Living in an apartment, I'm struggling to get good antennas set up, especially for HF. Worse yet, I'm not allowed external antennas (but am willing to try something low profile on the balcony), my apartment is built with stucco (with the "chicken wire" embedded in it), and my window frames are all aluminum. I have a balcony, but it's got a low (3') wall made of stucco inside and out (2 layers of mesh!) topped with a metal railing.
I tried a dual-hamstick dipole for 20M, but was placing it only 24" over the balcony floor, and this is below the edge of the wall. Tuning it was basically impossible, though I eventually could get 2:1 at one frequency. The balcony is about 4 feet deep and 10 feet long, and there is another floor above mine (so no roof access, and there's poured concrete above).
So how far away does metal have a significant impact on my antennas? I'm assuming it's design- and frequency-dependent, but I'm looking for resources that can help me figure out approximate clearances that are needed. Mostly I'd like to work at 20M, 10M, and 2M. (My balcony is way too small to think about 40M and lower.)