Last month we were pleased to welcome back Peter G3RZP, a well known speaker at amateur radio functions – at local, national and international levels.
The title of his talk was “Clean Signals” and he proceeded to tell us why transmitters, and SSB transmitters in particular, emit signals which are much wider than they should be – and so cause interference to stations on adjacent frequencies. SSB transmitters are essentially “linear” amplifiers, amplifying a signal generated at low-level up to the desired power required for transmitting a signal to wherever. However, the amplification process is not actually truly linear because of the inherent characteristics of the active devices, be they thermionic valves or solid-state devices. In an SSB transmitter the non-linearity gives rise to what are known as “intermodulation products” which spread out either side of the main signal. Intermodulation products are, in the simplest case, caused by two discrete signals being amplified mixing together, because of the non-linearity, to produce low-level signals spaced from each other by the difference in the frequency of the two signals. The further away from the main signal the weaker these products become. But even low-level “spurii” can cause QRM to near by stations.
So the way to minimise such interference – sometimes called splatter – is to make the transmitter/amplifier operate as linearly as possible. This can be done by making sure that the equipment is not over-driven – ie by not turning the mic gain up too far, by using minimal amounts of ALC and by making sure that the transmitter sees the correct load impedance. In the design stage negative feedback can be introduced to improve linearity, but this can cause other problems if not done carefully. One way to check that the transmitter is being operated corrected is to inject two audio tines into the mic socket and look at the RF output with a suitable ‘scope, the tops and bottoms of the waveform seen should be nicely rounded. Peter’s view was that in general valve amplifiers are inherently more linear than solid-state ones.
The talk was well received to judge by the number of questions asked. Many thanks to Peter for travelling from Swindon to see us once again.
How clean is your transmitted signal? What is meant by a “dirty” signal? What causes a dirty signal to be transmitted? What are “inter-modulation products”? Do you know how to adjust your transmitter so that the PA is not over-driven? For the answers to these and no doubt many other relevant questions come along to our July meeting to hear Peter Chadwick G3RZP talk on the subject of “Clean Signals”. Peter is a well known and highly qualified engineer who has given many talks to clubs over the years – including CARA. He has served as a technical advisor on national and international committees, eg IARU and ITU, so knows his stuff! We are pleased to welcome him back again for another of his thought provoking and informative presentations. Please attend and make his trip from Swindon worthwhile.
7.30 for 8pm on Thursday 19th July at the Brizen Young Peoples Centre, Up Hatherley Way – located between Morrisons and the Shurdington Road.
Beware of the height barrier when entering the car-park.
Coffee, tea and biscuits should be available as usual.
Visitors and prospective members will be welcomed, as always.
A good turn-out of members enjoyed an interesting display of exhibits from members’ radio rooms (or shacks if you prefer!). The exhibits ranged from early types of detectors of radio signals to a modern piece of kit to power radios from solar energy. Also on display were a selection of old QSLs cards, an Elecraft Miniature ATU, a once popular IC-202 portable 2m SSB rig from the past, a Retevis RT82 DMR hand-held, a portable station in a box and a dog! To add further interest to the evening Mike G4GUG gave a short talk on how to set-up a rotator for satellite working.
Many thanks to everyone who brought along something to show, to Mike for his talk and to Peter G3YJE for manning the kitchen (yet again!).
Several members attended the Thursday evening meeting to listen to Colin G3VTS and Jon M0JMM talking about the construction of their antennas.
Colin who was awarded the G5BK cup at the Constructors’ Exhibition for his 5 band cobweb antenna. He described his cobweb as 5 dipoles in a square shape with 1⁄4 wave sides with the feed point in one corner. The impedance was found to be about 12 ohms needing a 4:1 balun to match 50 ohms coax cable. Colin made good use of a projector and photographs he took during the construction period which took place in his garage and then testing it on the garage roof. He found that the bandwidth was narrow and although no DX was worked, Colin will no doubt be tweaking it leading to further trials in the future.
The second speaker Jon, talked about his 2 metre 3 element Yagi antenna manufactured from a Tape Measure, plastic water pipe, Jubilee hose clamps and a small piece of wire to form a match to 50 Ω coax cable. The tape was cut to lengths suitable for the driven, reflector and director elements, with the driven element cut in half to form the dipole. They were mounted on the plastic tubing to form the Yagi antenna using the hose clamps to hold them in place. At the dipole centre, a hairpin match and the coax cable were soldered to the centre of the dipole. For more information on construction and dimensions of this antenna, look up “Tape measure antenna”.
Jon said that it was ideal as a DF antenna for fox hunts because it was portable, light weight, had a good front to back ratio, a reasonable gain and above all cheap and easy to make.
The talks were well received by the members and it was good to see that the spirit of ‘homebrew’ is still alive !
The meeting started with a mini-auction of kit from the shack of the late M0ITF. There were bargains to be had and several members went home with some useful goodies. However the reserves were not met on the more valuable lots so these will be sold by other means, including rallies. Many thanks to everyone who supported the auction, and particularly to Mike G4GUG for doing a super job as the “porter” holding up and describing the various lots.
The second part of the meeting was a session by Barry M0HFY on PSK. He spoke about the various forms of this particular data-mode and illustrated how it could be used with the HRD logging program. A very interesting talk, supported by good screen-shots, even for those like me with little or no intention of trying the mode! The talk greatly improved our knowledge of PSK and we’re very grateful to Barry for his time and trouble.
A brief survey of members’ opinion on the suitability of the venue for our regular monthly meeting indicated a preference to remain at Brizen.
Thanks also go to Colin G3VTS for manning the kitchen.
April’s meeting will NOT be held at the usual venue nor on the usual date.
Instead we’ll be meeting on Wednesday 18th April 2018 at The Annex, St Margaret’s Hall, Coniston Road, Hatherley GL51 3NU.
See map at: http://stmargaretshall.co.uk/5.html
The meeting will commence with a short auction of SK stuff and this will be followed by the second part of the Data-modes talk by Barry M0HFY, covering PSK.
Doors will be open by 7.30pm for an 8pm start.
Visitors and prospective members will be made welcome, as always.
On Thursday 15th March we held our annual Constructors’ Exhibition preceded by a short auction. It was a really great meeting, with one of the best attendances for a very long time.
The meeting commenced with an auction of 15 lots from the shack of late CARA member Ian M0ITF. Our Chairman Tony G3YYH did a super job as auctioneer extracting bids with much skill and humour. Every lot was sold, some at higher than expected hammer prices. Ian’s family were well pleased with result.
The Constructors’ Exhibition was fairly well supported but with fewer exhibits than in recent years. Nevertheless there was much of interest to look at. The Morse Keys made from clothes pegs by Jon M0JMM attracted much attention and really should have won a prize had there been a suitable category! Two pieces of home-brew from the early 1970s, built by Ken G3XSJ, also attracted much attention – as being examples of construction techniques from yesteryear. The “741” op amp constructed from discrete components by Simon G4SGI was considered an excellent teaching aid. The Tuna Tin transmitter for 40m fascinated many.
The well deserved winner of the Bill Brown G5BK Cup for “Best in Show” went to Colin G3VTS for his home-constructed 5-band cobweb type antenna – exhibited as photos rather than the real object! The Pat Moore memorial Cup for “Innovative Construction” and the Roger Kendall G0UPU Memorial Cup for “Best Software or Computer Related Project” were not awarded as there were no suitable qualifying exhibits. The judges were Peter G4ENA and Ken G3LVP to whom go many thanks.
Here’s a full list of exhibits, in no particular order:
HF2V shorting strap for 30m operation – Tony G3SNN.
Selection of Morse Keys made from clothes pegs – Jon M0JMM.
KRC-5 80m direct conversion receiver – Derek G3NKS.
5-band cobweb antenna – Colin G3VTS.
Audio modules for 6cm wide-band FM transceiver – Stewart G0LGS.
160m valve transmitter 1971 – Ken G3XSJ.
PW Clubman receiver 1970 – Ken G3XSJ.
Tuna Tin 300mW transmitter for 40m – Walt G3NYY.
Discrete “741” op amp from a Mad Scientist kit – Simon G4SGI.
On display but not in the formal Exhibition:
An unfinished 4-band 45W transceiver – Colin G3TA.
Many thanks to everyone who helped to make it such an enjoyable meeting, and particularly to auctioneer Tony G3YYH, and to Peter G3YJE, Alan M0NRO and Smurf M0URF for manning the kitchen.
The meeting will commence with a short auction of SK items, for members only.
The main event will be the annual Constructors’ Exhibition – members may enter items as many items as they wish. These may be small or large, simple or complex, hardware or software based, use vintage or modern components and techniques. Anything which has been constructed, or heavily modified, to do with amateur radio will be welcomed and these may be built from a kit of parts, based on a magazine article or entirely original. Items must have been constructed within the last few years and not have been exhibited before.
Three awards are on offer:
The Bill Brown G5BK Memorial Cup for the “Best in Show”.
The Pat Moore G3IKR Memorial Cup for “Innovative Construction”.
The Roger Kendall G0UPU Memorial Cup for “Best Software or Computer Related Project”.
Thursday 15th March at the Brizen Centre, Up Hatherley Way, located between Morrisons and the Shurdington Road.
Doors will be open by 7.30 for an 8pm start. Beware of the height barrier when entering the car-park.
Visitor and prospective members will be warmly welcomed.
The meeting started with a short auction of Silent Key items. There were about a dozen lots including an elderly FT-102 HF transceiver, an external VFO and speaker units for the FT-102, two scopes, two AVO multi-meters and two power supplies. A couple of the lots failed to reach their reserves but the rest went for good hammer prices (for the benefit of the family).
The main activity that evening was the “Table Top Sale” where members brought along items no longer wanted to sell. There was plenty of interesting stuff up for grabs, some at very low prices, Andy M0JLY was disposing of several of his home-brew projects several of which were quickly snapped up. Other sellers included Tony G3SNN, Colin G3VTS, Jon G0FJT and Peter G3YJE (apologies to anyone missed). Reports suggest that both sellers and buyers were pleased with the outcome even though some of the tables didn’t empty as well as was hoped. As usual there was lots of chat – it’s great to see members getting together seeking advice, swapping stories and enjoying each others company. Surely that’s what clubs are all about! Finally, many thanks to Colin G3TA for manning the kitchen.
The CARA Foundation Licence course being held on 3rd/4th March is now full.
Expressions of interest in future courses should be sent to email@example.com