Ships Nostalgia - View Single Post - Mimco Crusader Transmitter
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Old 8th July 2019, 08:44
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Ron Stringer Ron Stringer is offline
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Organisation: Merchant Navy
Department: Radio Officer
Active: 1960 - 1966
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Originally Posted by Paul Braxton View Post
... and the metal parts of the earphones would often burn your ears a bit on the old 'Serenia'...
It wasn't just the use of 'open' antenna feeders (copper tubing) between the transmitter and the external antenna that caused such phenomena. Satisfactory grounding (earthing) of the transmitter was a cause of many problems that technicians, including those working in the installation planning department of MIMCo, didn't fully appreciate. For the Oceanspan range, tinned copper wire 7/.029" connecting the transmitter cabinet to a brass bolt screwed into the metal deck of the radio room was deemed adequate and, with the low powered Oceanspan range, did not exhibit any problems. This continued with Worldspan and Globespan and I can confirm that in hot weather, sitting at the key wearing only a towel around the waist, you had to be damned careful to keep the headphone cord away from your sweaty body if you wanted to avoid a painful reminder that RF does burn skin.

For the Crusader, after many initial problems, the earthing arrangements were improved somewhat to 6-inch wide, 0.4mm thick copper strip replacing the wire. With the higher power of that transmitter however, problems persisted, made worse by the changes in ship design that enforced the use of short, inefficient antenna arrangements. On some ships the length of copper tubing within the radio room was a significant portion of the overall antenna length.

To try to improve matters when Conqueror came along, I won a long struggle to convince the installation planning department, and others, that a minimum 12-inch strip must be used for the grounding. The transmitter cabinet was provided with appropriate connection arrangements for the wider strip and a specific grounding diagram was produced and included in the installation manual. Despite the power increase of Conqueror over Crusader, we had very few reports of operational problems with unwanted effects. A couple of years after the Conqueror entered service, I received a grudging acknowledgement from the head of the department that the transmitter had given them fewer problems than its predecessor.

Of course the real solution was to re-locate the transmitter's antenna tuning/matching arrangements out of the radio room, to the base of the antenna, being connected to the transmitter's 50-ohm output using co-axial feeder. Then all was calm within, as the transmitter's RF output was conducted into the antenna instead of bouncing around the radio room. This had become the standard for most manufacturers by the time that the radio room disappeared from ships and communication was transferred to the Bridge and the cabin.

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