The in situ or on-site measurement of antenna performance is very challenging in analog Land Mobile Radio (LMR) FM systems. It turns out that the conventional antenna gain measurement techniques are hard and also not quite relevant for understanding the in situ performance. In this work, we present a a novel approach for directly measuring the in situ performance of antennas in analog LMR systems.The procedure involves receiving an FM signal simultaneously using the antenna under test (AUT) and a reference antenna. Both received signals are demodulated to audio using separate but identical receivers. Then a convenient method for characterizing the audio signal quality is to analyze the private line (PL) tone. The PL tone signal-to-noise ratio (SNR) is calculated by measuring the power of the tone relative to the sub-audio noise power. The PL tone SNR for both antenna systems is compared as it provides a "bottom line" evaluation of the antenna performance. The audio SNR can also be mapped to RF SNR using a theoretical method. From simulation and experimental studies, we conclude that the estimate RF SNR is within 0.5 dB of the actual value for RF SNR values between +3 and +36 dB.