Monday, March 18, 2013

Improvised Amateur Television Downconverter

The only W6ATN ATV repeater I have a semi-clear shot to is Oat Mountain. It has an output on 919.25 Mhz. I have an old analog TV that goes up to cable channel 125, which is right around 800.25 Mhz . How convenient!

So, with a 900 Mhz yagi on the roof fed with heliax, I plugged in an SMA mixer (that I'm supposed to be building a 1296 transverter with) and dialed up my K5BCQ DDS signal generator up to 119.0 Mhz, and connected it to the TV.

Didn't quite work. I could tell there was something there that said W6ATN if I stared long and hard at the snowy screen. ;)

Made the following improvements.

1.) Moved the antenna higher. ;)

2.)  I connected a cavity bandpass filter. Used my 8924c (and a mini-circuits directional coupler as an improvised return loss bridge) to put the "dip" right at 919.25. Dunno how much gain/loss/rejection it came out to, I'm still learning in that dept. ;) Either way, inserting this helped, noticeably.

3.) fished out a DEMI MMIC kit (10 bucks!!) from the bin, this one had a MAR-6. Soldered together and installed SMA connectors, and inserted between the Bandpass filter and the mixer.

Pretty good:

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Here you can see the components: The water heater-lookin' thing is the BPF, MAR-6 preamp is floating in front of it with the green cable connected, and the mixer is floating off to the right with the three SMA's in a row. DDS VFO is on the bottom with the knob and the LCD that you can't read in this picture. Thats it!

Antenna: You can see I have my DVB-T dongle connected, used that to aim and peak.