I'm a General Class Licensee, and due to pulmonary disease I cannot operate voice. I am also about 540 feet from a 24/7 1 kW AM station. The signal is so pronounced that most audio devices in the house pick up the station. Every shortwave receiver I've tried overloads (without an antenna) from 1-30 MHz. Any suggestions?
First, find a transceiver which is properly shielded. When connected directly to a dummy load, there should be no breakthrough on any frequency (except the MW transmitter itself). Portable broadcast radio receivers are completely out of the question, but fairly recent ham gear should be ok.
Then, try a highpass filter to reduce the impact of the MW. I'd suggest starting on one of the higher bands, 15 m or 10 m. A simple highpass filter can be made with a quarter wavelength short circuit stub. On 15 m that's about 2.2 m of coax. Ideally for the stub you should use double screened coax like RG223, RG400 or hardline etc. Solder the stub into the radio end of a short jumper lead. A better filter can be made with more stubs or with discrete components.
Finally, take some precautions to reduce the pickup of the RF in the shack. Keep loop areas small - run coax, 12 V and power cables close to each other. Make a filter or trap for common-mode currents on both coax and power lines that enter the rig. Ground the coax shield to shack ground as it enters the shack. (forget about ground stakes, you're trying to keep differential voltages to a minimum). Use a horizontal dipole antenna, broadside to the MW source, as the MW fields are mostly vertical, and still somewhat radial as you're so close. Use a balun with good choking well below your HF frequency.
You may still be in trouble, if there are any non-linear effects in any gutters, downpipes, fences or electric cables outside anywhere near the transmitter. They could cause significant harmonics and intermods on your HF frequency, which cannot be filtered out.
This is not necessarily an impossible problem. You might approach the problem iteratively, with a simple monoband receiver and expand from there as you see what works. Assuming you are able to solder, you could start with a 40m SoftRock receiver kit for $21, which plugs into your computer line-in sound card input for decoding of digital modes. On 40m (7MHz) you'd receive stations hundreds to thousands of kilometers away with a simple outdoor antenna, particularly on sensitive digital modes like WSPR and JT65. I would build that receiver inside a metal box, and start with a few meters of wire as an antenna.
If you don't have interference with no antenna and short antennas, but longer antennas bring interference, consider building a medium-wave bandstop filter, like these demonstrated to attenuate AM broadcast by over 50 dB.
Finally, consider that after filtering the fundamental on-channel AM transmitter, the N-th harmonics of that transmitter will be present and will have N times the bandwidth of the fundamental channel. If the AM transmitter was on 1000 kHz, it will have significant modulation energy from about 995-1005 kHz (more if using HD Radio transmission). The second harmonic will spread from 1985-2015 kHz and so on.
Why not try a good small transmitting "magnetic loop" (like LNR's W4OP Loop, the AlexLoop, the AlphaLoop, etc.) along with some RFI protection around your transceiver? A narrowband directional antenna like that should help with half the problem, so you can focus on cleaning up the other power/data/microphone "inputs" on your transceiver.
While your situation definitely sounds dire, with a careful station design it should not be insurmountable.
Consider that a typical 2m repeater shares one single antenna between its transmitter and receiver. That is, it is simultaneously pumping a 25–100W transmission into the antenna over the very same coax used to receive the teeny tiny signal from a handheld in the next town. Not to mention, those two signals (the 25W output and the picowatt-level input) are half a percent away from each other, frequency-wise!
[Aside: If you changed the tuning of a guitar or violin the same relative amount, you'd barely hear the difference. If we kept track of notes up that high, the difference between 146.2 and 146.8 MHz would be only 7 musical cents. This is hardly past the average audible threshold of five cents. Put another way, there's about 100 cents between a piano's middle C and the black key right next to it — the entire US 2m allocation could fit twice within that musical half-step and the 0.6MHz offset just a tiny offset within.]
All that to say, in many ways a repeater inherently deals with much more severe interference than what you face. It's true that expensive duplexers and often additional filters are needed there, but in your case you have the benefit of signals that should be several octaves apart! It's a whole lot easier to keep a 550m or a 175m signal apart from a 40m signal, than keeping a 2.044m signal separate from a 2.052m one.
There's two things you need to avoid:
- signals overloading within your receiver
- passive intermodulation around your house
The first is completely within your control:
- You need a well-shielded receiver, which will hopefully be designed against RF signals coming in via the power supply or accessories but may need some "shoring up" in those regards due to your situation. Troubleshoot this via a dummy load or cap on the antenna port.
- Then you need to make sure interference doesn't come in via your antenna system. The most obvious way is that your antenna itself will pick up the signal. Adding a good filter (shielded and capable of handling the broadcast power levels) should help in the case of a broadband antenna. There are also some antennas that are inherently very "narrow band", such as a resonant small loop.
- A less obvious way can enter your antenna system is if your antenna is unbalanced.
This and more is all described in a much more authoritative way in "A Ham's Guide to RFI, Ferrites, Baluns, and Audio Interfacing" by Jim Brown K9YC. I've been trying to learn this stuff for a couple years now, and I still find that reference somewhat intimidating — when he says "You should study this tutorial thoroughly" he means it! — but he ends up boiling things down to practical actions. That guide is a perfect fit for your situation, though!
The other part is a bit out of your hands:
Even with a good station design, you'll still need to avoid that "Rusty bolt effect" of passive intermodulation. This one isn't as much in your control, since you may have little "transmitters" on all sorts of frequencies sprinkled throughout your neighborhood. A directional antenna will help here, and/or some additional "notch" filters for signals that are "in your way" on a particular band.
You might also try strike up a friendship with the broadcast engineer of the station. He might be just as interested in tracking down strong sources of intermodulation around their transmitter, or at least have more advice. His radio shack is 540 feet closer to the broadcast than you are!