The /ɑ/ vowel is usually spelled <ar> but we also find <a> in ta. The schwa, of course, can be spelled in a great many ways, not all of which are uniform in widespread usage. Thus the unstressed indefinite article an can be shown as 'n or un or 'un, while the hesitation sound /həm/ is represented in this dictionary as h'm but might just as well be hem or hum.
Since the schwa is always unstressed, while /ɑ/ is usually prominent, there are very few minimal pairs. Only two are proper minimal contrasts of dictionary words, namely forearm/forum and radar/raider. In the case of the hesitation word h'm, it is likely that the inflected forms would be pronounced with /hʌm/ rather than /həm/. In the other cases we have a contrast between a word and a weak form, i.e. the form of a function word that would occur in rapid speech. Isolating such weak forms for a contrastive exercise is a completely artificial endeavour.
With such a short list it makes no sense to calculate a density value or semantic loading.
ah a arm 'em cars cos forearm forum forearms forums harm h'm harmed h'mmed harming h'mming harms h'ms radar raider ta to
John Higgins, Shaftesbury, November 2009