RadCom February 2016, Vol. 92, No. 2

48 / 100

February 2016 48
Technical
So to get best accuracy, I carefully removed
the insulation (using an absolute minimum
of force to avoid squashing it) then, with a
tiny amount of solder and liquid flux, tinned
the wire so solder flowed into the strands but
contributed nothing to overall diameter. Using
a pair of vernier callipers the two dimensions
of my cable were D = 2.18mm, d = 1.1mm.
The oft-quoted equation for twin line is
Zo = 276 * vf * LOG(2D/d) but this is only
accurate for wide spaced line where D >> d.
Here the more accurate equation Zo = 120
* vf * COSH-1(D/d) has to be used. Now we
need to know the velocity factor to get Zo. So
again, guesswork or looking at tables is the
only way.
For my twin feeder, knowing vf = 0.7:
Zo = 120 x 0.7 x COSH-1 (2.18mm /
1.1mm) = 105Ω (which for my mechanical
engineering / measurement skills is pretty
impressive!)
(COSH-1, otherwise known as ARC-COSH,
is available on many scientific calculators by
pressing something like INV-HYP-COS, or
ARC-HYP-COS or HYP-COS-1. In the Microsoft
Excel spreadsheet, it is the obtained using the
function ‘ACOSH’).
So the twin feed is nominally 100Ω as
sold by many suppliers. Not what I wanted,
but useful to have on the shelf.
WEBSEARCH
[1] Clicklock, locking a soundcard to 1 PPS GPS
signals:
FIGURE 2: Network analyser plot of a 513mm test piece of twin feeder of unknown characteristic
impedance terminated in a 50Ω load.
News continued from p14
British Scientists Award
Throughout 2016, Phoenix ARC and other UK clubs
will commemorate the lives and achievements of
over 20 British scientists with short on-air events
on anniversary days and sometimes from related
locations. The programme started on 8 January
by commemorating the birthday of Prof Stephen
Hawking and will culminate on Christmas Day
2016 with Sir Isaac Newton. Bronze, Silver and
Gold Awards will be available for 5, 10 or 15
contacts made with the commemorative stations.
Various club and special event callsigns will be used. More information is available
online at
New QRP
Award
The 1000 Miles Per Watt award
is now taken to a new level with the
introduction of the KMPW Century
Club. Available from November 2015, it
is awarded to those who have contacted
100 countries where each QSO would
qualify for at least 1000 miles per Watt!
The first person to be awarded this
new certificate is JH1GNU who worked
all 100 stations on 10MHz with CW.
There is no requirement for QSL
cards or any electronic confirmation.
The award is issued free to active
members of QRP ARCI or $10 for non-
members.
Youth Night
As part of the Southport and District ARC’s
initiatives to support young people coming
in to the hobby, Rebecca, M6BUB, has
organised a Youth Night hosted on the
North Wales 2m repeater GB3MP. Sixteen
year old Rebecca is a member of the
RSGB’s Youth Committee and sees this
initiative as a way of bringing the young
amateurs of the area together on a weekly
basis.
The youth net runs every Wednesday night from 7 to 8pm and Rebecca, M6BUB
will be the net controller. Several of the SADARC committee have volunteered to keep
an ear out to ensure that the youngsters are kept safe. For further information visit
ThinkingDay 2016
World Thinking Day will take place on 20
and 21 February. This is when members
of Girlguiding use amateur radio to try to
contact other members throughout the
world as part of the celebrations of their
founders’ birthdays. For further details and
to add your station to the list of participants
please go to
or contact Liz, M0ACL by email to