Magdapur. Low Power Operation

John Leary
10th August 2005, 20:45
This story might be of interest only to radio anoraks and if you are not one of them then I suggest you skip to an other part of the Site.

All of the ships that I sailed on had transmitters of moderate power. Typically these generated something in the order of a hundred watts of power equivalent to a moderately bright light bulb. I used to listen in envy to stories of the one-kilowatt plus transmitters fitted to some Greek vessels.

However as in other aspects of life having the biggest is not necessarily the best and what you do with it is probably more important. This was particularly true when operating on the shortwave bands at night when very long distances could be achieved with moderately powered transmitters, particularly if the operators communicating over the link were experienced.

On the Magdapur the High frequency (shortwave) transmitter was an antique piece of equipment but one that proved to be extremely reliable during the time I was on the ship. I cannot remember who manufactured it but what I do remember was that it was relatively small and compact and sat on the table to the left of the operators chair. It was rectangular in shape and had perforated metal screens fitted to its sides that enabled you to see its internals. The controls were relatively simple and easily accessible. I also remember that IT had a single 813 type valve as its final power amplifier and that you were able to see this valve through the ventilation holes.

On the day in question, the Magdapur was on its return journey home from Calcutta in India and I believe that we were about half way across the Indian Ocean heading for the Gulf of Aden and the Red Sea.

In the late evening the captain handed me a telegram that he wanted sending to the Head Office in Liverpool. The normal practice was to try to send such a message directly to the UK via the Area Coast Station at Portishead just outside of Bristol. If that failed the alternative was to send the message via the Area Coast station that served the part of the World that you were sailing through. In my case that would have been the station located at Mauritius in the Indian Ocean.

There were a few preliminaries to be concluded before sending a telegram such as doing a word count for payment purposes and adding a preamble that included such details as destination address but once that had been done all that was necessary was to listen out for the preferred coast station and try to make contact.

After finding Portishead working on an HF channel it would then be necessary to tune the ships transmitter to its calling frequency.

Tuning the transmitter was usually straightforward because like most Radio Officers I kept a note in an exercise book of the preferred settings for optimum performance on the various short wave bands. Therefore after moving the control switches to the predetermined positions all that was necessary was to tweak the loading and coupling controls to achieve maximum power into the aerial, usually accompanied by getting the maximum current reading on the aerial ammeter.

Now a radio officer would become familiar with the sound a transmitter made when it was working correctly. On the Magdapur’s transmitter a further confidence check was provided by the blue ionisation that surrounded the anode on the 813 final amplifier. This came about because the high-tension voltage on the valve was in excess of 1500 volts. It was possible to see this glow from the operating position.

I remember calling Portishead unsuccessfully over a period of about fifteen minutes, changing bands throughout the time to see whether propogational conditions were more favourable on alternative frequencies. No matter what I tried I was having no success whatsoever. Plan B, routing the message via the Area station at Mauritius was therefore invoked.

I listened out for Mauritius, gave them a call and received an immediate reply. After offering the station one of the ships working frequencies, we both moved to new frequencies in order for me to transmit the message.

The message was unusually long and because there was a fair amount of static I believe that I reduced my sending speed to improve the prospect of successful delivery.

Halfway through sending the message, the voice of the transmitter changed completely and looking up there were no power indications on the meters, no aerial current reading and no familiar glow from the power amplifier. I had the option of breaking off communications to investigate the fault or continue sending the message although there would be very little prospect of it being received successfully because how could it be transmitted when there was apparently no power being radiated.

Without stopping or interrupting my transmission I continued to send the message. Imagine my incredulity therefore when the operator at the other end gave me an acknowledgement without any request for repeats. It was amazing. I had sent the second half of the message with a virtually dead transmitter.

In signing off with Mauritius I explained what had happened and the operator wished me well with my faultfinding. I sat back absolutely stunned. If I had been a betting man I would have wagered anyone that completing the message after the transmitter failed would have been impossible.

I reverted to listening watch and informed the captain that the message had been successfully cleared.

At the end of the watch when the automatic alarm was switched on I started to fault find the failed unit.

What I quickly discovered was that the 813 power amplifier had gone soft. In none technical terms this meant that the vacuum within the glass envelope had been overcome by air entering through a crack. The glass must have weakened over time and probably the heat generated through my unsuccessful attempts to contact Portishead had lead to its demise. Fitting a replacement valve and giving the internals of the unit a bit of a dust restored it to full working condition.

When I thought about the situation afterwards I concluded that what had worked in my favour was that I had already started to send my message and that the operator at Mauritius was highly skilled and was able to read my signals albeit at a very reduced power.

Secondly although the 813 valve had failed there was sufficient coupling through it from the transmitters drive stages to permit a very low level of radiation from the ship’s aerials. Possibly a fraction of a watt but that was sufficient.

Some radio amateurs specialise in low powered communications and after my experience on the Magdapur I have to salute them for their skill.

John Rogers
10th August 2005, 21:21
I'm not a sparky but that was a interesting story. Question..Was the practice of using "Skip" thought of back then.
John R.

thunderd
11th August 2005, 00:11
A fascinating story John, I've dabbled a bit in radio and have known amateur operators who specialise in QRP (low power) and it really is a skill. These same operators also sometimes go what I think they call "long path" where, using a directional antenna, instead of sending the shortest route from A to B they go right round the world using the longest route.

It's must be funny to people like yourself to now see the modern radios which are now slightly larger than a loaf of bread compared to the room full of equipment you were used to.

I believe that many of the greatest technology advances in things like satellites were due largly to the efforts of amateur operators.

John Rogers
11th August 2005, 00:54
True story. When I was stationed in Alaska we had one hell of an earthquake,the shock wave knock out our communication with the lower 48 states,the missile battery's were off the net and the only people that could get through to the closes main command in Seattle was the Ham Radio operators using skip.
JohnR

thunderd
11th August 2005, 01:06
I have noticed the use of the term "anorak" lately, what does it mean?

John Rogers
11th August 2005, 01:39
Dave you went and ruined my vision of a "Anorak" I thought it was a little four legged animal with a hard shell and a long nose.
John.

thunderd
11th August 2005, 04:51
Dave you went and ruined my vision of a "Anorak" I thought it was a little four legged animal with a hard shell and a long nose.
John.
No John you're thinking of the haggis whose left side legs are shorter than the right side ones so it can walk over hills and stay balanced.

Thanks for the explanation Dave. Interestingly enough I've always found that listening to a true hobbyist speak to be most enjoyable, they seem to have an enthusiasm that makes the mundane interesting. I mean look at us we rave on about ships and there are probably some people who think we are a lot of plonkers, I know you'll find that hard to believe.

R651400
11th August 2005, 08:43
Nice story John.
To drive a 813 you need a fair bit of skelp and the only answer is the driver stage managed to get some rf past the sick 813 to the aerial.
My guess Magdapur's gear was IMR?
On the subject of skip, way back in the dark ages, the powers that be who thought long wave was the answer to long distance comms deemed,
"Amateurs, let's see, stick 'em on 200 metres and down."
Later a complete volte-face when they discovered amateurs attaining enormous distances on the shorter waves using very little power.
No one then knew the reason why.
Malcolm/F5VBU/GM3UIN

thunderd
11th August 2005, 08:44
Dave I never ever stuff up a good story with the truth

trotterdotpom
11th August 2005, 09:11
The 'Area Scheme' was great for British ships but upon it's demise, it was murder competing with those high power transmitters of the Norwegians, etc. We had to rapidly revise radio propogation theory and, occasionally, put down the gin to go and send a message off in the night, returning to pick it up again of course.

'Anorak' is the Eskimo (Inuit) word for 'Anorak' and therefore, by definition, it must be 'cool'.

Re 'Plonkers' - they used to say that there were two kinds of people on ships, Plonkers and Liars!

John T.

pat
11th August 2005, 11:27
exactly the same thing happened to me while on a ship in the Persian Gulf many years ago while operating on 22 mhz the main PA valves gave up the ghost and ended up sending my traffice to Portishead only the Drive tubes which can only be around 1 or 2 watts getting thru to the Ant,but of course 22 mhz when its open can provide world wide communication with very lettle power as any Ham operator will tell you,

PAT/oa3ezj

thunderd
11th August 2005, 11:39
During the good skip cycle in the early eighties I spoke to countries all over the world regulary on a 27mhz CB radio with a yagi beam, it was great stuff.

trotterdotpom
11th August 2005, 11:57
During the good skip cycle in the early eighties I spoke to countries all over the world regulary on a 27mhz CB radio with a yagi beam, it was great stuff.

"Yagi Beam"? Is that a missprint?

BooBoo
Jellystone Park

thunderd
11th August 2005, 12:02
"Yagi Beam"? Is that a missprint?

BooBoo
Jellystone Park
Oh we're quick tonight Mr T.

jordiboy
11th August 2005, 22:37
The Aussie lease on that little URN is nearly over thunderd

Santos
11th August 2005, 22:58
"Yagi Beam"? Is that a missprint?

BooBoo
Jellystone Park


'Yagi Beam ' (?HUH) = Aerial in old money. and please bring back Huck and his mate Yogi, my all time favourites.

thunderd
12th August 2005, 00:52
The Aussie lease on that little URN is nearly over thunderd
I can't bear to think about it

thunderd
12th August 2005, 00:53
'Yagi Beam ' (?HUH) = Aerial in old money. and please bring back Huck and his mate Yogi, my all time favourites.
You're just a boy at a bad age Chris LOL

Santos
12th August 2005, 20:28
You are dead right there Derek, young in thoughts but old everywhere else. Still who cares, I am still enjoying myself, thats the main thing. When people ask me how I am, I say, " Well I woke up this morning, so thats a good omen ". (LOL)

Happy Days. (Thumb)
Chris.

Marcus Cardew
15th August 2005, 18:52
Interesting Story from John Leary,
Whilst I was just a humble deck apprentice, to supplement my income I would work in a Tv and Radio repair shop when ashore to pick up a few extra 'scrubies'. One machine I had to fix, was a physiotherapist's RF 'Deep Heat' Unit, which had a 318 like PA stage. An earthy lead was leant against it, and ' electron beam machining' had drilled a neat hole through the glass envelope (more than a bit soft!)... They were NOT happy with the repair cost........