Another good method to ease measurement of harmonics is to build a simple diplexer (a highpass filter behind a 10dB or greater attenuator will also work) and separate out the fundamental from the harmonics; this will greatly reduce the dynamic range requirements on your spectrum analyzer.? If you put the diplexer on the low-power side of an attenuator its power requirements are reduced, so cheap components will work..
Of course this is impractical for a 160-6 meter radio, but for a single band rig is simple.? Of course you will need to calibrate out the losses but you would do that anyway.
73, Don N2VGU
Donald S Brant Jr