Like all things man-made there is always a prototype made and tested before the volume manufacturing starts.  Naval equipment is no exception and before we talk about the FH4 System, let us have a brief look at the FH4X System, the 'X' standing for EXPERIMENTAL.

The handbook on the experimental set has not survived and all we have are the operating instructions and a couple of line drawings of part of the system, and one of them a components layout of the FHA power supply which we will give a miss to.  The original experimental receiver was the FHA which was also called a B71 receiver. The front panel is quite different and there are three fewer operator controls on the FHA than on the FHB but several more  switches and knobs on the left and right hand sides of the receiver than on the FHB.

This file tells you what each control is called FHA NUMBERED INDICATORS TO EACH SWITCH.pdf and this file shows the FHA FHA.pdf.

The following description CLICK and this photograph
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will be familiar to all who understand H/F D/F which was colloquially known as Huff Duff. The 'UA Series' of D/F sets were referred to as SHUFF DUFF, but, MUFF DUFF was never coined to mean the 'FM Series' !

HOWEVER [and FULLY applicable to other outfits, FM12 for example] the picture above IS NOT A FH4 ! 

It is a picture of a Receiver called the FHB [in the case of the FM12, the equivalent receiver is the FMB,  where 'H' means H/F and 'M' M/F]. So what is the FH4 ?

An FH4 is a SEA-GOING HF D/F OUTFIT comprising of the FHB, its aerial {always sited to ensure that its polar diagram was virtually free of nulls throughout 360˚of azimuth, implicitly suggesting on top of the tallest mast in the ship }, and its power supply unit.  Whilst the FH4 Outfit/System bagged the best place onboard for its aerial, the Type S25B, the rest of the parts making up the system had to be accommodated in a suitable space in the ship known variously as the HUFF DUFF Office, the EWO [Electronic Warfare Office] the 3rd Office, and others.

There was a requirement for such an OUTFIT to be fitted ashore on terra firma, and one such user-requirement was at a Royal Naval Air Station [RNAS]. 

 When fitted ashore {in Royal Naval Air Stations for example} there was enough room to give the system its own space and operating environment, so they built a building specifically for the purpose of Huff Duff. Into that building came the Receiver FHB now with its own purposely designed operating desk etc., and the aerial was placed on top with a goodly advantaged position. This shore system was called D/F OUTFIT AH6.  Here is a picture of the building and its environment as created ashore.  There are no dimensions, but clearly the system operator {or the FHB operator} sits in the "hut" sited  in the middle of a fenced compound.

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The following two files show the full AH6 system and elaborate on the description file shown above covering more fully the operator controls fitted to the Receiver FHB.  AH6 OVERALL SYSTEM.pdf and AH6 FHB DESCRIPTION.pdf.

In the early post war years, this nomenclature could have been confusing for some users, one of whom was the Fleet Air Arm {FAA} pilot seeking to ascertain his position vis--vis an air-field, be it a RNAS or an aircraft carrier. The following small input is taken from the 1949 edition of the Manual of Naval Airmanship AP [N] 71 {Air Publication [Naval] 71}.

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Was the pilot using an Outfit FH4 operator or an Outfit AH6 operator for the bearing, the reciprocal of which he would fly down until reaching a safe haven ?   Obviously he couldn't have cared less what the answer was as long as he was back in the wardroom sipping his pink-gin at the close of play.