Monday, August 24, 2009

Final TX configuration set

I've set up the transmitter in what I believe will be its final configuration. I put a 6 dB attenuator on the output of the exciter to raise the output power setting required so that it would be in the middle of its range rather than on the end. Getting the exciter output to an average power of about 6 dBm now means setting the output power to 10 instead of 3. I traded some mail with the guys at SR-Systems and they said that the "sweet spot" is anywhere between 5 and 13. Anyway, with 6 dBm of power, the amp is supposed to generate 15 watts of power, which means an output ERP of about 75 watts. I put an N to UHF adapter in the input of the spectrum analyzer and then put a big nail in the center conductor socket and laid the coax next to it that was hooked up between the amp and the dummy load. It may be less than perfectly accurate, but my hope is that it's close enough to score. Anyway, with that setup, I could see the "ramps" of out-of-channel noise well enough to check how strong they were relative to the ATSC pilot (which, again, is 11.3 dB down from the channel average power). At a power setting of 10, the skirts were just about 47 dB down from the average power, which is right where the part 73 channel mask would have them. At a power level of 11, they come up to closer to 40 dB down. Now, the part 73 channel mask is not of any particular relevance for part 97 operations, but we amateurs are required to reduce spurious emissions in accordance with good engineering practice. Besides, limiting the output power can't help but make the amplifier just that much happier, which means it is likely to last longer - particularly given that it will be in a repeater shack that is not climate controlled.

I have a 20 watt 20 dB attenuator arriving in the mail tomorrow, so I'll be able to run the output of the amp directly into the SA for a final check tomorrow evening. Not only will I be able to directly measure the out-of-channel emissions, but I'll be able to get an accurate power output reading as well.

So it will put out 75 watts of ERP (perhaps it might drop down to 70 or so with coax losses when all is said and done). That still should be receivable everywhere the analog signal of K6BEN is receivable today.

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