[hpsdr] Analogue TV broadcast aliasing observation

[Erich Heinzle]

both myself and Dean VK5LB have noticed a strong bimodal signal
starting around 52.85 MHz using stock standard Mercury receivers,
but nothing is heard on analogue rigs.

The smaller of the two signal peaks is a wideband FM signal centred
at about 52.878, which with NFM mode and wide filtering sounds like
the local Channel 2 analogue TV station broadcasting on 64.26MHz

( http://www2b.abc.net.au/reception/frequencyfinder/asp/details.asp?transmissionid=53&presdir= )

This apparent audio subcarrier is a few MHz below the larger signal,
i.e. an inverted PAL B/G video signal with +5.5MHz audio subcarrier

We are both pretty close (within 20k or so) to the local Channel
2 PAL B/G broadcast tower.

I am using a G5RV

Simple maths gives Mercury receive clock (122.88) minus TV frequency minus audio subcarrier offset (i.e. minus 64.26 + 5.5) = 53.12

This looks roughly about right, as it is about where I see the inverted PAL B/G signal with WFM subcarrier. Someone correct me if I am wrong.

I guess this is a good demonstration of Nyquist x2 sampling rate having
trouble excluding stuff just above half the sampling rate.

I guess the wideband antenna and 327kW TV transmitter in the same grid square doesn't make things any easier for the onboard filter....

A good reason to get Alexiares....

73

Erich Heinzle
VK5HSE