QRP
THE ------- COLUMN OCTOBER 2000
80/40
by Dave Gemmell and
the Low Power Bunch
This Column is being typed /compiled on a my daughter-in-law’s computer and e-mailed to our illustrious Editor Wolfy. Yes, occasionally I do make use of home labour-saving devices but what makes this task more pleasurable is that merely turning my head I can see a nice rural setting (horses and cows grazing the grass in Surrey) from the window of my son’ s computer room.
When planning this holiday some months ago I decided to “design/build” a QRP transceiver to take along. Now that SA is a member of CEPT I reckoned that it would be a “chinch” to get on the air! I built the transceiver but didn’t take it, mainly because it was built into a plastic sandwich box and might have looked, to those uneducated in matters electronic, like a bomb or what have you! It did occur to me that my XYL and I would be passing though at least four custom check points and trying to explain the joys of low power operation didn’t really appeal to me. Especially after spending an almost sleepless night with some 350 or so happy fellow travellers!
Now let’s look on the bright side!!! The next time I’m going to build a QRP rig to fit in the top pocket of my jacket!
There was quite a lot to be learnt from this exercise! Getting the parts together, building this rig and getting it to work, was fun in itself. The circuit I used is an old favourite of mine. Who knows, maybe I will be able to publish the circuit diagram in the next issue of RADIO-ZS! You should be able to draw your own circuit from the following description.
Choice of RF “power” transistor 2N1711 type as well which one operated the best. In fact, I used a socket for the “PA” power transistor this made it easier to select the best transistor for the job. It was amazing to find out the variation in the value of the power delivered by the thirty or so 2N1711’s in my “junk box”. Don’t forget to use a heatsink or you might burn out your entire stock of your transistors! A lot of similar transistors seem to be able to with-stand over heating for a short time but not so the 2N1711!
A 2N2222 was used for the oscillator to drive the power amplifier as well as the receiver’s diode mixer. The receiver RF amplifier was built around a BF115 which was followed by a double diode mixer the output of which is connected to a two stage directly coupled AF amplifier which gives enough gain and power to drive high impedance earphones. I tried to use the common collector circuit mentioned in a previous column be have no time to iron out some silly problem which cropped up.
Deciding what antenna to use was not without it’s “points of interest”. For the same customs and excise reasons plus the luggage weight limit set by the airline conpany, I wasn’t going to take a wire dipole and coax along so I was going to buy suitably priced copper wire at the nearest hardware shop and use ordinary table lamp “rip-cord” for the feeder line. I had been led to believe that this “rip-cord” had a characteristic impedance, Zo of 75 ohm but I thought I’d calculate it just the same using the formula Zo = 276 logD/d. After a few measurements and calculations, I ended up with the figure 230 ohms. Then I tried measuring the “rip-cord’s” Zo and obtained 120 ohms ???? If this type of cord was used irrespective of the value of Zo the output impedance of the transmitter could be adjusted to suit by varying the values of the capacitors in the Pi-coupler. The main problem is coupling to the antenna. I though of the windom fed with 300 ohm tape. So why not connect the feeder at a point where the impedance matched that of the “rip-cord”. This idea will definitely be investigated at the earliest opportunity.
Deciding what other accessories to take or not to take along was a very interesting exercise. The power requirements are 12 volt , receive cut about 30 mA and transmit current about 500 mA depending on the RF power transistor used. Normally a rechargable 12 volt battery, could be dry cells but these turned out to be a very expensive alternative if bought in the UK. Using a mains power supply seems to be the logical answer but its going to take a bit of experimenting to get something light in weight. Anyway I have a year or three to do something about this.
I have since learnt that if a South African Ham is staying in the UK for less than 3 months he/she may operate on the amateur bands without special permission or any payment. This situation changes when the stay is longer than 3 months when the SA Ham must apply in writing for a special licence which will be granted on payment of £15-00. (About R155-00 at the present current rate of exchange.)
How to call on the ham bands is laid out concisely in booklet BR 68, page 14, section 10(2):-
CEPT Amateurs shall transmit their home callsign after;
a)the United Kingdom callsign prefix “M”;
b)followed by the appropriate Regional Secondary Locator (if any); and
c)followed by the symbol “I”.
So if yours truly was operating from Scotland I would have to use MMI/ZS6AAW but from Wales it would be MWI/ZS6AAW or Northern Ireland MII/ZS6AAW. The regulations when operating anywhere in the UK seem to be almost the same as in SA but check first the Licence Requirements BR 68 which can be obtained from the UK Radio Communications Agency. It may be a good idea to check for any subtle changes before you start operating there. Secondly check with the relevant authorities about correct procedure to getting your equipment, home brew or commercial, through customs at each end of the route.
Here’s hoping the usual contributions from the QRP Bunch, the Radio Scouts (and Guides !) plus the Museum News have survived their trip down the digital tubes of
e-mail! But, seriously chaps, this Column (and others!) seriously need news and contributions from you the Reader. Please, please send in any item of low power news. It doesn’t matter how small. Even a sentence or two on the back of a post card can be a BIG help! The address is as usual – P.O.Box 77, Irene, 0062.