CVA Aerial System
By Adrian (M3LCR)
(Graphic Intense Page - May be slow to load)
After a site visit, it was time to decide what the aerial system should be. Initial thoughts centred on a 256 Foot Centre fed dipole with open wire feeder. This assumed that we would have sufficient supports to lift this aloft.
Alas,
although “Big Bertha” had been delivered (Big Bertha is my 70 Foot Pneumatic
Scam mast) it was soon realised that this was impossible to get it to the site.
In addition to this I was somewhat concerned that erecting this monster may
actually be a quite dangerous activity, since I had not had a chance to check
it over. (I didn’t really want 300lbs of Aerial mast coming down on some one’s
head / car / teenager / vintage aircraft etc).
Anyway,
as it happened we still used my 30-foot mast. Initially we had an Inverted V
Dipole, fed with RG58. This proved useable, but after some scratching of heads,
pacing out the field and working out how much wire we had to play with, we came
to the conclusion that a Vee beam was possible.
After
getting permission from the site owners, we sent ‘ZMD aloft (it was agreed that
Steve and myself posed too much of a threat to the well being of the tree to
climb it) so up went ‘ZMD. Each leg of the Vee beam was 100 Metres in length,
fed with Open wire feeder.
Technical stuff….
A
long wire aerial, two wavelengths long, has a lobe of maximum radiation at an
angle of 36 degrees to the wire. If two such wires are erected horizontally in
the form of a V, with an included angle of 72 degrees, and if the phasing
between them is correct, the two pairs of lobes will add, fore and aft along a
line in the plane of the aerial and bisecting the V. Remaining lobes do not act
in this way and so this provides, what is essentially a bi-directional beam,
although minor lobes will occur away from the main beam.
The
Vee beam we erected was somewhat different to the “perfect” one described
above, firstly, we couldn’t get it at an optimum height, secondly, the trees we
used as supports, dictated a North – South beam heading and thirdly the angles
between the two legs was 56 degrees. Nevertheless, the aerial proved to work
well.
Below
are the Aerial modelling diagrams for various bands:
This
is the plot for 80 Metres, centred on 3.650 Mhz. The red lines indicated the
vertical component, and Black, the horizontal. The two elements are fed at the
centre of the diagram, in Blue.
At
7Mhz, 40 Metres, the main lobes are now visible, and unfortunately, since we
couldn’t get the angle correct, the two lobes aren’t combining. Never the less,
as will be shown later, the aerial was very effective on this band.
At
14Mhz, the main lobes are concentrated due North and South. At this point the
aerial, performed well, in these directions, but failed to perform elsewhere.
This is even more evident at 21 Mhz, see below.
Propagation
Below is the 24-hour
prediction received Friday 14 June, for a 24 hour period, received from NW7US,
and the HF Propagation Study website.
SFI: 131 down from 133 | A:6 down from 8 | K:1 down from 2
(7 nT) at 2100 on 14 June.
Observed: No space weather
storms were observed for the past 24 hours.
Expected: No space weather
storms are expected for the next 24 hours.
Sun Spots: 126 as of 13
Jun 2002
Aurora Level: 6
Solar Wind: 360.9 km/s at
3.6 protons/cm3
Global HF Propagation
Conditions
for on 14 Jun, 2002
Low Latitude: Normal
Mid Latitude: Normal
Hi Latitude: Normal-fair
Following this
information, and using VOACAP as the propagation prediction tool, it was
decided that Europe was our main target and that Geneva was likely to be a
fairly central location to base the prediction software on.
If we now map the polar diagrams onto a map
of Europe, we can see, from the propagation prediction, and from the log books,
that the Vee beam was performing exactly as it should, furthermore indicating
that the propagation and aerial modelling tools are accurate.
This
diagram illustrates the areas of Very Strong signal reports, on the 7Mhz band
around 1700 – 1800 UTC (in Red indicate 5/9 +20 Db, Green 5/9). The aerial performed
exactly as predicted, the propagation software was also accurate. The black
line indicates the North – South line of the aerial.
Conclusions
A
number of conclusions can be drawn from this document:
Although
a Vee beam is an efficient, directional aerial, its directional properties will
need to be assessed in terms of their usefulness for such an event in the
future. While it may be a requirement to “put down” a massive signal in certain
events, and in particular “point to
point” working, in this environment the requirement was to work as many
Europeans and, in particular, Museum stations, as possible. With this in mind,
the Vee beam was not really the correct choice (if we had one anyway!).
If
the Vee beam was to be used again, it may be wise to shorten the legs by 50% to
achieve a more radial pattern, and therefore wider audience. This would
obviously be at the expense of received signal strength.