantly, a good DVB analyser gives measurement of not just signal strength, but carrier to noise (C/N) ratio (which is actually RF S/N), Bit Error Rate (BER) and Modulation Error Ratio (MER), the last two very important statistics in optimisation and validation and not available on an ordinary Spectrum Analyser.

MER is calculated as the sum of the squares of the magnitudes of the ideal symbol vectors is divided by the sum of the squares of the magnitudes of the symbol error vectors. The result, expressed as a power ratio in dB, is defined as the Modulation Error Ratio (MER).

MER is a good overall single statistic for quality, but BER is more sensitive to occasional errors, so they are both important.


Above, the DVB analyser (the red device) enables a view of measurements whilst adjusting the antenna. The analyser here is connected to the masthead amp output and of course powers the masthead amp.

The channels of interest on Knights Hill are 35, 36, 37, 38 and 39. There is a significant slope across the channels, it may be just the uneven gain vs freq of the log periodic array. The weakest channel is 39 and best is 35, so the pointing for best signal strength, observing carefully BER and MER was found for both channels. They coincided (not surprising), but what was a little surprising was that the azimuth was 5° less than the bearing to Knights Hill. Perhaps the antenna is asymmetric, perhaps the obstructions create the effect. Anyway, an improvement of 3.2dB in C/N ratio was obtained.

The analyser does not have facility to save measurements (quite a shortcoming), so pics were taken of a couple of screens at the main TV of the worst channel (39) as an archival record. The chain includes a LP antenna, filter, 43dB masthead amp, and two cascaded splitters. Loss from the masthead amp out to the main TV cable is about 12dB, ie level at the masthead amp is about 12dB higher than shown below.


Above, the detail measurement screen.


Above, the constellation screen, the detail related to the MER measurement.