When the station is on, try disconnecting the antenna. Then try grounding the antenna input. That should give you some idea if it¡¯s coming through the antenna.
Clark Martin
KK6ISP
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On May 8, 2018, at 8:57 PM, Bruce MacKinnon KC1FSZ <bruce@...> wrote:
Hi:
I've scratch built my own BITX using Manhattan construction on copper clad PCBs. At the moment there is no enclosure or shielding beyond some attempt to shield the digital controls from the RF parts of the rig. Everything works great and I've logged 100's of QSOs.
Except I'm having one problem: during the evenings here on the East Cost of the US I sometimes hear what I believe is a Spanish language broadcast coming through my audio output. It fades in and out and only on occasion does it get loud enough to obstruct a QSO, but it can be annoying. When I disconnect the antenna the signal goes away.
I've always assumed the issue was overload interference from a local broadcast station. A few times I've turned on an AM broadcast radio and tuned around trying to match what I was hearing without any luck.
The important thing that recently dawned on me is that the interference isn't impacted very much by the setting of the VFO. When I hear the interference I can hear it across the entire band (for the most part). Which got me thinking: perhaps the problem is an interfering signal that is entering around the product detector (since that part of the radio is operating at a fixed frequency).
Might there be a station transmitting around 12000 kHz (the BFO frequency of my rig) that is being demodulated by the back-end of my receiver? Or maybe some harmonic thereof?
This evening I tuned an AM radio to 12000 kHz and, sure enough, I could hear what sounded like a Spanish station! The interference wasn't very bad today so it was hard to be completely sure, but I believe the interference I hear through my BITX is the same thing I hear when I tune an AM receiver to 12000. In some ways the second half of the BITX receive path is like a direct conversion receiver permanently tuned to 12000 kHz.
A few questions:
1. Has anyone else ever had this problem?
2. Does it seem plausible that there is a path from the antenna into the BFO/product detector part of the receiver?
3. Does anyone know what the 12000 kHz station might be?
DISCLAIMERS:
1. I'm not using the standard BITX-40 or uBitx modules. This post is not raising complaints about any product manufactured by the HFSignals Corporation. I'm sure the real boards are free from whatever defect I'm experiencing.
2. I don't speak Spanish and I'm not much of a linguist. It's possible that the interfering station is some other language like Portuguese or something else. Hopefully I've not offended anyone.
