A newer connector that resembles the UHF series but is smaller is called the Mini-UHF series. (These connectors typically do not have Mil. Spec. or generic part numbers and have only manufacturer part numbers.) The Mini-UHF connectors are now seen in many lower-power and more compact radio devices.
In boating, the DIGITAL brand antennas have begun to use Mini-UHF connectors. The DIGITAL brand antennas often have a Mini-UHF cable end jack crimp connector installed on the antenna transmission line. DIGITAL then supplies an adaptor, a Mini-UHF plug to UHF plug, permitting the antenna transmission line to be connected to the UHF Jack on the radio. The pre-installation of the Mini-UHF jack on the transmission line relieves the boater of the task of installing a connector. The small outside diameter of the Mini-UHF jack connector permits the pre-installed connector and transmission line to be routed through existing conduits and bulkhead openings without having to enlarge the conduit or bulkhead openings much more than would be required for just the RG-58C/U transmission line diameter (nominal OD of 0.2-inch).
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The max OD of the connector is just 0.375-inch.
The Mini-UHF cable end jack connectors are usually crimp-type connectors, so one must have the appropriate crimp tools to properly install them. (More on crimp tools in a future addendum to this thread.)
Adapting a Mini-UHF jack to a UHF jack requires a male-to-male adaptor, a Mini-UHF plug to UHF plug. This adaptor is hard to find. After several minutes of searching, I found one source at RFPARTS.COM: an adaptor part number RFU645 for retail price $3.75.
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This same vendor also stocks the Mini-UHF cable end plug crimp connector, Amphenol part number 182112 for a very reasonable $2.85. (For details about this connector, you can download a PDF specification sheet.) I believe this connector has a solder-on center conductor contact, and a crimped-on outer conductor connection. This may be an obsolete part, as I don't see it listed in the latest AmphenolRF catalogue, and that may account for its low price.
For comparison an AMPHENOL brand Mini-UHF cable-end jack crimp connector for RG-58/U-size cable is their part number 81-116. MOUSER, a large retail distributor, has the 81-116 connector in stock at $14.33 in single-unit quantities. I don't know why this connector is so much more expensive. It appears to have a smaller overall OD than the 182112 connector, and is only 0.375-inch OD.
A no-brand equivalent Mini-UHF cable-end jack crimp connector for RG-58/U-size cable can be found for much less. For example, a website SHOWMECABLES.COM is offering these connectors for $3.49.