In 1943 this was a highly confidential set. This, the equipment handbook points out that the Operational Use was described in another confidential book - CB03067/42 which the Museum does not have/does not have access to. Moreover, the Type 91 was a transmitter and its associated receiver, the Type P29 [RAF] Navy Pattern No W4794, was described in yet another separate handbook, H.469, also not held.

Essentially, the Type 91 was a VHF {of those days - now very much VHF, UHF, EHF} set operating in the band 200 to 600 Mc/s as a BEACON or TRANSPONDER of a sort, and at these frequencies [line of sight] for aircraft.

The total wavelength [of a frequency band 200-600Mc/s] is 50 150 cms and wavelength adjustment is dealt with in two stages. Referring to the picture below.  For 50-100cms, the LINKS, one in each aerial cone [more in a minute] are removed, and for 100-150cms are in place. What appear to be broom heads [just to the right of the links] are called SLUGS, and their positions, in or out, short circuit the transmission lines by a measurable amount thus altering the wavelength [frequency].  Both the LINKS and the SLUGS form the coarse tuning of the set. To the left of each link there is a variable control which acts as the fine tuner.

The picture beneath is the complete wiring diagram

The set produced an un-keyed, automatic [when HT was switched on] MCW emission modulated by a oscillating valve circuit with a frequency of 1Mc/s. 

It would appear from the following drawing that the RAF receiver mentioned above, the P29, with a 360 degree wrap-round aerial, became the FV1, the first of the VHF D/F sets used in the air world especially in aircraft carriers and NAS. Here, in the following picture you can see the Type 91 TX [W3747A on the left] attached to its HORN aerial and the Associated RX [W4794 on the right] attached a HORN aerial mounted 4-foot above the TX Horn as close to the top of the ship as possible as well as to the FV aerial.  The outfit W4262 selects the desired aerial for the the P29.  The listening earphones are attached to the RX.

Notice the Rank, a Signal Lieutenant Commander