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Old February 13th 16, 06:37 AM posted to rec.radio.amateur.moderated,rec.radio.amateur.equipment
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Default [IW5EDI] Ultra-portable 5ele 144MHz Pocket Portable Yagi


IW5EDI Simone - Ham-Radio

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Ultra-portable 5ele 144MHz Pocket Portable Yagi

Posted: 12 Feb 2016 04:33 PM PST
http://www.iw5edi.com/ham-radio/2067...-portable-yagi


Please feel free to build or distribute the information on these antennas.
G4DHF retain the copyright to the designs and ask that they are not
manufactured for commercial gain.
Introduction





This is a design for a 5 element 2 Metre beam with a forward gain of 8dBD
and a front to back ratio of over 24dB.Â*As with my fishing pole yagis, it
has a feed impedance of around 50 ohm andÂ*so the Driven Element only
requires a simple unbalanced to balanced feed. The antenna utilises
flexible plastic coated wire for the elements, which are supported by fibre
spreaders and kite flying cord. When assembled the antenna forms a
rectangle of only 1 x 1.51 Metres. When not in use the antenna collapses
into a very small package, which can be carried in a medium sized pocket or
small bag making it ideal forÂ*backpacking.Â*I have successfully carried
these antennas as hand luggage during air travel and worked some quite
remarkable DX.

The inclusion of the 30cm ruler is for visual reference only




Antenna Modelling using YO Software by K6STI








This yagi is designed using highly flexible 16g plastic coated wire with an
outer diameter of 3mm and a stranded inner conductor of 1.5mm and is of the
type readily available from most electrical retailers. The plastic coating
has a significant effect on the resonant length of the elements due to
changes in the velocity factor when compared to free space values. The
value of the velocity factor was determined by trail and error over a
period of many weeks and several yagis. A correction factor of around 7% to
5% from the free space values gave repeatedly good results and a value of
5% has been applied in this design. If a different wire diameter is used or
the wire is non-insulated changes to the length of each element will be
required to compensate. As the elements are very thin it was expected that
the operating bandwidth would be limited to around a few hundred kHz at
144MHz. As the operating frequency was designed around 144.1 MHz this
should be of little concern to those operating in the European DX section
of the band. In fact, the beam will give creditable performance up to
around 144.6 MHz after which the 50 ohm feed impedance, gain and front to
back ratio rapidly begins to deteriorate.

Cut the element lengths to these dimensions
Cord Framework



The elements are held in position along a string frame, supported by four
6mm fibre spreaders. A number of string types were tried, ranging from
1.5mm polypropylene line to the more conventional household twine. Almost
all exhibited an unacceptable memory affect, which turned the beam into
tangled wire when the supports were removed. The ideal material was found
during a visit to a kite shop when 2mm flying line was purchased. This
consists of hundreds of fine nylon strands bound together in a woven cotton
outer sheath. This cord is highly resilient to stretching, has minimal
memory affect and is extremely strong. Once used there was no going back to
the types used in the prototypes.


Securing the Elements



The end of each element is secured into lengths of 5mm plastic hollow
tubing, obtained from model and craft shops. Each element end support has a
different length in order to preserve the shape of the supporting frame.
After the tips have been drilled each element is threaded along the length
of the cord to their correct position. The end of each element is at a high
RF voltage potential so care needs to be taken to ensure that surrounding
objects and materials due not exhibit detuning effects. Four small lengths
of 2mm plastic rod terminate each of the supporting frame by providing tie
points for the cord.
Element End Support Dimensions




Driven Element



The Driven Element consists of a simple dipole. The feed impedance for a
conventional yagi is usually considerably lower than the desired 50 ohm
feed and requires some form of matching network to compensate. This design
sacrifices a few tenths of a dB of forward gain so that by careful
attention to the length and position of the first director in relation to
the dipole the feed impedance is raised to the desired value. This concept
is not new. Indeed, at least one well-known commercial manufacturer of
V/UHF antennas has used this technique successfully over many years,
including matching UHF folded dipoles directly with 50 ohm cable. These
parameters are, however, quite critical and so careful attention should be
given to ensuring that the recommended dimensions are followed. The centre
of the dipole passes through a small plastic support and is terminated with
standard spade connectors, as weight at this point needs to be kept as
light a possible to reduce element sag. The dipole requires an unbalanced
to balanced balun, which is described below.
Unbalanced to Balanced Coaxial Balun



This balun serves to provide an unbalanced to balanced 50 ohm match, which
helps reduce RF currents on the outer of the feed. Omit this balun at your
peril as the antenna may exhibit false resonance at the desired frequency
or high SWR due to the presence of circulating currents. For RG-58U coax
with a velocity factor of 0.66, cut a 34cm length and trim back the braid
5mm at en end. Note that the two lengths run parallel to each other and
that the braid and inner are isolated at opposite ends.


Construction



The end of each element is cut to the corrected length and secured into
plastic element spacers, which are drilled 5mm from the tip and threaded
onto the two outer lengths of kite cord. The element positions are marked
out onto a length of wood and the cord stretched between two sets of panel
pins. Thread one end of the Reflector element first, followed by the
plastic end cap that holds the fibre support. When the beam is assembled
these caps hold the Reflector and 2nd Director tight against the end
supports. Next thread the Driven element, Director 1, another end cap and
finally Director 2. Repeat the process for the opposite side. When the
fibre supports are inserted into the end caps the beam assumes its physical
dimensions. The element ends can be glued along the cord, but leave
Director 1 free until adjustments are complete.
Fibre Supports



The shape of the yagi is created when the four fibre supports are inserted
into a central hub and the four plastic end caps at each corner of the
beam. Lengths of 6mm fibre rod are available from a DIY stores and kite
retailers. I have even successfully used the types that support bicycle
flags. The total length of each support is around 865cm, which I cut into
three sections for portability. There is an additional 4cm of hidden length
to be added when they are fitted into the central hub. I placed small
lengths of 8mm (6mm ID) aluminium tubing on each end of all the second
sections to enable the rods to be compression fitted together. A word of
caution is necessary when handling and cutting these rods as fibres can
easily become embedded in the skin causing irritation so gloves must be
worn. When the rods have been cut each end should be dipped in Super Glue
to prevent the fibres from peeling. Furthermore, the addition of heat
shrink sleeving makes for safer handling. It is not recommended to use
wooden dowel for the supports as the tension required to form the structure
is quite high and this will either distort or snap the wood.
Supporting Hub and Mast Connection



This yagi has to be lightweight, strong and portable. Metal fixings are
almost eliminated. The supporting plates consist of two 9cm x 7cm sheets of
3mm acrylic. The geometry of the structure is scored on one of the sheets
to determine the centre. Mark the 67-degree angle created by the fibre
supports. The sheets are held together with the markings on top so the
centre-supporting hole can be drilled. If two stacked antennas are planned,
supported by a 6 metre to 9 metre fibre pole, the top antenna has a hole
diameter of 2.5cm while the second, located some two metres lower has a cut
out of 3.5cm. The four 4cm lengths of 8mm aluminium tube that house the
fibre spreaders are then pop riveted in place. Turn the support over and
secure the reverse. Two small L shaped aluminium brackets (or square open
section) are formed and secured either side of the plates. These support a
length of ribbed rubber matting, which, with the additional of cable ties,
increases the grip between the central support and the mast support. There
may be concerns regarding the effect of sunlight on acrylic over time but
because the yagi is intended for temporary use the trade off in weight,
strength and ease of assembly more than compensates for this potential
problem.


Alignment



Attach the antenna to an insulated mast at a comfortable working height of
between five to six feet above ground in a clear environment. Attach the
ends of the coaxial balun to the Driven Element and connect a length of 50
ohm cable to either an antenna analyser or SWR meter connected to a low
level (1 watt max) 2 Metre signal source. Note the impedance or amount of
reflected power. If the dimensions of the beam have been followed closely
these will be reasonably low and close to 50 ohm. Moving only the position
of the first director in 2mm steps, usually towards the Driven Element,
note the change in impedance until the desired match is achieved. The
antenna can now be raised to a more suitable operating height to confirm
alignment. When complete, the position of D1 is marked and secured. Lengths
of lightweight plastic placed either side of the dipole centre help to
maintain this distance, which is critical, when the elements sag slightly
in operation. Once aligned and the distance between D1 and the Driven
Element has been secured power levels of up to 300W have been used
successfully.
Stacking Antennas



As has been previously suggested, stacking two or more of these beams is
perfectly viable due to their low weight. Stacking two antennas at a
distance of one wavelength (6) should yield a forward gain of about 11dBD,
which makes for quite a potent one-person system.
Results



Readers may be interested to know just how effectively this type of antenna
can perform. While testing one of the prototypes in the garden of my QTH
near Spalding (IO92UU) in May 2005 we had a Sporadic E event late in the
afternoon. I worked CN8LI (IM63) at a distance of 2163Km, EA9IB (IM85)
1966Km and several EA7s running only 20W. Given the high QRM levels and the
number of G stations who were active, these results speak for themselves
and should encourage potential builders to get busy. I would be interested
in receiving comments and details of your operating experiences using this
type of antenna.



Author G4DHF



The post Ultra-portable 5ele 144MHz Pocket Portable Yagi appeared first on
IW5EDI Simone - Ham-Radio.


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