CTARC Forthcoming Meeting - 28 May '16

The May 2016 Cape Town Amateur Radio Centre members meeting, (usually held on the fourth Saturday of each month), is on 28th May, at 14h00, at the CTARC clubhouse in Rondebosch.

Directions to the CTARC clubhouse are [here].

At the meeting, Tom / ZS1TA will present a talk about E-Mail using HF Radio, something extremely useful to long-distance seafarers and overlanders alike.For some years now, yachties have used this facility to stay in touch with their loved ones, report their daily positions, make arrangements with suppliers at their destinations, etc. It promises to be a very interesting presentation indeed.

Further details will be announced on the Sunday morning CTARC club bulletins transmitted on the 145.750 MHz repeater at 08h30, for listeners in the Cape Town area.

All club members and interested parties are most welcome to attend.

CTARC Report-Back on February Meeting - 27 Feb '16

On Saturday, 27 February 2016, the Cape Town Amateur Radio Centre held its regular monthly members' meeting at 14h00 at the CTARC clubhouse in Rondebosch.

The meeting was well attended. Our Chairman, Rob /ZS1SA started with a report back on the Committee Meeting (held on the Monday beforehand). Our clubhouse manager, Noel / ZS1FW, announced there will be a Work Party on 19 March to do essential antenna and feed-line maintenance work.

Rob also took a straw poll to see how many members would be interested in attending our 70th Anniversary Function to be held later on this year - the response was very promising.

We then proceeded to the main topic of the afternoon, which was a talk ably given by Rob on his Tri-Bander Coaxial Trapped Dipole antenna. This useful antenna, which Rob has been developing and working regular HF contacts with over the past year, is unique in that the traps are constructed of coaxial coils only (no capacitors are used, as the capacitance is inherent in the loops of RG58 coax cable traps, wound around sections of plastic plumbers pipe).

The antenna gives you three bands - 20m, 40m and 80m. The first two bands are fully covered, with an SWR of 2:1 or better over the full bandwidth of each band. However, the trade-off is that the 80m bandwidth is quite narrow - typically just 25 kHz wide. However, that is not too bad performance considering the antenna, when used in an inverted-V configuration, is just 29m wide.

Rob showed us a freeware calculator programme by VE6YP which calculates the dimensions and windings of the coils for certain frequencies. He also discussed the design and construction of the antenna, with the use of stainless steel bolts and wingnuts on the traps to facilitate easier adjustment of the various elements to tune it to perfection. It was noted that the copper wire used has a tendency to stretch over time and in the wind (Rob's QTH is located in a Wind Tunnel on the Table Mountainside...) and lower the resonant frequencies involved. However, once readjusted to accomodate this, the antenna's resonant frequencies have stabilised over time.

The antenna is fed with coaxial line, that connects into a 1:1 balun at the central feed point on the apex. Rob showed us how the resonant frequency of the traps can be altered by stretching out or narrowing up the coaxial coil winding on its former. A surprisingly wide variation in frequency can be obtained. Once adjusted for resonance, the coax trap coils can be fixed in place with a marine varnish, which doesn't appear to alter the frequency of the trap once dry.

Rob demonstrated how to test the trap for its resonant frequency using an MFJ Antenna Analyser with a homebrew coupling coil that is placed just inside the coil former.

The power handling of the antenna is limited by that of the central balun at the feed point. In the case of the antenna Rob handed around for us to see, that was rated at around 150W, but full legal limit could be handled if a more robust balun (or possible an "Ugly Balun") is used. One point Rob emphasized is that inexpensive coax provides inconsistent results when it comes to tuning. Far better (and more predictable) results were obtained by using Military Spec coax cable for the traps.

An emphatic comment from the floor (by John /ZS1AGH) stressed that one must NOT use electrical conduit for the coil formers. That material is conductive (as can be proven by testing a section of it in a microwave oven with a glass of water inside). If it heats up, the former is conductive. (The glass of water is to protect the microwave oven itself! Thanks To Frank /ZS1CM for that advice).

Thanks to Rob for delivering a fascinating talk indeed - one that inspired us all to go out and build our own antennae!

You can download Rob's Powerpoint slide show of his talk [here].

Our trusty chairman Rob started the meeting with a welcome to all present,
as well as making some announcements

Noel / ZS1FW announced a work party to be held at the clubhouse on 19 March

The reason the antenna was considered in the first place was the G5RV in use
was causing QRM & TVI at the QTH

The basic concept and structure of the trapped inverted-V dipole is explained

The traps are made of easily-obtainable materials, but the coax used should be
of Mil-Spec quality. No capacitors are required, the necessary capacitance is
obtained from the spacing of the coil windings of the coax

The VE6YP freeware software enables the designer to customise the traps

Rob constructed his own RF probe to use with his MFJ Antenna Analyser,
which proved very useful in tuning the traps to specification

The traps are robustly constructed, yet lightweight. Stainless steel bolts,
washers and wing nuts are used to prevent corrosion.

Once dismantled, the antenna is quite compact

The audience had the chance to handle the antenna and get a sense of its weight

Paul /  ZS1S brought along another trap dipole
antenna for comparison

Rob demonstrates how to use the MFJ Antenna Analyser to tune the trap

After the meeting there were the usual discussions among fellow hams

CTARC Report on September Meeting of 26 Sep '15

On Saturday 26 September 2015 at 14h00 the CTARC held its September meeting, which was reasonably well attended in spite of rather cold and windy weather.

The afternoon began with our chairman, Rob / ZS1SA, calling us to order. He gave us a report on the recent committee meeting of 21 September, and the various forthcoming events with our club will be involved:

• The club has disposed of its old trailer by selling it to one of our members. The trailer had been sitting unused for several years and was accumulating fines for unpaid vehicle registration fees;

• On the weekend of 17/18 October, the CTARC will run a JOTA station for the Bergvliet Scouts and Guides. Noel / ZS1FW is driving this project;
  
  
• Our Saturday 24 October monthly meeting will be the screening of the video of a DX-pedition to Chatham Island, kind courtesy of Barry / ZS1FJ via Paul / ZS1S;
• We also plan to operate ZS1CT during the forthcoming Field Day exercise;
• Our final monthly meeting of the year will be on Saturday 28 November, which will be the CTARC End-of-Year function;
   
• Next year, the CTARC celebrates its 70th anniversary. We eagerly await members' input and suggestions on how to commemorate this auspicious milestone.

Further details on the above forthcoming activities will be published on this blogspot and announced in the regular CTARC Sunday morning bulletins.

In addition to this, Rob mentioned our club had recently been featured in an article in the Southern Suburbs Tatler newspaper. (See that article here)

We then proceeded with the main event of the afternoon. John / ZS1AGH gave us a fascinating lecture on Decibels - what they are, and how they apply to amateur radio. His talk was illustrated with a PowerPoint lecture, which may be downloaded here.

Those CTARC members with grey hair had to scratch our heads (and watch what hair we have left turn even greyer) at the complexity of it all, having last had exposure to such mental calisthenics when at technical college or university (many years ago...)! At the time, John made it all appear so clear and simple!

John also showed us the tools of the pre-calculator epoch - logarithm tables and a slide rule. They may appear cumbersome to us now, but in experienced and adept hands they were tools of great productivity before cheap digital calculators became widely available.

Hans-Jurgen / ZS1HJH brought along a very useful card published by the German Funkamateur club, of a comparison between decibel values, Voltage and Power levels, and S-Units. He very kindly allowed us to make it available, so you can download it as a PDF file here.

Afterwards we broke for the usual natter over coffee and biscuits. All in all, another great CTARC meeting. We look forward to the forthcoming JOTA field station and October meeting.

Rob / ZS1SA opens the meeting and discusses forthcoming events

John / ZS1AGH begins his lecture

A series of calculations on a loooong sheet of paper!

John's lecture was ably illustrated by a Powerpoint show

The meeting was reasonably well attended, in spite of cold weather

Hans-Jurgen and Paul in discussion

Tea and biscuits and chewing the rag after the meeting