80-Bus News

  

April–June 1982, Volume 1, Issue 2











Page 17 of 55











7

A2D amplitude modulated double sideband a.f.s.k by data J2d amplitude modulated single sideband suppressed carrier a.f.s.k by data F2D frequency modulated a.f.s.k. by data

I know the Home Office say that it was not the intention to prohibit data transmission (and RTTY) by ‘Amateur B’ Licence holders, but on paper they have, blame the loon who wrote the schedule! Perhaps they’ll get it right on their third try due in September.

As Pete notes, an aerial input power of 100 Watts and no limitations of the type of aerial used should be adequate for some sort of data transmission country wide. I also agree with his comment that anyone who knows which is the hot end of a soldering iron stands a reasonable chance in passing the RAE exam.

The underlying purpose of my original article was to desuade people from attempting to try data transmission on CB, whilst the recent inclusion of data transmission within the scope of the amateur radio licence (albeit very badly defined) opens up a wholly practical field for experiment. Lastly, sorry Pete, your advice to me to put away my Chicken Brain set comes far too late. I’m still waiting for the HO to perform a bit of ‘digitus extractus’ in my case (Pete will know what I mean). I can listen to Brum via ‘’VA’ at present, and it should be Ok simplex on SSB when I get my big beam up, so I’1l give you a call some time.

More about Amatuer Radio and Non-disks

mr ale tH Sa‘ Se ZS ts OL ee Se tN O

The following letter on the subject was recieved from C. A. Graham close to the copy date. From the latter half of his letter, it seems he reads the editorals, and has a form of precognition of what Peter Kendall is going to ask.

Radio Data Transmission I was interested to read Dave Hunt’s article on data transmission on the CB

channels, but concerned that one or two of the comments made in the article, with reference to the Amateur Transmitting licence, were not quite correct.

It is true that the licence restricts teleprinter operation on the h.f. bands to the CCITT code No.2 (Murray code), at a rate of either 45.5 or 50 baud. However, amateurs are [Now – D-H. | also allowed to transmit data on all frequency bands above 144 MHz, provided identification (i.e. callsign) is sent in morse or telephony at least once every 15 minutes and that a bandwidth no greater than that used for telephony be occupied.

I have conducted a number of experiments into this form of transmission on channels within the 144-146 MHz band, with Steve, G6BLF, and found that 100% reliable service can be achieved over a distance of 4-5 miles at a transmission wate of 300 baud. The equipment at both ends of the link was a Nascom 2 linked to a 10 watt FM transceiver, in my case, a home built synthesizer controlled rig; and the interfacing required?..... WNone!! (except a couple of leads and level controls).

The Nascom’s audio tape output is coupled to the microphone socket on the transceiver and the loudspeaker output from the latter coupled to the Nascom audio tape input. This arrangement has the advantage that the Nascom thinks it is sending files to and receiving them from a tape recorder; thus programs and data may be sent from within Nas-Sys, BASIC or any other program that uses audio tape I/0. The audio shaping in the transceiver takes care of bandwidth occupancy and with a little optimization might well allow a 1200 baud rate, although this was not achieved in my case – an error rate of approximately 2% being recorded at this speed. .

The question of mutual interference with other stations does arise, especially in an area of high activity. However, it is usually possible to find a clear channel (there is even a data-transmission calling channel reserved) and most other stations are content to listen to the proceedings without feeling the need to put a carrier up. If this does happen, another band may be sought; possibly the 70 em band. This has the advantages of low occupancy and small antenna size at high gain and directivity (you can squirt your signal in one direction, and receive best


This is an OCR’d version of the scanned page and likely contains recognition errors.











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