Hi,
MFJ makes a noise bridge called the MFJ-212 for $99.
Vectronics makes a similar product whose model number I do not know for $99.
These emit a pulsing noise which goes away when y ou have tuned the tuner.
They claim a match of 1 to 1 SWR when you find the null.
These go between your external tuner and transmitter.
Eric
KU3I