15 Nov 2018 – Reflections of Early Radar, a video from the RSGB

Hugh Griffiths, G4CNV, said that radar has a long and fascinating history, with some surprising twistsand turns. This lecture gave an account of radar from its first demonstration through to the present. The main emphasis was on radar development in World War II. Making use of recently declassified material from the Public Records Office at Kew, Hugh told of the invention of radar, Watson Watt’s contribution, and the Daventry experiment with developments of Chain Home. Also covered was early airborne radar and the Bruneval raid. Hugh explained the use of early radar in the detection    of V-2   rockets.

The German Wurzburg Radar

8 Nov. 2018 – “My Old Rig” Topic for Discussion

Kenwood 599S twins (demonstrated by Paul G1GSN)

The idea for this topic was put forward by Bryan Bourne last year. Members were asked to  bring in old rigs or photos to tell of rigs they had owned or known.  The discussion was started with a slide show of rigs that were ‘known but not owned’. That was mainly because the rigs were used while serving in the military during the 1950’s.  This continued on to more modern rigs with a description as to why they were changed to something more up to date. Then other members presented slide shows of their rigs through the years. This was followed by members who brought in their actual equipment from past years. All were of a manageable size and weight, i.e. no ‘Boat Anchors’   Members said later during the tea and biscuits that the evening was very interesting and fun.

In photos below, the RX is a US Navy command set CCT-46104 dated Feb 1942 modified for plug in coils covering 160,80,and 40. Extra audio stage and heaters wired for 12 volts. TX is home brew 6 valve (includes voltage stabiliser for VFO) 160m AM/CW 10 watts from 5763 PA and 6BW6 AF.   HT supplied by motor generator under bonnet with home brew soft start circuit, used in his car  (from Ian G3ORG)

(Shown below) For mobile use the antenna was home brew 7ft 6in base loaded whip with 1ft adjustable top for tuning 160 band.  Loading coil 3in diameter 12in long filled with 10swg wire. Used a chest mike and foot switch (PTT) under the clutch pedal!  (from Ian G3ORG)

 

1 Nov. 2018 – Feeding Antennas, pt2 by Richard Porter, G3NII

Part two began with the use of ladder line in place of coax. An explanation followed as to why variations in design affect the characteristic impedance.  It was pointed out that window line is relatively easy to make for little cost.

Richard then moved on to the subject of the Aerial Tuning Unit, Aerial Matching Unit and the commercially produced ‘Matchbox’  All ultimately do the same thing but with losses. A point to look out for when building your own. The work done by Steve, G3TXQ was cited as a good reference.

This then led naturally on to SWR or VSWR and how the ATU is used to get the best transfer of power from rig to antenna.  Losses in coax led back to ladder lines and how they can be used successfully to transfer power with the least amount of loss.

Baluns were explained as well as the difference between a Voltage balun and a Current Balun. Here again Steve, G3TXQ website was cited as an excellent source of information. Richard showed some photos of baluns from the internet that were of poor construction although being presented as ‘how to do it’ projects.  The reasons why they were poor was explained. Considered also were choke baluns and numerous diagrams were shown and the  information presented on line by GM3SEK was recommended.

Richard ended his presentation by saying “Perhaps one of the most important points I want you to keep in mind is    DO NOT BUY CHEAP COAX”

25 Oct. 2018 – Feeding Antennas, pt1 by Richard Porter, G3NII

The programme began with an introduction to Oliver Heaviside, 1850 – 1925 who first produced screened cable which formed the basis for our coaxial cable. Richard detailed Oliver’s life which was beset with problems, no less than contracting Scarlet Fever at the age of eight leaving him deaf. In spite of this he became an acknowledged electrical engineer, mathematician and physicist, a member of the Royal Society and  predicted the ionosphere in 1920.

Richard stated that “now that coaxial cable had been invented, lets take a closer look at what’s involved and how it works”  There followed an in depth look at the construction and how that affects the overall characteristics. The various types were listed and how and why they were named. After this discussion it was pointed out that one should never buy cheap coax! Richard passed around samples of good and bad and explained why they were good and bad.

It was noted that coax feeding a dipole or any other balanced antenna will suffer from ‘common mode’ currents and further information was discussed as to why and how to deal with the problem.  Losses in coax were calculated according to coax type and the radio frequency being used.