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The 8405A Vector Voltmeter measures voltage vectors in terms of both magnitude and phase over the frequency range from 1MHz to 1GHz.  The 2 Probes have a high input impedance of 100KΩ.  The 8405A cost $4,975 in 1982.
Front view with attached twin probes
Rear view
Top view with cover removed showing modular construction
Bottom view with cover removed 
Frequency Range: 1MHz to 1GHz in 21 overlapping octave bands.
Tuning: Automatic within each band, 10ms approx.
Voltage Range Channel A: 1.5mV to 1Vrms 1 to 10MHz; 300µV to 1Vrms 10 to 500MHz; 500µV to 1Vrms 500 to 1000MHz.  Can be extended by a factor of 10 with 11576A 10:1 Divider.
Voltage Range Channel B: 100µV to 1Vrms 1MHz to 1GHz.  Can be extended by a factor of 10 with 11576A 10:1 Divider.
Input Impedance: 100kΩ shunted by 2.5pF.  1MΩ shunted by 2pF with 11576A 10:1 Divider.  100kΩ shunted by 5pF with 10216A Isolator.
Isolation between Channels: >100dB 1 to 300MHz; >80dB 300 to 1000MHz.
Maximum ac Input: 2Vp or 4Vpp.
Maximum dc Input: ±50V.
Voltmeter Ranges: 100µV to 1Vrms in 10dB steps. 
Phasemeter Ranges: 360° indicated on centre zero meter with end scale ranges of ±180°, ±18°, ±6°.
20kHz IF Outputs: Reconstructed signals with 20kHz fundamental components, having the same amplitude, waveform and phase relationship as the input signals.  Impedance 1kΩ in series with 2nF.
Recorder Output Amplitude: 0 to +1Vdc open circuit proportional to voltmeter reading.  Impedance 1kΩ.
Recorder Output Phase: 0 to ±0.5Vdc open circuit proportional to Phasemeter reading.  Impedance 1kΩ. 
November 2011
I had not intended to buy one of these units but eBay had one going at a bargain price.  I will see if it provides any useful measurement capabilities against the 8407A & 8410C Network Analysers, both of which are 50Ω systems. After approx 30 years, the 8405A is in good condition with no mechanical damage and the probes are both intact.  The inside of the 8405A is very clean but the ouside case and panels have a lot of stickers and tape to clean off.  Downloaded (US Army) Technical Manual.
February 2016
More cleaning-up.  Downloaded Operating and Service Manual but missing Section 7 Schematic Diagrams, although the Technical Manual includes them.
Need to test with Section 3 Operating Instructions and Section 5 Maintenance.  Power-up checks ok.  Adjustments throws up a few problems. See Repair HP 8405A Vector Voltmeter for details.
The Probes have a blue ring to identify Channel A and a white ring to identify Channel B. Tips are screw on 5020-0457.  Other accessories which would be useful are the: 11536A 50Ω Tee; 11576A 10:1 Divider; 10216A Isolator; and Grounding Clips.
The 20kHz IF Outputs for amplitude and phase are useful for displaying waveforms on an Oscilloscope.  This is a simple way of looking at waveforms of too high a frequency to normally see on an Oscilloscope.