I live in an attic apartment, just acquired a JRC NRD-545, and am somewhat limited to what I can put outside (within reason).
I have an old folded dipole CB antenna that I might get away with mounting just outside my window. Comments, suggestions?
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When only receiving HF, the design of the antenna does not matter much, unless your receiver has notably bad filtering.
The most valuable characteristic of a receive antenna is being located away from local noise sources and structures that absorb/reflect the signal. The second thing to consider (for HF in constrained spaces) is making it as big as you feasibly can (up to 1/2 wavelength).
I encourage you to try using any and all antennas you have on hand, or can construct from wire and a connector, and compare their performance. (You can't damage a receiver by using the wrong antenna. Unless it's so wrong it touches energized metal.)
Since you say you live in an attic apartment, unless your roof is metallic, you should try running a wire dipole antenna indoors along the ceiling as close to the ridgeline of the roof as you can reach. This may well be the best antenna you can put up.
You might try using a receiving end fed balun at the end of a 45 ft wire strung by 2 insulators on each side of the wire. Don't forget a surge protector in line to protect the receiving front end of your JRC NRD-545. I have a receiving antenna for my Icom IC-746 Pro using regular RG-6U TV Coax fed about 50 ft. between the radio and the antenna. You have to search for 2 PL259 Male to F female adapters for connecting the TV cable. The nice thing about this is that it's pretty stealth with 14 AWG black insulated wire. Also the nice feature is the antenna does not need to be tuned because it's broadband from 50 KHz to 30 MHz. You might find one on ebay. I must mention there is book for Receiving Antennas by Joe Carr.