Chelmsford Amateur Radio Society
G0MWT, GX0MWT, GB5HF, GB100MWT, G100RSGB & GB5RVA

CARS Meetings   July - September 2019


Award for All


July Meeting
Tue 2-Jul-2019, 7.30-10pm
Oaklands Museum, Moulsham Street
"Three Short Talks"
By CARS Members

The July meeting featured the CARS 'Three Short Talks' format for a real variety:-

  • OFDM and All That – Phil Pearson G0UIB
  • Wire Aerial Construction – Tony Gilbey G4YTG
  • Using Trees as Antennas – Peter Bridgeman G3SUY

    OFDM – Phil Pearson G0UIB
    The evening started with one of the most well animated slide sets ever presented at CARS as Phil explained all about OFDM.

    This started with how a series of sine wave harmonics can add up to create a square wave, as well as how their phase relationship can be ‘orthogonal’ (out of phase). Multi-carrier frequncy multiplexes are the basis of a raft of datamodes from FreeDV to ADSL, 4G-LTE/5G, DAB/DVB and more recent Wi-Fi standards.

    The basic scheme in most OFDM uses relative slow symbol rates (sometimes just QPSK) on lots of separate but close-spaced carriers (with plenty of error correction spread across them as well), making them relatively to synchronise and resilent to multipath and transient interference.

    Creating or receiving these in a radio uses a techncique that goes back to Joseph Fourier (of Transform fame) in the 19th Century, but its only the advent of modern DSP that make it so practical. Spectrally it looks like this:-

    In practice the ideal waveform can be affected by non-linear PAs, poor frequency sources (with phase noise), Doppler as well as differential fading etc. In the worst case, the decoding process may get data from the wrong carrier as illustrated below.


    OFDM Signal reception can be impeded by frequency offsets and phase noise


    Wire Antennas – Tony Gilbey G4YTG
    Next was a wire antenna tour with Tony G4YTG, CARS (and former BT) antenna expert. No Powerpoint for this one – out came pen and paper!

    Tony illustrated volts and current on some common antennas including the Doublet, W3DZZ 5-band trap dipole, and the famous G5RV.

     
    Tony G4YTG with freehand drawings of how various antennas work

    Our thanks to Tony – and for the handy paper and coloured markers!


    100 Years of Antennas – Elaine Richards G4LFM, RSGB
    Finally after the break it was ‘100 Years of Amateur Aerials’ by Elaine Richards, G4LFM, RSGB RadCom Editor & Archivist which is one a set of talks originally created for the RSGB centenary.

    Bob G4MDB kindly offered to read the presentation notes for a fascinating series of slides from the start in Marconi days when wavelengths were rather longer and ‘fat dipoles’ and groundplanes were even more important than today.


    France 1922: Fat caged dipoles for 3MHz (100m) used by Deloy

     
    The shack and antenna system for Ernest Simmons 2OD, Acton 1912, for 1000m / 300kHz

    As the slides moved on to more recent years the G5RV, VHF and PW 10 GHz antennas all featured.

    All in all a most intriguing trio of talks – our thanks to all the presenters!

    Note: Offer of talks for other short slots are always welcome - please let Colin G0TRM know



    August Meeting
    Tue 6-Aug-2019, 7.30-10pm
    Danbury Village Hall
    "Constructors Competition"
    By CARS Members

    The August meeting was the annual CARS constructors competition. Whilst a regular event, the venue was a little unusual in that the large hall at Danbury was used, as Oaklands was not available. Three fine entries were presented:-

    • Andy Tyler G1GKN opened the evening with a 10GHz receiver for the QO-100 geostationary satellite
    • Kevin 2E0FEQ gave an inspiring tale of perseverance and self-training as he first tried to build a Chinese circuit design, only to find issues and then build his own test circuits as well to diagnose it.
    • Finally Alan M0IWZ presented how he had adapted an Arduino to be an antenna and network analyser.
     
    John G8DET commenting on the Satellite dish entry - and others looking at the entrants at breaktime

     
    Alan's analyser and Kevin's circuits

    CARS President Tony Gilbey G4YTG handled most of the honours and counting up all the votes during the break in what was a close run result.

    Cash Prizes were awarded - £15 for 1st, £10 for 2nd and £5 for 3rd


    Prizes were presented by President Tony G4YTG: 2nd - Kevin 2E0FEQ; 3rd - Alan M0IWZ


    And First prize - Andy G1GKN



    September Meeting
    Tue 3-Sep-2019, 7.30-10pm
    Oaklands Museum, Moulsham Street
    "Life as a Radio Officer"
    By Dave Ellis G4AJY

    Local CW enthusiast Dave Ellis G4AJY served in the Merchant Navy as a Radio Officer in his younger days.

    His is frequently asked what he had to do to become an R/O, what the R/Os job was like, and what life was like after leaving the sea.

    In a talk entitled "My Time as a Radio Officer" Dave will answer these questions and much more.




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