RadCom February 2016, Vol. 92, No. 2

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February 2016 97
The Last word - Letters
Not suited for use at less than 10MHz
because that’s what the designers intended.
Of course mag loops can be designed to
operate well on 40m. How efficient they will
be depends mostly on loop circumference,
conductor material, and conductor diameter.
PEER REVIEW
Barry Green GW4HYZ
I don’t mind if some articles contain an error
or two, now into my eighties I have learned
to be tolerant of the odd mistake now and
then, unless of course they impinge on the
health/safety of the unwary. Some errors
sometime give me an insight to the reasoning
and thoughts of the presenter. I consider that
the present methods of vetting are more than
adequate.
Is it possible to check the accuracy of an
instrument using cheap components? In my
experience reference devices are extremely
expensive and usually more costly than the
value of the instrument itself.
I hope that the previous correspondence
on the subject will not deter future would be
contributors from submitting articles because
I, and many others I suspect, look forward
to reading them, warts and all, thereby
enhancing the knowledge and enjoyment of
the hobby.
HEAT TRANSFER AND X-RAY TUBES
Dr Mark R.StJ Foreman
Regarding Steve (2E0EUR) Milner’s letter,
I am glad that he likes my letter but I
would like to point out that acceleration
is a change of velocity. Velocity is a vector
rather than a scalar quantity so change of
direction which causes no change of speed
is still acceleration. A medical LINAC (linear
accelerator) often generates a beam of high
energy electrons that strike a metal target,
rather like what happens in a normal X-ray
tube. It is interesting that if a beta (fast
electron) radioactive source is placed inside
a metal container then it becomes an X-ray
source. The higher the atomic number of the
metal the more likely that the deflection of
the path of the electrons will be occur, and
thus X-rays will be generated. If the same
beta source is placed in a plastic (low atomic
number) container then far fewer X-rays will
be generated. The atomic nucleus is a very
small object it can be regarded as a small
spherical capacitor that has an electric field.
The larger the atomic number (number of
protons) the greater the charge and thus the
greater the electric field will be.
I have some experience of the 2kW X-ray
tubes used in X-ray diffraction equipment.
I am sure that convection cannot occur
inside the tube as an X-ray tube contains a
very good quality vacuum. When air leaks
into a X-ray tube, the low pressure gas will
conduct electricity very well. This can lead to
a flashover inside the tube (between anode
and cathode), which causes the overcurrent
protection on the power supply on a generator
to trip.
It is normal to control the current by
altering the current flowing in the directly
heated cathode. These 2kW grounded
anode X-ray tubes tend to be cooled by a
constant flow of water, the majority of the
heat transfer between the part of the anode
(which is the target for the electrons) and
the cooling water is by conduction through
the block of copper that forms the anode.
An alternative for lower powered X-ray tubes
would be to immerse the tube plus the high
voltage electronics in oil. Here convection
will be important as the tube is immersed in
a liquid, but the transport of heat inside the
tube will still be a combination of radiation
and conduction.
TIME
Bob, G4PVB
I’ve been looking into clocks for coastal & ship
radio rooms (as well as time). Since early in
the 20th century, the radio frequency of 500
kilohertz (500kHz) has been an international
calling and distress frequency for Morse code
maritime communication. For most of its
history, the international distress frequency
was referred to by its equivalent wavelength,
600 metres, or, using the earlier frequency
unit name, 500 kilocycles [per second] or
500kc. Subsequently (regarding the Titanic
rescue radio chaos) periods were allocated to
monitoring the distress frequency.
As a visual memory aid, a typical clock
in a ship radio room would have the silence
periods marked by shading the sectors
between h+15 to h+18 and h+45 to
h+48 in red. Similar sectors between h+00
to H+03 and h+30 to h+33 are marked
in green, which is the corresponding silence
period for the later voice frequency SSB/AM
2182kHz.
In addition, during this silent period all
coastal and ship stations were required to
monitor the frequency, listening for any
distress signals. All large ships at sea had
to monitor 500kHz at all times, either with
a licensed radio operator or with equipment
that detected an automatic alarm signal.
Odd facts about time. Have you
considered that without the phrase “Turn
clockwise” we would find it difficult to convey
that instruction without gestures? British
Summer (day light saving) Time begins at
0100GMT on the last Sunday of March and
ends at 0100GMT on the last Sunday of
October. GMT is an absolute time reference
and doesn’t change with the seasons. UTC
(atomic based) is the successor to GMT and
for most purposes they are the same.
WHAT A WONDERFUL HOBBY
Julian Sims, G0RMA
What a wonderful hobby we have, access to
many frequencies and we can converse with
any licensed amateur radio operator from
anywhere in the world and indeed off world
to spacecraft if you are lucky.
But has that now changed? I am talking
about the emergence of digital platforms
such as DMR, D-Star and Fusion, where
even these divide in to smaller sub-groups
that are not currently linked to each other –
DMR South West, DMR Plus and D-MARC.
Now as numbers drop we have managed to
segregate in to smaller isolated groups.
I am a DMR user and have been since
early 2015, I am also lucky to have access to
a fabulous repeater network and I applaud the
time and effort my fellow amateurs continue
to devote to building and maintaining the
networks across all platforms. I am however
concerned that by sitting in one camp I am
excluding myself from the other, unless I
purchase multiple equipment types or am
able to change my location. I realise that for
the past 23 years I have not been able to
link to repeaters in quite the seamless way
as most digital repeaters are able, however
surely now is the time to ensure we get a
level of integration and connectivity.
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