Before I got use of a spectrum analyzer with a tracking generator, I used a UHF Rf modulator as a signal generator. They can be found on auction sites for $20 or less and most will tune from ch14-69+. They're by no means calibrated outputs but for doing comparisons they work just fine.
Mine was able to transmit a long enough distance to test far field readings on most UHF antennas, you might be able to use a disto amp to get a little more signal if needed.
For UHF you don't need to transmit too far to get valid measurements.
ATSC modulators are very expensive.