Tuesday, August 18, 2009

A major amateur ATSC milestone achieved

I just got back from a "DXpedition" of sorts. I went to the crest of a hill in Cupertino and transmitted ATSC at 910-916 MHz and successfully received a picture at home.

The path length, according to Google Earth, is 6.59 miles. I now claim that as the amateur ATSC DX record. I'll be happy to be proven wrong, but so far as I know, nobody apart from a professional broadcaster has transmitted an ATSC signal and successfully received it at that great a distance.

Alas, I don't have any recordings. In order to make this work, I connected the TVC-9S to the best tuner in the house - our brand new Samsung 52" LCD TV in the living room. I then drove off into the night and called Scarlet on the cell phone and asked her to watch. The reception wasn't perfect, but it was there for reasonable stretches.

The receive antenna was a 7 element 900 MHz Yagi I bought off the Internet for about $30. I'm still waiting for the 15 element loop Yagi from Directive Systems - that should give me an 8 dB boost in receive gain when it comes. So the total receive gain was on the order of 7.8 dBd, taking a small loss for coax.

On the transmit power side, that's a much grayer area.

Recall the following rules of thumb: If you can see the ATSC pilot on the spectrum analyzer, it's power level is approximately 11 dB lower than the channel average power level. The peak-to-average power ratio for ATSC is about 6 dB. So the pilot to PEP ratio is 17 dB.

I started out with the minimod power set to "3" on a scale of 1 to 15. On the spectrum analyzer, that sets the pilot at about -11 dBm, which means the channel average power is about 0 dBm. Add 6 dB for the peak-to-average ratio, and that's an input PEP of 6 dBm. The power curve of the 3370PAHS has that being an output power of about 47 dBm (50 watts), or an average power of about 12 watts. That's right between the 1 and 2 dB compression points for the amp. That signal didn't quite make it. I had actually made two trips out to the transmit location - the first time was with the HDHomeRun set up as the receiver, and that configuration did generate a signal strength of about 70, but a signal quality of 0. With the TV hooked up, I didn't have ready access to any signal metrics, so it was just whether it would decode at all or not.

In a fit of impatience, however, I decided to crank the power up, just to see. And sure enough, a power setting of "12" was what it took for decent reception.

Now, that's a pilot level of about 0 dBm, or an average input power of 11 dBm, or an input PEP of 17 dBm. The amp's specifications show it is fully saturated at 16 dBm input power, for an output power of 100 watts PEP.

I can't for the life of me imagine that the amp was very happy doing that.

I can't look at the output of the amp on the spectrum analyzer, since it exceeds the maximum input level of 1 watt. I do have a 25 watt 20 dB attenuator on order, however. When it gets here I'll be able to look and figure out exactly how terrible the splatter is from running the exciter that hot. If previous experience on 420 MHz is any guide, it won't be pretty.

What I can't quite wrap my head around, though, is that if the amp is being overdriven, how can it be that that actually improves reception? Surely an amp that's heavily clipping ATSC would make the eyes close up like a punch drunk boxer's.

We'll have to wait and see. In the meantime, I'm happy to claim some small success, and I'd like to publicly thank my lovely wife Scarlet for helping me verify this transmission. I couldn't have done it - or anything thus far - without her unending help, assistance and understanding.

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