I live in an apartment with a metal rail patio very similar to the attached photo. I want to put a Diamond X30A on the patio but the radials will be about a foot below the top rail. Will this work, or an I better of with a simple 1/4 wave?
Placing the antenna on a tripod on your balcony should not be a problem. It is always a good idea to perform an RF exposure assessment when locating a VHF antenna close to people.
How well the antenna will work is another matter. At VHF frequencies, most of the reliable communications comes from direct, line of site paths. When station antennas are in line of site, even very modest power levels and antenna gains will typically facilitate communications.
In some situations, reflections, refractions, and scattering may enhance or reduce the reliability of communications. Often these tend to be unreliable due to changing foliage conditions, weather effects, and changing landscapes.
Walls in buildings tend to reduce VHF communication ranges. While your balcony will provide a clear shot in one direction, the balance of your directions are to some degree attenuated by your building.
You may find it advantageous to use an HT from various locations in your apartment and on your porch to determine your effective repeater and simplex coverage - both transmitting and receiving. If you find that you have some degree of omni-directional coverage from your porch, your chosen antenna should be adequate. If on the other hand, you find that the only practical coverage is in the direction of the porch opening, you may wish to consider a small Yagi antenna in order to maximize your gain in that direction.
Definitely better than a 1/4 wave vertical if only because of the radial requirements, but also because of increased gain.
It needs very little groundplane.
A 1/4 wave antenna would require much more, and would intrude into and extrude outside the railing.
So the Diamond would be the better choice, except maybe for its 4 and a half foot height.
I would not suggest mounting in any way that hides part of it behind the railing. You sat the radials would be below the top of the railing so I presume you mean outside. That could work well.
If the top is touching anything I wouldn't suggest running the full rated 150 Watts through it. It may experience some detuning, at the least. This antenna is meant to be mounted in free space. For that matter, so is a 1/4 wave groundplane.
The reviews I find on it are all excellent.
It has more gain so should outperform a 1/4 wave vertical.
If it were me I might be tempted to paint it black if there is the possibility of neighbor complaints.
I agree that the Diamond X-30A would be the better choice. I think it should be mounted outside the metal rail using something like this off-set or cranked mast.
The lower half of the mast would be strapped to the balcony's uprights, preferably in a corner, and the antenna would then be mounted such that the ground radials would be level with the top of the rail.
I suggest the use of metal clips such as circlips to attach to the rail, cable-ties are prone to damage by UV and you do not want the antenna and mast to fall from the building dragging your radio with it :-) https://www.ebay.co.uk/sch/i.html?_odkw=circlipd&_osacat=0&_from=R40&_trksid=m570.l1313&_nkw=ubilee+clipsd&_sacat=0 Another possibility is to use U-Bolts around the upright and into the wall bracket provided with the crank-mast.
This may take the top of the antenna into view if there is a flat above, the dimensions are not that clear.
If the radials are a concern, consider using a J-pole so there won't be a need for radials. However, I think the bigger problem is the operating location. That balcony will pose a problem for any 2m/70cm antenna due to the close proximity of the safety rail and rebar in the concrete construction which is in the antenna's near-field. The antenna will of course radiate, but will not be as advertised from the vendor or in reference literature.
It is difficult to say with any certainty what the pattern or gain will be without modeling the setup or taking measurements. Modeling the antenna in the operating environment will be somewhat complicated and requires specialized software/tools and a knowledge of the building and materials near the antenna. Taking field measurements is simpler, but will require help from a couple of friends in different locations with an S-meter.