Frequency Range: 1 to 25 MHz
Manufacturer: PlesseySome notes about the FH-4 are provided by John Wise:
In the RN, outfit FH4 was being used ashore in 1943 and did not get accepted for shipborne use until later.
According to the FH4 handbook BR 1569, the system had five RF ranges: 1 to 2, 2 to 4, 4 to 8, 8 to 16 and 16 to 25 MHz with an actual RF coverage of 0.96 to 25.5 MHz, hence five sets of plug-in coils for the early sets. There were three IF stages all running at 450 KHz, although not stated, there is an equation which suggests that the low frequency oscillator tracks high. So if the RF was turned to 5 MHz then the LO would have been at 5.450 MHz. The test oscillator also operated at 450 KHz, so it would not directly interfere with any RF signal but it would be immediately accepted by the IF stages, regardless of the tuned frequency. An elegant solution.According to BR 1569, the system is designated FH4, the 4th High Frequency DF system using a B-T frame coil antenna. The individual components of which are the receiver - FHB (APW 6142 - without gyro, APW 6142A - with a gyro), the Power Supply Unit - PSU for FHB (APW 6143), the Voltage Regulator (APW 5981) and the Frame Coil - S25B (AP W6118A).
AP = Admiralty Pattern, and subject to confirmation, W = Wireless, a designator which was dropped by 1950.
It appears that at least four aerials were associated with the FH-3 and FH-4 HF DF systems. They are designated S16, S18, S25 and S25B. The only difference between the S16 and S18 is their size. The S16 was crossed diamonds with a 3 foot aperture, usually fitted on top of a pole structure. S18 had a 4.5 foot aperture and was often integrated into a mast structure. The S16 had a short 'spike' on top - this was not a sense aerial but a lightning conductor. S16 was used on corvettes, frigates, destroyers while S18 was fitted to cruisers, carriers, battleships and depot ships.
The FH4 had a cathode ray scope for diametric visual bearing indication and was superior to its predecessor due to the ability of being able to visually distinguish between the sky wave and the ground wave from the lissajous figures on the CRT. Its scope was as big as a pie plate, and was surrounded by a compass rose. Accuracy was variable from 2 degrees plus, dependent upon emitters range and the prevailing propagation. The unit was powered by a 230 VAC 50 Hz mains source with a voltage stabiliser. The FH4 was connected to a Bellini-Tosi aerial array consisting of fore/aft (F/A), port/starboard (P/S) loops and a vertical sense aerial. The initial sets were designed with five aerial/RF coils that had to be changed for different frequency ranges. In 1944, the set was improved by the addition of a pair of band changing switches, one each for aerial coil and RF coil selection. The twin channels shared a single intermediate frequency unit, which saved a significant amount of channel balancing in gain and phase.
VARIANTS
So far, three FH-4 variants have been identified. The FH-4 type 1 has a raised front panel (see B71 photo) without the two band changing switches. The second version, FH-4 type 2 has a flat front panel and no band changing switches. Neither the Type 1 or Type 2 originally had a coupled gyro repeater. This only came with the introduction of the Type 3 in early 1944. However, to overcome a serious lack of material shortages around that time (Collingwood Archives, ASE Bulletin RH600(1) March 1944, page 37), a modification kit to give the FH4 Type 2 a gyro was introduced.
Type 3 Variant has a flat front panel with two band changing switches (switched coils). These had to be switched in unison to ensure that the antenna and RF selections were the same for a given frequency band. Type 3 was a significant improvement in having both a gyro and aerial and RF band switching.
John Wise has compiled a summary listing of the FH4 DF variants in a JPG spreadsheet which can be viewed here. John is looking for pictures of some of these variants in order to confirm the findings and also to identify any APW, AP or simple Admiralty Pattern numbers that are appended to each type.
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| FH-4 basic installation. The power supply is at the top right and the mains voltmeter/variac is below the power supply. At the very right is the antenna junction box. At the top of the FH-4, that box is the bearing ring repeater drive from the main gyro, so that true bearings can be noted. (From FH-4 handbook BR1613). |
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| An FH-4 installation as seen aboard HMCS HAIDA in 1946. (Photo courtesy HMCS HAIDA National Historic site.) |
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| This FH-4 can be found in HMS Belfast. (Photo by Øyvind Garvik) |
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| On this B-71dual channel receiver/display S/N 6 , note the 1.5 inch raised section in the middle of the front panel. It is believed that the early models has this raised section, with later versions being flat.This unit is on display at the National Cryptologic Museum, Fort Meade Maryland. (Photo by Jerry Proc) |
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| The FH-4 series all used an external, stabilized power supply. They were usually blue in colour but sometimes came in light gray finish like this example. (Photo by John Wise) |
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| Rarely ever seen is the test signal injection unit used for the initial setup of the FH-4 and also for completing checks after any major work to the system such as the fitting new IF coils. This example is one that was recently salvaged and now resides at HMS Collingwood Museum. (Photo by John Wise) |
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FH-4 antenna. Click on image to enlarge. (Source unknown) |
OPERATING INFORMATIONRemarkably, Norman Langley, (e-mail: norman(at)langley45.freeserve.co.uk) was able to provide the FH-4 Operator's Drill procedure:
1. Check Ht and Grid Bias Neon have struck. Voltage to read approximately 200V. Check Gyro.
2. If attenuator at minimum. Check brilliance, focus and centralize spot.
3. All Comparator switches set to off.
4. Range switches to correct frequency.
5. Tuning dial to correct frequency. (Detune if voice signal).
6. BFO control upright.
7. Master Gain upright.
8. Master Phase upright.
9. IF attenuator to position, just giving noise indication on CRT.
10. Test Oscillator switch to "TAE" position.
11. Test Oscillator control switch to FULL
12. Adjust trace to full scale with Test Oscillator Output control.
13. Remove any ellipsing with Master Phase control.
14. Adjust trace to 45 degrees with Mater Gain control.
15. Switch TO to "BFO" or "OFF" dependent on signal.
16. Tune to signal.
17. Sense signal and note Relative and Gyro bearing.
18. Apply correction from chart and check percentage blurring, using relative scale.
19. Report corrected bearing.
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| FH-4 control description from B.R. 1442 Operating Instructions for D/F Outfit FH-4 with switched coils", dated November 1945. (Submitted by Øyvind Garvik) |
APPLICABLE FH-4 MANUALS1. SS 132 FH-4X (experimental (?)
2. BR 1382 FH-4 with plug in coils
3. BR 1442 FH-4 with switched coils
4. BR 1569 (ex CB 4333) FH-4 with plug in coils (and probably an update of BR 1382) (1943)
5. BR 1569 A (Addenda A) Update to the basic book (1944)
6. BR 1613 FH4 with switched coils.
AH6 DF OUTFIT
Initially the AH-6 was used for shore based DF work before the FH-4 was approved for use in ships. From 1947 onwards, the AH-6 was used by British Naval Air Stations. The pattern number was AP 55357 and the FHB power supply unit (Design 2) was AP 55663.
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| AH-6 Outfit. The position of the goniometer is most puzzling . Why is it at right angles to the display and so far away? Contact jerry.proc@sympatico.ca (From a BR manual) |
A word about Plessey, the manufacturer of the FH-4, from John Wise:The FH4 series was developed and built by Plessey at their Ilford Plant in the outer east end of London during WWII. The Germans knew about this production plant and bombed it quite early in the war. As a result of repeated bombings, the government was approached with a request to consider moving production underground.
At that stage, a new Central Line underground railway extension line was being constructed out towards the far east end of London and its eastern suburbs. The tunnel had been built but the rails had not been laid in and there were no plans to complete the work during WWII. Instead, approval was given for Plessey to move its production into 5 miles of the tunnel system.A brief contact with an ex-Plessey employee told me that Plessey management apparently became very touchy about the possibility of the Germans over-running the UK and considered that London might be their first objective. The result was that Plessey destroyed all development system paperwork and soon as an equipment went to production, and then destroyed production records as each product run completed. yet!
We may never actually find any solid information about the RL 135 receiver and whether it was a single or dual channel system sharing a single local oscillator like the RL66. (FH4). This information came from a single telephone conversation with an elderly ex-Plessey contact who I had hoped to meet but sadly he died a short time after our conversation.
During WWII, Plessey Ilford employed 11,000 personnel and by 1946 that number had dropped to just 6,000 because it lost so many military contracts at the end of the war. As a company, Plessey no longer exists.
Contributors and Credits:1) Øyvind Garvik <oygarvik(at)online.no>
2) Norman Langley, < norman(at)langley45.freeserve.co.uk>
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