My HF-225 is sufficently sensitive at 136 kHz to be operated on its own
without preamps and its 200 Hz CW filter is good enough. Have never noticed
intermods etc even putting a big antenna on it. Have used it quite a lot
away from base with only a small 2m whip which works well.
My ICOM IC-756PRO (Mk. 1) is just about sensitive enough on its own provided
I use a big antenna, but then it's very prone to intermod problems from LW
B/C. You can hear background chatter from the B/C's all the time. With a 25
dB preamp that has 500 Hz selectivity ahead of it it's fine, but the
outboard selectivity is essential to get rid of the intermod.
My Kenwood TS-870 is just not sensitive enough on its own even with a big
antenna (can hardly hear DCF!) so it needs the outboard preamp/selectivity
like the 756.
Guess the last two were never designed for LF. The HF-225 is my preference
considered as a bare-bones LF receiver.
I also have an AR88LF which is fine on sensitivity and selectivity but
drifts far too much to be usable for narrow-band stuff.
I've also tried an old R1155N which tunes down to 75 kHz on 136 but its
selectivity and frequency stability are awful. That's an old WW2 RAF
receiver for those who don't know it.
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