Saturday, August 22, 2009

New Amateur ATSC DX record - complete with video

I went up to the Sierra Rd QTH in the east foothills, and this time everything went perfectly. I was able to capture a video from up there, and with a good line of sight and 80 watts of ERP, I was able to get a signal strength of 100 and a signal quality of 75 or so on the HD HomeRun. This location is very similar to the location of the K6BEN repeater and is in, more or less, the same direction. Google Earth says it's 9.07 miles away from home. The coordinates are 37.413081° N, 121.824316° W.

Here's a screenshot from my phone's ssh client showing the status report from the HDHomeRun with the signal strength and qualty (the first line showing no reception was a control - the transmitter was unkeyed):

It was actually sort of tough to get the phone to cooperate. Cell phones operate in the 800 MHz range, and transmitting 85 watts of 900 MHz right next to the phone desensitized the receiver somewhat. I did manage to get working in enough fits and starts to check the signal strength and start and stop the recording, thank goodness.

The receive antenna was a 7 element (10 dBi) Yagi - about $30 off the Internet, and the reception was almost perfect. That pretty much proves that this transmitter is going to work. With a real antenna - like the 18 dBi loop Yagi I have on order - reception should be possible from perhaps 30 miles away with a good line of sight.

Unlike the first time, it was not necessary to adjust the power. The exciter was set to power level "3", which is an average power of about 0 dBm, or a PEP of about 6 dBm. That means an output power of about 16 watts or so, for an ERP of 80 watts. Of course, all of that is in theory. Still on my to-do list is to run this setup through a proper sampling wattmeter and dummy load to get a measured output power level and get a good measurement on how far down the out-of-channel emissions are. Part 73 contains an emissions mask requirement that says that the out-of-channel emissions must be reduced from the channel average power by at least 47 dB - that is, 36 dB below the pilot. That's a pretty reasonable goal to shoot for. At the power we're contemplating, it'd be less than a couple milliwatts. At 900 MHz, that ought not to be a problem.

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