Radar Interference Suppression (RIS)

A

RADAR INTERFERENCE SUPPRESSION (RIS)

© RN Communications Branch Museum/Library

There are quite literally scores upon scores of three digit groups in the R.N., nomenclature lists. Every Branch has them; some are great (FAB meaning Frequency Availability Broadcast) for example, some make you think of the dog back home (LAP meaning Line Access Protocol - either sitting in yours or at its drinking bowl)....no, not the wife - dog would be inside inverted commas to convey that meaning. All young (first time at sea) telegraphists sat down in front of a receiver (B28 in my case) which had either a switch or a rotary control knob clearly marked "R.I.S" Its meaning , once pointed out, was obvious, and gave to the new, nervous and yes, gullible operator, a device which could control the other end, the transmitting operator. Here we are talking about Morse Code, where the transmitting operator SHOULD HAVE sent his message at a speed commensurate with the reading speed-skill of the slowest operator, but that rarely happened, so this control got a lot of use. It could be turned to the left or to the right or could be gently nudged or pushed-in to take effect. When turned to the right it indicated to the distant operator that a RISE IN SPEED was required and to the left, a REDUCTION IN SPEED. If the distant operator ignored these commands, then, when he had finished, by pushing the centre of the control gently, the distant operator would receive the signal "REPEAT INDICATED SECTION" and you would add that part of the signal with your Morse key. As boys' in HMS Tintagel Castle, we were unlucky enough to get duff switches for whilst the short push in the centre worked, the turning motions didn't. Anyway, credit where due, we were told about this defect regularly, but no attempt was made to put new switches on the receivers. I will always remember that ZBM OP-SIG meaning that something {our switch I believe} is duff and should be replaced immediately ! The ship didn't sink, so we weren't doing anything terribly wrong !

If you have ever had the privilege of crawling over the upper deck [deck 1] and the decks above it [decks 01, 02 upwards etc] of a warship, you will be struck by the lack of space remaining and the numbers of pieces of kit already up there in terms of boats, superstructure, radio aerials [vertical and horizontal], radar aerials [all shapes and sizes], sensors, rigging, weapons, rocket firers, fan intakes, fan exhausts et seq. You may even have wondered about how it all operates and functions properly in such a restricted and claustrophobic environment, and if you did, I can tell you that it does, but not without a considerable amount of 'tweaking', additional extra and unseen equipment, and central management. Mainly because of our Museum's title, we are going to look at Radar and Radio to find how they lived and worked together, remembering that Radar 'squirts' a great deal of RF power around the place although its aerials are usually well above head height and cannot be touched. Radar aerials have three problems, namely the movement of a rotating piece of metal, the deadly RF BURN [just think of a microwave oven and your dinner tonight] because of the frequencies involved and the immense radiated power. Radio aerials don't move [our satellite dish{es} which is locked on to the satellite possibly many tens of thousands of miles up in the sky, moves so slowly that it often cannot be discerned, and anyway, it is covered over by a radome], and all aerials abreast and forward of the bridge are receiving aerials anyway. W/T transmitting aerials {aft of the bridge superstructure} have suitable deck paint-markings and bulkhead notices posted, that their two problems, power, albeit not lethal, and radiation, are not considered a health and safety issue even though they are very accessible if one so wished, if, and like everything else, the rules are obeyed.

Apart from power outputs [a radar in hundreds of kilowatts and a radio in hundreds of watts] there is a good gap in the frequencies they each use. The inherent interference caused frequency wise between radio and radar, is addressed by the ship designer, and the equipment is fitted into place using exacting placing and measurements employing hundreds of filters which require little if any maintenance over the whole of their lives, or until the next major refit when old equipment is replaced by new. Extensive trails are carried out at post build and at intervals thereafter, to assess the levels of interference within the ship, and these are called WEMIT's {Weapon Electrical Mutual Interference Trials}. What the designers/engineers cannot build-out of the ship, they are aware of, and it only remains to contain the nuisance and know of its bounds: if, by WEMIT's, the measurements concur with previous WEMIT's, then there is NO PROBLEM, even though, in reality, it would be better for all to have a 'DEFECT' FREE ship. This is the same criteria applied to TEMPEST which we briefly discussed under the RATT AT SEA IN THE ROYAL NAVY - PART TWO file.

Assuming that all is well and that the 'ailments bag' has not grown [it almost certainly will not have shrunk] then the RADIO man and the RADAR man can sleep easy and enjoy a drink together in the mess. That, believe it or not and from great experience, is the situation for 99.9999% of the ships commission.

Now, back to aerials and the ones that cannot be touched and those that can, but it would be fool-hardy to do so. Perched high on the fore mast, main mast and mizzen if fitted, at the end of each yardarm, sited upward looking and downward looking are UK/SRA 102 aerials, what we originally knew as AJE's. They are UHF aerials [225 to 399.9 MHz] which are fed to resonators for UHF common aerial working. Usually, the ones fitted on the fore mast are the receiving resonators and those on the main mast, feed the transmitter resonators. Throughout the period of STANDARD TWO ships [see transmitter page] COMIST, ICS1 and ICS2, the navy had a Type 965 warning radar [see Radar pages]. It was fitted to many ships [frigates upwards] and operated on a frequency of 216 to 224 MHz. At peak power, it radiated 450kW [that's in excess of 450 individual ship radio transmitters aerial power], and, because of its aerial weight [Type AKE1 or 2 - single of double bedstead - see Aerial page] it had to mounted on the mast which was the nearest midships mast, usually the main mast. From close up to this aerial it was enormous, and remember that it rotated in a continuous circular pattern and even in the roughest of weather. It weighed 2.5 tons, was 26 feet wide, 17 feet high and just over 6 feet deep. A formidable sight to see. The transmitting UK/SRA/102's were on the yardarms of the main mast, just a little bit underneath the line of the rotating aerial, but not within the circle transcribed by the aerials rotation. For very obvious reasons [frequency, power and proximity] these two aerials were going to affect each other, and no points are awarded for guessing which of the two would come off the worst. Fortunately the 965 AKE aerial only transmitted when it was rotating, so it never stopped directly opposite one of our UK/SRA/102 aerials. Thus, the interference, whatever it was, was intermittent and could be foretold by counting the rotation rate.

In 1961, our scientists developed the RIS [Radar Interference Suppressor] 6 unit which was to be an equipment sited in the 965 office attached to the trigger of the radar, whose output would be fed to the UHF receiver rooms [clearly it was not a problem affecting UHF transmission]. We had two types of kit which were styled UHF, one being the 691/CUH and the other 692/3/CUJ [see Transmitter page]. The frequency range of the CUH was 277 to 283 MHz [just a 6 MHz range] and was way outside the frequency range of the 965, 216 to 224 MHz, so was not affected. The CUJ however, at 225 to 399.9 MHz was much affected, and it was this receiver which had to be modified. The modification involved the re-routing of the IF circuitry and thereafter, to make room for a bolt-in unit, one to each CUJ, connected to the 965 {UHF office sited} trigger box. This file shows the overall block diagram of the system which includes the D/F set's receiver for the FU1. RIS 6 Block Diagram and this is a description of the system RIS6 Details and a bit more detail here RIS. This is what the Radar Office Trigger Unit looks like.

These next set of files (left and far right below) show UHF Aerial fits. Note the CUJ CAW outfits and the number of UK/SRA 102's fitted.

The second picture above, shows the block diagram run of RIS6 in a Leander Class Frigate in 1974. The third picture above, is of the unit which is built into each CUJ receiver. Note the gating switch on the bottom side panel. If the CUJ was used for covered voice with a BID150, this switch had to be out of circuit so that the cryptostream was not interrupted by the 965 Radar, while the temporary interference was accepted. At all other times the switch had to be ON so that the IF of the CUJ was gated and therefore the receiver was safe.

In 1960, and for many years before, Radar Interference was not thought to be a common problem for the W/T Office. The B40 had a Noise Limiter built in which could be switch on and off to protect the receiver from PULSE TYPE interference, like Radar for example, but in all my years, I can never remember using it or seeing it used and this was truly a 'bread-and-butter' main receiver. In those days, we knew every switch/control/block diagram and circuit diagram, so its lack of use wasn't because of ignorance but because it was never required to be used. Before the B40/B41, the B28/B29 had RIS facilities built in to the receiver cabinet but the input for a feed from a radar set was never wired or employed. To use the facility, as in all cases of Radar [or RDF] the receiver, in some way or other, had to be attached to the offending radar set.

It wasn't that the navy didn't have such a 'thing' as a RIS Unit, far from it, because in 1943, after just a few short years from the introduction of Radar into the Fleet, they brought out the RIS1, RIS2 and RIS3 all in one go. RIS1 was to eliminate Radar from MORSE circuit: RIS 2 to eliminate Radars with high PRF's up to 2000 c/s, and RIS 3 to eliminate Radar from R/T circuits. To confirm my understanding of the RIS situation in the B28 and B29 {which comes from the 1951 issue of the BR222] there is no mention of these receivers, but others are mentioned, which, by and large pre-date the B28/B29 or which are commercial receivers vulnerable to radar. This is a picture of the first RIS machine the navy had.

and this file Outfit RIS1 dot's the i's and crosses the t's for lots of other drawings, pictures and a short description.

RIS 3 put out its own handbook which was dated 1943.

and this file contains drawings, pictures and text for the RIS 3

RIS 4, when it came along in 1944, was designed, like RIS 3 to plug into the AF output stage of any communications receiver to suppress interference from Radar equipment operating with a PRF of 50 c/s, such as Radar Types 279 and 281 [see Radar page]. RIS 5 was different from RIS 3/4, in that it was designed for high PRF's - Radar Type 291 for example. The early RIS sets could be plugged into the AF stages of receivers without the need to modify them, but the RIS5 incurred a modification and was introduced into the RF stages of the receiver and not the AF stages. This is a picture of a RIS 4

and this file contains some details of the RIS4 and RIS5. Regrettably, we don't have a picture of a RIS5.