Modern Frigates....
The Nat Turner class and John Brown class ....[/quote]
Robert_A_Woodward wrote:...But the only navies they can defeat in battle have seriously obsolete ships.
Weird Harold wrote:You mean any Navy using SLN tech?

Really the problem with Frigates is this - you cannot hold sufficient CMs and defenses in a traditional Frigate sized hull to reliably survive a combat scenario in a laserhead environment and still carry sufficient anti-shipping weapons and decent cruise capability, without reaching the size and cost of a Destroyer.
So, to make a minimum sized surviveable, long cruise combatant in 1900, the minimum mass rose above the traditional definitions of a Frigate sized hull. Small Combat oriented destroyers also disappeared for the same reason and the minimum survivable combatant became the size of what previously was a small light cruiser, with all the mass brackets sliding up 15-20 Kilotons to accomodate, with a multifunction Destroyer becoming the smallest minimum combatant, and Light Cruisers starting ~100-120Ktons in 1900s builds.
This of course has been thrown out the window by 1920, with more size creep required to counter/mount long range munitions and pods, further making Frigates outdated in 1st tier navies.