We find in general th a t th e synchrotron lum inosity is only a few percent of th e je t power. This ra tio is in d ep en d an t of th e radio pow er for our small sam ple of sources. Rawlings (1987) has estim ated jet powers for a large num ber of 3C R radio sources. He finds a linear relationship of je t pow er w ith synchrotron lum inosity over th ree decades in radio lum inosity. Rawlings also finds th a t the ra tio of radio lum inosity to je t pow er varies betw een 1% and 10% depending on th e environm ent of th e radio source. R adio sources in rich environm ents w here th e inferred density of th e interg alactic m edium is large, have a larger fractio n of synchrotron lum inosity to je t pow er th a n do radio sources in less dense environm ents. All th e sources we have considered, except PK S 1254-300 an d PK S 2104-256, are field sources. Hence our inferred ra tio of synchrotron lum inosity to je t pow er of a few percent is in excellent agreem ent w ith th e results of Raw lings (1987). We n ote th a t th e ra tio of sy n ch ro tro n lum inosity to jet pow er for PK S 2104-256, a t ~ 16%, is som ew hat larger th a n th a t found for any o th e r source in o u r sam ple an d in th e sam ple of Rawlings. This m ay indicate th a t PK S 2104-256 is in a p articu larly rich environm ent. A nother explanation is suggested by th e very long tails in this source (C am eron 1988) which m ay be evidence for a very strong lobe backflow w ith respect to th e in terg alactic m edium . It is not unreasonable th a t o u r energy budget analysis has u n d erestim ated the je t pow er for this p a rtic u la r source because of o u r neglect of th e backflow.
Bicknell et al. (1989) analyse th e energy budgets of a selection of class I ' adio sources and find ratio s of radio lum inosity to je t pow er of 10% up to about %. This is a very m uch higher fraction th a n in th e class II sources studied by Raw lings (1987) an d ourselves. M oreover we note th a t our results agree extrem ely well w ith those of Rawlings (1987) even tho u g h our class II radio sources lie closer to th e class I /I I tra n sitio n region. Given this observation an d th e observation th a t Raw lings notes no a p p aren t tre n d in Tradio/Fe w ith radio pow er, we suggest th a t th e ratio of radio lum inosity to je t pow er is in d ep en d an t of radio power (at least in th e class II sources) an d depends p rim arily on radio m orphological class. We acknow ledeg th e dependance of Tradio/^E w ith environm ent found by Rawlings (1987) b u t note th a t th is a p p ears to be only be a secondary effect.
This concludes ou r energy budget analysis for th e selected radio sources. T he rem ainder of th is ch ap ter deals w ith th e radio cores, an d th eir relationship w ith th e large scale p aram eters of th e radio sources in ou r sample.
The core components.
In c h ap te r 3 we p resen ted VLA fluxes and sp ectral indices of the arcsecond scale cores. We also p resen ted the results of long-basline interferom etry on a subsam ple of radio sources. O ur m ain conclusion th ere is th a t th e cores w ith flat sp ectral index ( a < 0.5) have sim ilar flux-densities as observed w ith the VLA an d w ith th e P arkes-T idbinbilla Interferom eter (P T I). T he flat sp ectru m cores are unresolved on arcsecond an d m illiarcsecond scales. Conversely, th e steep sp ectru m cores ( a > 0.5) have system atically lower P T I fluxes indicating th a t th ey are largely resolved on m illiarcsecond scales. We also gave evidence th a t th e d istrib u tio n of core sp ectral index is bim odal, suggetsing th e existence of two d istin ct p o p ulations of radio core. In this section we discuss th e tw o classes of radio cores in relatio n to o th e r p ro p erties of th e radio sources.