This paper deals with the achievable spectral efficiency and outage analysis of short burst frequency hopping (FH) mobile radios under heavy jamming scenarios. With the use of outage probability analysis based on cut-off rate, the maximum spectral efficiency (bits / dimension) and the required number of radio frequency bursts (RFB) (or degrees of freedom) is determined in order to transmit a message reliably for a given target outage probability, number of information bits (message information length) and constellation. A Cut-off rate approach is exploited in order to take the influence of constellation into account, and outage metric is obtained depending on the jamming state, fading, number of RFB and constellation. On-off jamming with a given probability assignment is assumed for each burst. It is shown that the message length limits the achievable rate of the system. An upper bound on the code rate as a function of message length is derived for given outage probability, constellation, and channel statistics.