G'day
Many thanks to all those who offered advice on my question as to whether I
should pursue a rotatable Flag or a rotatable 2-element end-fire array (as
per the Titanex and the ON4UN array on page 7-63 of his Low Band DXing Book.
These were to replace the now (almost) defunct VK6VZ beverage antenna - the
neighbour who let me use their land unfortunately moved...
After considering the mechanical problems of such an antenna on my
relatively limited sized block (half an acre is small for 160 - really!)
and feedback from friends/the reflector, a somewhat different approach is
being taken at VK6VZ, which may be of interest to others who want a
high-performance dedicated 160m rx array in a small space.
The experimental array design (the APHaser) has been designed by my friend
Phil VK6APH, is of the 2-element end fire type and is essentially an
'active antenna'. It was inspired by the ON4UN/Titanex ideas and consists
of two shortened dipoles made of wire (12m long), spaced 8m apart and each
fed with equal lengths of RG58 coaxial cable via a link-coupled parallel
tuned circuit. The low-gain loaded dipoles are fed into two W7IUV pre-amps
(special thanks to Greg ZL3IX for his help) which are fed into the main
and auxillary inputs of a modified MFJ-1025 noise canceller, which is used
as a phasing device.
As the arrival of 160m DX signals at VK6VZ is apparently often high angle
and horizontally polarised, the dipole elements will each be sloped at 45
degrees and placed parallel to each other. This allows reception of the
high-angle horizontally polarised signals, as well as low-angle vertically
polarised ones. If the antennas were made vertical, this would, of course,
make it a low-angle vertical array, which would probably suit most people
on this reflector.
In ELNEC, the 3-d pattern of the APHaser array (with its dipoles at 45
degrees) looks something like an upside-down cereal bowl as the result of
its vertical pattern responding to signals arriving at low angles, whilst
in the horizontal plane it responds to high angle signals. The overall
pattern of the two elements when phased together is basically a cardioid.
The two wire dipoles will be suspended under a tree, using a simple boom
made of plastic conduit and lots of polypropylene rope.
Using the MFJ-1025, I should basically be able to set its gain controls so
each dipole element has an identical gain. The phasing control on the
MFJ-1025 should then be able to be rotated to either peak a wanted DX
signal, or in the case of an interfering noise source, the noise peaked
with the phasing control and then the PHASE Normal/Invert button on the
MFJ-1025 pressed (to invert) to remove the noise.
Work has been completed on the MFJ-1025/W7IUV preamps and the dipole
loading/tuning circuits. The next step is to physically build the array,
tune and test it, so there is still a long way to go. If the APHaser array
works as hoped, it will be reported on the reflector and written up for
publication.
Once again, many thanks to all those who shared ideas/information with me
(especially Doug NX4D).
Vy 73,
Steve, VK6VZ
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