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style="MARGIN-BOTTOM: 1px; TEXT-DECORATION: ; FONT-FAMILY: ; WHITE-SPACE: normal; WORD-SPACING: 0px; TEXT-TRANSFORM: none; COLOR: ; PADDING-BOTTOM: 1px; PADDING-LEFT: 8px; LETTER-SPACING: normal; LINE-HEIGHT: normal; BACKGROUND-COLOR: rgb(255,255,255); TEXT-INDENT: 0px; -webkit-text-stroke-width: 0px"
align=left><FONT face=Tahoma><FONT style="FONT-SIZE: 15pt" color=#39617b><FONT
style="FONT-WEIGHT: normal">70cm mixed mode helix</FONT></FONT></FONT></H3>
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style="MARGIN-BOTTOM: 10px; FONT-FAMILY: ; WHITE-SPACE: normal; WORD-SPACING: 0px; TEXT-TRANSFORM: none; COLOR: ; PADDING-BOTTOM: 5px; PADDING-TOP: 0px; PADDING-LEFT: 10px; MARGIN-TOP: 0px; LETTER-SPACING: normal; LINE-HEIGHT: 17px; PADDING-RIGHT: 10px; BACKGROUND-COLOR: rgb(255,255,255); TEXT-INDENT: 0px; -webkit-text-stroke-width: 0px"
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<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><FONT
style="size: 2" face=Arial><FONT style="COLOR: black">This is an experimental
omni-directional antenna for 70cm which has both horizontal and vertical gain,
which is now in use with the Southampton University Wireless Society </FONT><A
style="TEXT-DECORATION: ; COLOR: ; PADDING-BOTTOM: 1px; PADDING-TOP: 1px; PADDING-LEFT: 3px; PADDING-RIGHT: 3px"
href="http://websdr.suws.org.uk/" target=_blank><FONT style="COLOR: #c13c50"
size=+0>WEB SDR</FONT></A><FONT style="COLOR: #414141"
size=+0> </FONT></FONT></P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><FONT
style="COLOR: black" size=2 face=Arial>Please note that the radiation pattern is
optimised to peak toward the horizon, with a gradual reduction in gain towards
the overhead sky. This should give good results for Satellite operation and High
Altitude Balloon tracking, whilst still providing maximum gain towards the
horizon for terrestrial operation using both horizontal and vertical
polarisation. This makes it ideal for use on bands such as 2m & 70cm, where
both polarisations are commonly used.</FONT></P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><FONT
style="COLOR: black"><FONT style="size: +0"><FONT face=Arial><FONT
style="size: 2">As a starting point I had been experimenting with single
turn<SPAN class=Apple-converted-space> </SPAN></FONT><SPAN
style="FONT-FAMILY: "><SPAN><FONT style="FONT-SIZE: 10pt">'</FONT></SPAN><FONT
style="FONT-SIZE: 10pt">twisted halo' design, and decided to try stacking them
to see if I could achieve more gain. Modelling suggested that a stretched 3 turn
helix with a helix circumference of approx 1/2 wave length and an overall length
of 1/2 wave at 70cm, and fed with a gamma match at the centre would offer
reasonable gain, an omni-directional pattern and mixed polarisation. Adding more
turns changed the propagation mode towards that of an axial mode helix. Less
turns reduced the gain in both planes of polarisation, and caused an imbalance
between the horizontal and vertical
gains.</FONT></SPAN></FONT></FONT></FONT></P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><FONT
style="face: arial"><FONT style="size: 2"><FONT style="COLOR: black"><FONT
style="size: +0"><SPAN style="LINE-HEIGHT: 14px">Since originally designing this
antenna, I subsequently found a paper entitled 'The Helical Antenna'<SPAN
class=Apple-converted-space> </SPAN></SPAN><SPAN
style="LINE-HEIGHT: 14px">written by<SPAN
class=Apple-converted-space> </SPAN></SPAN><SPAN
style="LINE-HEIGHT: 14px"> </SPAN><SPAN style="LINE-HEIGHT: 14px">J. Kraus,
W8JK (SK) which was published in the Proceedings of the IRE </SPAN><SPAN
style="LINE-HEIGHT: 14px">Volume: 37 Issue 3 - </SPAN><SPAN
style="LINE-HEIGHT: 14px">1949 Page(s): 263 –
272.</SPAN></FONT></FONT></FONT></FONT></P>
<P class=MsoNormal
style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><SPAN
style="LINE-HEIGHT: 13px"><FONT style="face: arial"><FONT style="size: 2"><FONT
style="COLOR: black"><FONT style="size: +0">In this article a distinction is
made between<SPAN class=Apple-converted-space> </SPAN></FONT>the Normal
(Omni) mode of radiation when the helix circumference is smaller than about 1
wavelength, and the Axial (Beam) mode of radiation when the helix circumference
is about 1 wavelength.</FONT></FONT></FONT></SPAN></P>
<P class=MsoNormal
style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><SPAN
style="LINE-HEIGHT: 13px"><FONT style="face: arial"><FONT style="size: 2"><FONT
style="COLOR: black">In the normal mode, depending on the helix geometry, the
radiation may, in theory, be elliptically, plane, or circularly
polarised.</FONT></FONT></FONT></SPAN></P>
<P class=MsoNormal
style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><FONT
style="face: arial"><FONT style="size: 2"><FONT style="COLOR: black"><SPAN
style="LINE-HEIGHT: 14px">For circular polarisation to occur the ratio of<SPAN
class=Apple-converted-space> </SPAN></SPAN><SPAN
style="LINE-HEIGHT: 14px">π</SPAN><SPAN style="LINE-HEIGHT: 14px"><SPAN
class=Apple-converted-space> </SPAN>x Diameter of Helix has to be equal to
the Square root of 2 x the Spacing between turns x wavelength. So
any helix which has a suitable Diameter to turn Spacing ratio will produce a
circularly polarised radiation pattern at right angles to the length of the
helix. However this formula starts to break down when length of the helix starts
to exceed that of a single ½ wave dipole.</SPAN></FONT></FONT></FONT></P>
<P class=MsoNormal
style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; LINE-HEIGHT: 14px"><FONT style="COLOR: black" size=2
face=arial>Any deviation from this ratio would result in elliptical or linear
(only horizontal or vertical) polarisation.</FONT></SPAN></P>
<P class=MsoNormal
style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><FONT
style="face: arial"><FONT style="size: 2"><FONT style="COLOR: black"><SPAN
style="LINE-HEIGHT: 14px">In the case of my antenna is about the same length as
a dipole and the Diameter to Spacing ratios as defined in the formula is fairly
close 0.377 ≠ 0.387 – </SPAN><SPAN
style="FONT-FAMILY: ; LINE-HEIGHT: 18px"><FONT style="FONT-SIZE: 12pt">So only
65 years after Kraus published, I manged to hit upon the same ratio
!</FONT></SPAN></FONT></FONT></FONT></P>
<P class=MsoNormal
style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><IMG border=0
src="http://webzoom.freewebs.com/g8jnj/70cm%20helix%20wires.png" width=382
height=251></P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><FONT
face=Tahoma><FONT style="FONT-SIZE: 9pt" color=#414141></FONT></FONT> </P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><FONT
style="COLOR: black" size=2 face=arial>Next a plot showing the modelled gain in
both planes, when mounted 10m above ground and fed with coax.</FONT></P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><FONT
face=Tahoma><FONT style="FONT-SIZE: 9pt" color=#414141></FONT></FONT> </P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><IMG
border=0
src="http://webzoom.freewebs.com/g8jnj/3%20turn%2070cm%20helix%20radiation%20pattern.png"
width=715 height=402></P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><FONT
face=Tahoma><FONT style="FONT-SIZE: 9pt" color=#414141></FONT></FONT> </P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><FONT
style="COLOR: black" size=2 face=arial>First attempt at a practical design -
approx 300mm long 100mm between turns</FONT></P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><IMG
border=0
src="http://webzoom.freewebs.com/g8jnj/prototype%203%20turn%20helix.png"></P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><FONT
style="size: 2" face=Arial></FONT><FONT face=Tahoma><FONT style="FONT-SIZE: 9pt"
color=#414141></FONT></FONT> </P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><FONT
style="face: arial"><FONT style="COLOR: black"><SPAN
style="FONT-FAMILY: ; COLOR: "><FONT style="FONT-SIZE: 10pt">Fixed the antenna
to a short length of plastic tube in the workshop, and checked with a hand held
fluorescent tube to see if the voltage distribution looked even between turns.
Got the lamp to strike and then wound the power down to about 5 watts so that I
could see the areas of the tube that remained lit up more clearly. Observed four
distinctive bright spots of similar luminance (although it's not that clear in
the photo) at each of the high voltage points.</FONT></SPAN></FONT></FONT></P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><IMG
style="HEIGHT: 663px; WIDTH: 132px" border=0
src="http://webzoom.freewebs.com/g8jnj/3%20turn%20helix%20light.png" width=208
height=1031></P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; COLOR: "></SPAN><FONT face=Tahoma><FONT
style="FONT-SIZE: 9pt" color=#414141></FONT></FONT> </P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><FONT
style="COLOR: black" face=arial><SPAN style="FONT-FAMILY: ; COLOR: "><FONT
style="FONT-SIZE: 10pt">Decided to check the difference in field strength
between Horizontal and Vertical polarisation, by connecting the antenna to the
VNA output and a small dipole to the VNA input. Held the dipole about 2
wavelengths away and rotated it between horizontal and vertical polarisation.
Although this isn't the ideal environment for antenna measurements, the received
signal levels on each polarisation are remarkably similar. Looks like about 8MHz
bandwidth between 3dB points, not too bad for elements made from 2mm diameter
wire. Larger diameter tubing would give a much wider operational
bandwidth.</FONT></SPAN></FONT></P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><IMG
style="HEIGHT: 462px; WIDTH: 719px" border=0
src="http://webzoom.freewebs.com/g8jnj/H%20and%20V%20gain.png" width=889
height=602></P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><FONT
face=Tahoma><FONT style="FONT-SIZE: 9pt"
color=#414141></FONT></FONT> </P><FONT style="COLOR: black" face=arial>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><FONT
style="size: 2" face=Arial>Final version built from 22mm plastic water pipe and
5mm copper car brake pipe.</FONT></P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><FONT
style="size: 2"><IMG border=0
src="http://webzoom.freewebs.com/g8jnj/70cm%20pipe%20Helix.png" width=645
height=498></FONT></P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><FONT
style="size: 2"></FONT><FONT style="FONT-SIZE: 9pt"></FONT> </P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><FONT
style="face: arial"><FONT style="size: 2"><FONT style="COLOR: black">The turns
of copper pipe are held in place by a single turn of thin copper wire wrapped
around the pipe and soldered close to the plastic pipe. The antenna is quite
high Q so requires some fine tuning once built. I made the last turns slightly
longer than required and then cut them to resonance. Painting the antenna will
lower the resonant frequency slightly. Very fine tuning can be achieved by
bending the gamma match wire.</FONT></FONT></FONT></P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><FONT
style="face: arial"><FONT style="size: 2"><FONT style="COLOR: black">Two or
three suitable ferrite beads also need to be placed on the coax inside the boom
arm near the feed point, in order to choke off common mode currents and create a
balanced feed.</FONT></FONT></FONT></P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><FONT
style="face: arial"><FONT style="size: 2"><FONT style="COLOR: black">The support
boom was made with an inner of 22mm water pipe and an outer of 25mm plastic
electrical conduit. A plastic conduit straight coupler can be persuaded to fit
over the 22mm Tee by warming it with a heat gun.</FONT></FONT></FONT></P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><FONT
style="COLOR: black" size=2 face=arial>The whole assembly is not waterproof, so
the coax connection to the feed point needs to be protected with something like
epoxy resin, hot melt glue or silicone sealant. A small hole also needs to be
drilled in the bottom of each length of plastic tube, to allow any water that
does collect inside is able to drain away.</FONT></P>
<P class=MsoNormal
style="PADDING-LEFT: 5px; MARGIN: 0cm 0cm 10pt; PADDING-RIGHT: 5px"><FONT
style="face: arial"><FONT style="size: 2"><FONT
style="COLOR: black"></FONT></FONT></FONT><FONT
style="FONT-SIZE: 9pt"></FONT> </P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><FONT
style="size: 2"><IMG border=0
src="http://webzoom.freewebs.com/g8jnj/70cm%20Helix%20dimensions.png" width=693
height=1252><BR></FONT></P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: "><FONT style="size: 2"></FONT></SPAN><FONT
style="FONT-SIZE: 9pt"></FONT> </P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: "><FONT style="FONT-SIZE: 10pt">Graph showing the measured
input impedance and SWR.</FONT></SPAN></P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><FONT
style="size: 2"><IMG border=0
src="http://webzoom.freewebs.com/g8jnj/70cm%20helix%20plots.png" width=676
height=588></FONT></P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><SPAN
style="FONT-FAMILY: ; COLOR: "><FONT style="size: 2"></FONT></SPAN><FONT
style="FONT-SIZE: 9pt"></FONT> </P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><FONT
style="size: 2">So far so good,everything seems to work as predicted. This
antenna is now in use on the<SPAN
class=Apple-converted-space> </SPAN></FONT><A
style="TEXT-DECORATION: ; COLOR: ; PADDING-BOTTOM: 1px; PADDING-TOP: 1px; PADDING-LEFT: 3px; PADDING-RIGHT: 3px"
href="http://websdr.suws.org.uk/" target=_blank><FONT style="COLOR: #c13c50"
size=2>Southampton University Wireless Society uWave WEB SDR</FONT></A><FONT
style="size: 2">.</FONT></P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><FONT
style="size: 2"></FONT><FONT style="FONT-SIZE: 9pt"></FONT> </P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><FONT
style="size: 2"><IMG border=0
src="http://webzoom.freewebs.com/g8jnj/70cm%20Helix.png"></FONT></P></FONT>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><FONT
style="COLOR: black" face=arial></FONT><FONT face=Tahoma><FONT
style="FONT-SIZE: 9pt" color=#414141></FONT></FONT> </P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><FONT
face=Tahoma><FONT style="FONT-SIZE: 9pt" color=#414141></FONT></FONT> </P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><FONT
face=Tahoma><FONT style="FONT-SIZE: 9pt" color=#414141></FONT></FONT> </P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><FONT
face=Tahoma><FONT style="FONT-SIZE: 9pt" color=#414141></FONT></FONT> </P>
<P style="MARGIN-BOTTOM: 13px; PADDING-LEFT: 5px; PADDING-RIGHT: 5px"><FONT
face=Tahoma><FONT style="FONT-SIZE: 9pt"
color=#414141></FONT></FONT> </P></DIV>
<DIV style="FONT-SIZE: 12pt; FONT-FAMILY: 'Calibri'; COLOR: #000000">73's de
Carlos CT1GFQ<BR>SKCC#466C www.skccgroup.com<BR>REP#1406
www.rep.pt</DIV></DIV></DIV></BODY></HTML>