Thanks for the responses guys. Sounds like a 10m full-wave loop will be quiet, have gain and wide bandwidth, and fit on my balcony. That's a pretty darn good combination. Let's hope propagation conditons cooperate. This weekend I'll try to whip up the loop and a toroidal (iron powder, not ferrite) transformer on a T50-6 core I have lying around.
You won't be putting this antenna in an ideal location if it's close to the building, but doesn't the 817 have an internal antenna matching module?
Unfortunately no. There's an Icom I think with similar specs to the FT817 that has a built-in tuner, but the Icom is larger and has less coverage than the FT817.
The noise pickup for the length of coax you'll use won't be much of a problem if you aren't plagued by noise now. [...] Don't get cheap and try to tune a loop for one band on another band. The bandwidth over which you can match will be narrow and you'll spend more time adjusting the matching network than operating.
How about for wide-band receiving - would it make sense to try and tune a 10m loop for reception on 20m or even 40m?
The reason I ask is that indoor random wire antennas (about 2-3m) at my location connected to the FT-817 are incredbily noisy (I saw S8 and above at some frequencies). I can't receive anything with random wires. On the other hand, when I connect my two-turn, shielded, small loop antenna cut for 40m and 80m (http://web.archive.org/web/200608051813 ... gloop.html
), the receiver goes blissfully silent and I can hear acceptably well on 40m and 80m - lots of SWBC and CW. However I have never heard anything on the FT-817 above about 10 MHz when using the loop - in particular, 20m always seems dead though I know there must be hams on that band when I can hear 40m hams. So, for reasons not apparent to me at this stage of my antenna study, my shielded loop is very quiet but can't receive 20m and up, while random wires are very noisy.
Thus my desire to find a good wideband receiving antenna. Can a 10m full-wave loop be tuned for good reception on other bands? If so, would something like a T-tuner be good for this?
Pick one band and stick to it while you learn operating practices. You'll make plenty of contacts on 10 M.
Sounds like good advice. The seductive thing about the FT-817 is its offering of all modes and all bands in a hand sized package. That makes it more than a little tempting to try to make an all-band antenna system as well. I think remotely-tunable magnetic loops could possibly be a good all-band antenna, and hope to investigate them more later.
One more question about operating QRP (5w or less) in the field. Let's say I have a wire antenna (dipole, quad loop, end-fed-halfwave) cut for resonance that works at home. If I take that into the field, the environment and probably the resonant frequency will change. Is this something to worry about in practice (requiring a tuner and maybe some kind of antenna analysis instrument), or does one generally just deploy home-cut wire antennas as-is in the field?