repair manual
Harold Adams
does not transmit clearly. Psk transmissions all jibberish and broken
up. Want to make some checks under the hood. Any help would be
appreciated. By the way if anyone hears me on PSK-31 and I don't
answer its because of my lousy antenna. I'm not anti-social. LOL
73 de Harold w2aco
jimfra...@comcast.net
Perhaps my recent BPSK31 troubleshooting and repair event will be of some help.
My outside ground connection was deteriorated and caused the VSWR to spike up and at times erratically go up and down. Solution was clean up the ground connection, remove rust, etc, from the connection point on the grounding rod. Included sanding the top of the grounding rod, the grounding wire, and cleaning up the "bug" that attachs the grounding wire to the rod.
At first, my brother, WA3VYP, and I assessed the root cause was my antenna. It is a DX-33 Alpha Delta wire dipole set up in the inverted position. The antenna was > 18 years old so we thought the erratic VSWR excursions was due to changes to the antenna input impedance due to degraded connections at the feed or perhaps degraded loading coils on the 40 m element wire. Needless to say after 2.5 days dedicated to over-hauling the antenna and re-installing, it was a mighty disappointment to find out the problem on BPSK31 wasn't resolved.
Then the light bulb came on, it must be a grounding issue. The symptoms did not lead me to suspecting a grounding problem initially because all other communications modes I use, phone Lower SSB and phone Upper SSB were apparently working fine. And a quick ground resistance check didn't initially reveal any unwarranted concern. But then when the antenna repair action didn't resolve the spiking VSWR, only experienced on BPSK31, it was time to scratch the head again.
So why does a degraded ground connection appear to impact BPSK31 communications more so than phone SSB? Here is my explanation, it is all about the instantaneous 180 degree shift that occurs everytime a "0" bit is transmitted. Some technical ham folk equate the effects to CW key clicks. In short, PSK, a digital modulation scheme, is susceptable to large intermodulation distortion products in its transmission spectrum which you and I limit by ensuring we don't over-drive the transmitter, But, when you have a poor ground, like I had, the IM products and natural frequency spectral spreading characteristics of the BPSK31 signal is exasperated. In my case the intermodulation distortion (IMDs) may have become so significant with a bad ground most of the energy was being reflected back due to the out-of-band carriers created by the PSK signals. It now makes sense why the antenna tuner VSWR measurement was so high.
The moral to the story is a very good ground connection is required to communications using binary phase shift keying or any phase shift keying modulation scheme. Instantaneous 180 degree phase shifts and poor grounds make for nasty IMD.
73, KC5RUO
Jim Frazier
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