[amsat-bb] FT-847 and third harmonic

Kevin M n4ufo at yahoo.com
Wed Nov 12 11:03:08 UTC 2014

Hi all,

I got some new sat antennas up yesterday. I'm not quite operational yet as there is no power to the preamps and the tracking interface is not wired up, etc. But I was up early this morning and when I checked N2YO I noticed that a couple birds were about to come over. Wee hours of the morning, best time to fool around and not bother anybody. =^)

Once I figured out the "upside down" azimuth for AO-7B, I flipped the beams over and gave it a shot. Very happy to hear my own signal coming back through the bird! Tracked it a few minutes and played with the doppler to make sure I was actually going THROUGH the bird. No one on and FO-29 was coming into range. So I flipped the beams back over and this time turned the azimuth around to greet it.

I did hear find my own signal on FO-29, but through a lot of splattery noise that seemed to be some type of desense. I managed to tune in what sounded like a spanish language voice in the passband also a bit odd sounding. I think this might have been an AM or FM signal, likely not intended for sat reception. (Mexican taxi?) But between that and copying the beacon, I was convinced I was hearing the bird. Long story short, I could make out my own whistles and my voice through the noise right up through LOS. But even after LOS the noise was still there whenever I transmitted. I was quite disappointed, as I thought I had taken precautions to prevent desense.

Now mind you, I'm only testing. There is no power to the mast preamps yet. I have diplexers in both lines just before the preamps to act as filters. But I did have my rig's internal preamps on. I turned the rigs transmit power way down to almost nothing and the noise was still there. I calculated the third harmonic of my transmit frequency and tuned up there... then it dawned on me while trying to tune it in. The splattery quality of the 'noise' was simply me trying to listen to my third harmonic on the OPPOSITE sideband. X^o  When I changed sidebands there was my own signal, clear as a bell, 40 over S9. MAN! Can't believe my precautions didn't work, etc.

Then I had a thought... unplug the UHF feedline from the back of the rig. No change... I checked the other sat frequencies and nothing heard on mode B, just Mode J... so, definitely third harmonic. I turned the internal preamps off and while greatly reduced, the third harmonic was still heard while tuned to the FO29 passband. So now to my questions...

Is this NORMAL for an FT-847? Is this normal for any full duplex rig and can't be avoided? I assumed there was filtering to prevent this, etc.

Perhaps when I had up my previous set of antennas last year, I just didn't notice it before, but could something have happened to my rig in the meantime to cause this? Anything to check/fix? Or should I just hope that once I get the mast preamps working and turn off the internal ones, the signals will be stronger and the internal desense will not be enough to notice?

I'm just not experienced enough with this rig (or operating linear sats for that matter) to know if this is normal.

Thanks in advance!

Kevin N4UFO

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